Evolution of mammals
Encyclopedia
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The evolution of mammals within the synapsid
lineage (sometimes called "mammal-like reptile
s") was a gradual process that took approximately 70 million years, beginning in the mid-Permian
. By the mid-Triassic
, there were many species that looked like mammal
s, and the first true mammals appeared in the early Jurassic
. The earliest known marsupial
, Sinodelphys
, appeared 125 million years ago in the early Cretaceous
, around the same time as Eomaia
, the first known eutheria
n (member of placentals' "parent" group); and the earliest known monotreme
, Teinolophos
, appeared two million years later. After the Cretaceous-Tertiary extinction wiped out the non-avian dinosaurs (birds are generally regarded as the surviving dinosaurs) and several other mammalian groups, placental and marsupial mammals diversified into many new forms and ecological niches throughout the Tertiary
, by the end of which all modern orders
had appeared.
From the point of view of phylogenetic nomenclature
, mammals are the only surviving synapsid
s. The synapsid lineage became distinct from the sauropsid ("reptile") lineage in the late Carboniferous
period, between 320 and 315 million years ago, and were the most common and largest land vertebrates of the Permian
period. But in the Triassic
period a previously obscure group of sauropsids, the archosaur
s, became the dominant vertebrates and one archosaur group, the dinosaur
s, dominated the rest of the Mesozoic era. These changes forced the Mesozoic mammalaiforms
("nearly mammals") into nocturnal niches, and may have contributed greatly to the development of mammalian traits such as endothermy, hair
and a large brain
. Later in the Mesozoic mammals spread into other ecological niche
s, for example aquatic, gliding and even preying on dinosaurs
. Non-mammalian synapsids are often called mammal-like reptiles despite diverging very early from the other reptilian lines.
Most of the evidence consists of fossils. For many years fossils of Mesozoic
mammals and their immediate ancestors were very rare and fragmentary, but since the mid 1990s there have been many important new finds, especially in China
. The relatively new techniques of molecular phylogenetics
have also shed light on some aspects of mammalian evolution by estimating the timing of important divergence points for modern species. When used carefully, these techniques often, but not always, agree with the fossil record.
Although mammary glands are a signature feature of modern mammals, little is known about the evolution of lactation
, and virtually nothing is known about the evolution of another distinctive feature, the neocortex
region of the brain
. This is because these soft tissues are not often preserved in the fossil record. Hence, most study of the evolution of mammals centers around the development of the middle ear bones
from components of the ancestral amniote
jaw joint. Other much-studied aspects include the evolution of erect limb posture, a bony secondary palate
, fur
and hair
, and warm-bloodedness.
Living mammal species can be identified by the presence of milk-producing mammary gland
s in females. Other features are required when classifying fossils, since mammary glands and other soft-tissue features are not visible in fossils.
Paleontologists
therefore use a distinguishing feature that is shared by all living mammals (including monotremes) but is not present in any of the early Triassic
therapsids: Mammals use two bones
for hearing that all other amniotes use for eating. The earliest amniotes had a jaw joint composed of the articular
(a small bone at the back of the lower jaw) and the quadrate
(a small bone at the back of the upper jaw). All non-mammalian amniotes use this system including lizards, crocodilians, dinosaur
s (and their descendants the birds), and therapsids. But mammals have a different jaw joint, composed only of the dentary (the lower jaw bone, which carries the teeth) and the squamosal
(another small skull bone). In mammals, the quadrate and articular bones have become the incus
and malleus
bones in the middle ear
.
Mammals also have a double occipital condyle
; they have two knobs at the base of the skull that fit into the topmost neck vertebra, and other vertebrates have a single occipital condyle. But paleontologists use only the jaw joint and middle ear as criteria for identifying fossil mammals, as it would be confusing if they found a fossil that had one feature but not the other (e.g. a mammalian jaw and ear but a non-mammalian single occipital condyle).
Due to the incremental changes in transitional fossils, it has been said
s were amniotes — their eggs had internal membranes that allowed the developing embryo
to breathe but kept water in. This allowed amniotes to lay eggs on dry land, while amphibians generally need to lay their eggs in water (a few amphibians, such as the Surinam toad
, have evolved
other ways of getting around this limitation). The first amniotes apparently arose in the late Carboniferous
from the ancestral reptiliomorph
s.
Within a few million years two important amniote lineages became distinct: mammals' synapsid
ancestors and the sauropsids, from which lizard
s, snake
s, crocodilians, dinosaur
s and bird
s are descended. The earliest known fossils of synapsids and sauropsids (such as Archaeothyris
and Hylonomus
resp.) date from about 320 to 315 million years ago. Unfortunately it is difficult to be sure about when each of them evolved, since vertebrate fossils from the late Carboniferous are very rare, and therefore the actual first occurrences of each of these types of animal might have been considerably earlier
.
Synapsid
skulls are identified by the distinctive pattern of the holes behind each eye, which served the following purposes:
Early Permian
terrestrial fossils indicate that one synapsid group, the pelycosaur
s, were the most common land vertebrates of their time and included the largest land animals of the time.
and took over their position as the dominant land vertebrates. They differ from pelycosaurs in several features of the skull and jaws, including larger temporal fenestrae and incisor
s that are equal in size.
The therapsids went through a series of stages, beginning with animals that were very like their pelycosaur ancestors and ending with some that could easily be mistaken for mammals:
Only the dicynodonts, therocephalians and cynodonts survived into the Triassic
.
were the most primitive and pelycosaur-like of the therapsids.
ns ("terrible heads") were large, some as large as a rhinoceros, and included both carnivores and herbivores. Some of the carnivores had semi-erect hindlimbs, but all dinocephalians had sprawling forelimbs. In many ways they were very primitive therapsids, for example they had no secondary palate and their jaws were rather "reptilian".
s, survived almost to the end of the Triassic
. But anomodonts were very different from modern herbivorous mammals, as their only teeth were a pair of fangs in the upper jaw and it is generally agreed that they had beaks like those of birds or ceratopsians.
bone tightly gripped the skull's very small quadrate bone
. This allowed a much wider gape, and one group, the carnivorous gorgonopsia
ns ("gorgon faces"), took advantage of this to develop "sabre teeth". But the theriodont's jaw hinge had a longer term significance — the much reduced size of the quadrate bone was an important step in the development of the mammalian jaw joint and middle ear.
The gorgonopsians still had some primitive features: no bony secondary palate (but other bones in the right places to perform the same functions); sprawling forelimbs; hindlimbs that could operate in both sprawling and erect postures. But the therocephalia
ns ("beast heads"), which appear to have arisen at about the same time as the gorgonopsians, had additional mammal-like features, e.g. their finger and toe bones had the same number of phalanges (segments) as in early mammals (and the same number that primates have, including humans).
The cynodonts, a theriodont group that also arose in the late Permian
, include the ancestors of all mammals — one sub-group, the tritheledonts, is widely regarded as the most likely to contain mammals' ancestor. Cynodonts' mammal-like features include further reduction in the number of bones in the lower jaw, a secondary bony palate, cheek teeth with a complex pattern in the crowns, and a brain which filled the endocranial cavity.
Multi-chambered burrows have been found, containing as many as 20 skeletons of the Early Triassic cynodont Trirachodon
; the animals are thought to have been drowned by a flash flood
. The extensive shared burrows indicate that these animals were capable of complex social behaviors.
slightly more than 250 million years ago killed off about 70 percent of terrestrial vertebrate
species and the majority of land plants.
As a result, ecosystems and food chains collapsed, and the establishment of new stable ecosystems took about 30 million years. With the disappearance of the gorgonopsians, who were dominant predators in the late Permian, the way was open for the cynodonts to compete with another previously obscure group, the archosaur
s, for dominance of the carnivorous niches. The archosaurs include the ancestors of crocodilians, dinosaur
s and bird
s.
The archosaurs quickly became the dominant carnivores, a development often called the "Triassic takeover." Their success may have been due to the fact that the early Triassic
was predominantly arid and therefore archosaurs' superior water conservation gave them a decisive advantage. All known archosaurs have glandless skins and eliminate nitrogenous waste in a uric acid
paste containing little water, while the cynodonts probably excreted most such waste in a solution of urea
, as mammals do today; considerable water is required to keep urea dissolved.
But the Triassic takeover may have been a vital factor in the evolution of the mammals. Two groups stemming from the early cynodonts were successful in niches with minimal competition from the archosaurs: the tritylodonts
, who were herbivore
s, and the mammals, who were small nocturnal insectivore
s. As a result:
The problem of the lack of good fossils of nearly-mammals is a part of the general problem that Mesozoic
mammals were, until recently, known almost exclusively by teeth and jaws. Since the 1980s, the number of Mesozoic fossil mammals has increased decisively, from 116 genera known in 1979 to about 310 in 2007, with an increase in quality such that "at least 18 Mesozoic mammals are represented by nearly complete skeletons".
As a result:
So the evolution of mammals in the Mesozoic is full of uncertainties, although there is no room for doubt that true mammals did first appear in the Mesozoic.
and the quadrate
bones have become the middle ear's malleus
and incus
). But more recently some paleontologists have usually defined "mammal" as the crown group
mammals, i.e. the last common ancestor of monotremes, marsupials and placentals and all of its descendants. The need to address the animals that are more mammal-like than cynodonts, but less closely related to monotremes, marsupials and placentals, led to erecting the group mammaliaformes
to accommodate these primitive forms. Mammaliaformes is a paraphyletic taxon, representing the early radiation of mammals after the jaw-ear criterion. Although this now appears to be the majority approach, some paleontologists have resisted it because it simply moves most of the problems into the new taxon (a paraphyletic one at that) without solving the original problem; the Mammaliaformes includes some animals with "mammalian" jaw joints and some with "reptilian" (articular-to-quadrate) jaw joints; and the newer definition of "mammal" and "mammaliaformes" depends on last common ancestors of both groups, which have not yet been found. Despite these objections, this article follows the majority approach and treats most of the cynodonts' Mesozoic descendants as mammaliaformes.
Multituberculates (named for the multiple tubercles on their "molars
") are often called the "rodents of the Mesozoic" but this is an example of convergent evolution
rather than meaning that they are closely related to the Rodent
ia. At first sight they look like mammals: their jaw joints consists of only the dentary and squamosal
bones, and the quadrate
and articular
bones are part of the middle ear; their teeth are differentiated, occlude and have mammal-like cusp
s; they have a zygomatic arch
; the structure of the pelvis
suggests that they gave birth to tiny helpless young, like modern marsupials. And they lived for over 120 million years (from mid Jurassic
, about 160M years ago, to early Oligocene
, about 35M years ago), which in terms of clade longevity would make them the most successful mammaliaformes ever. But a closer look shows that they are very different from modern mammals:
tidae first appeared in the late Triassic, about 205M years ago. They are an excellent example of transitional fossils, since they have both the dentary-squamosal and articular-quadrate jaw joints. They were also one of the first discovered and most thoroughly studied of the mammaliaformes
, since an unusually large number of morganucodont fossils have been found.
The most notable member of the docodonts
is Castorocauda ("beaver tail"), which lived in the mid Jurassic
about 164M years ago and was first discovered in 2004 and described in 2006. Castorocauda was not a typical docodont (most were omnivores) and not a true mammal, but it is extremely important in the study of the evolution of mammals because the first find was an almost complete skeleton (a real luxury in paleontology) and it breaks the "small nocturnal insectivore" stereotype:
The consensus family tree above shows Hadrocodium
as an "aunt" of crown mammals, while symmetrodonts
and kuehneotheriids
are more closely related to true mammals. But fossils of symmetrodonts and kuehneotheriids are so few and fragmentary that they are poorly understood and may be paraphyletic. On the other hand there are good fossils of Hadrocodium (about 195M years ago in the very early Jurassic
) and they have some important features:
mammals are the extant mammals and their relatives back to their last common ancestor, variously called 'crown mammals' or 'true mammals'. This part of the story introduces new complications, since the crown group are the only group that still has living members, enabling both anatomical and DNA
analysis:
is a group name that has been given to some rather puzzling finds that:
Australosphenida
is a group that has been defined in order to include the Ausktribosphenidae and monotremes. Asfaltomylos (mid- to late Jurassic
, from Patagonia
) has been interpreted as a basal australosphenid (animal that has features shared with both Ausktribosphenidae and monotremes; lacks features that are peculiar to Ausktribosphenidae or monotremes; also lacks features that are absent in Ausktribosphenidae and monotremes) and as showing that australosphenids were widespread throughout Gondwanaland (the old Southern hemisphere super-continent).
But recent analysis of Teinolophos
suggests Teinolophos (about 115M years ago) was a "crown group" (advanced and relatively specialised) monotreme, so the basal (most primitive) monotremes must have appeared considerably earlier; that some alleged Australosphenids were also "crown group" monotremes (e.g. Steropodon
); and that other alleged Australosphenids (e.g. Ausktribosphenos, Bishops, Ambondro, Asfaltomylos) are therefore more closely related to and possibly members of the Therian mammals (group that includes marsupials and placentals, see below).
is Teinolophos
, which lived about 123M years ago in Australia
. Recent (2007, published 2008) analysis suggest that it was not a basal (primitive, ancestral) monotreme but a full-fledged platypus
, and therefore that the platypus and echidna
lineages diverged considerably earlier and that basal monotremes were even earlier.
A more recent study (2009), however, has suggested that while Teinolophis was a type of platypus, it also was a basal monotreme and predated the radiation of modern monotremes. The semi-aquatic lifestyle of platypuses prevented them from being outcompeted by the marsupials that migrated to Australia millions of years ago, since joeys need to keep attached to their mothers and would drown if their mothers ventured into water. Genetic evidence has determined that echidnas diverged from the platypus lineage as recently as 19-48M when they made their transition from semi-aquatic to terrestrial lifestyle.
Monotremes have some features that may be inherited from the cynodont
ancestors:
Unlike in other mammals, female monotremes do not have nipples and feed their young by "sweating" milk from patches on their bellies.
Of course these features are not visible in fossils, and the main characteristics from paleontologists' point of view are:
("beasts") is a name applied to the hypothetical group from which both metatheria
(which include marsupials) and eutheria
(which include placentals) descended. Although no convincing fossils of basal therians have been found (just a few teeth and jaw fragments), metatheria and eutheria share some features that one would expect to have been inherited from a common ancestral group:
are all marsupials ("animals with pouches"). A few fossil genera
such as the Mongolian late Cretaceous Asiatherium may be marsupials or members of some other metatherian group(s).
The oldest known marsupial is Sinodelphys
, found in 125M-year-old early Cretaceous
shale
in China's northeastern Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.
Didelphimorphia
(common opossums of the Western Hemisphere
) first appeared in the late Cretaceous and still have living representatives, probably because they are mostly semi-arboreal unspecialized omnivores.
The best-known feature of marsupials is their method of reproduction:
Although some marsupials look very like some placentals (the thylacine
or "marsupial wolf" is a good example), marsupial skeletons have some features that distinguish them from placentals:
Marsupials also have a pair of marsupial bones (sometimes called "epipubic bones
"), which support the pouch in females. But these are not unique to marsupials, since they have been found in fossils of multituberculates, monotremes, and even eutherians — so they are probably a common ancestral feature that disappeared at some point after the ancestry of living placental mammals diverged from that of marsupials.
Some researchers think the epipubic bones' original function was to assist locomotion by supporting some of the muscles that pull the thigh forwards.
("true beasts") are all placentals. But the earliest known eutherian, Eomaia
, found in China
and dated to 125M years ago, has some features that are more like those of marsupials (the surviving metatherians):
Eomaia also has a Meckelian groove
, a primitive feature of the lower jaw that is not found in modern placental mammals.
These intermediate features are consistent with molecular phylogenetics estimates that the placentals diversified about 110M years ago, 15M years after the date of the Eomaia fossil.
Eomaia also has many features that strongly suggest it was a climber, including several features of the feet and toes; well-developed attachment points for muscles that are used a lot in climbing; and a tail that is twice as long as the rest of the spine.
Placentals' best-known feature is their method of reproduction:
It has been suggested that the evolution of placental reproduction was made possible by retrovirus
es that:
From a paleontologist's point of view, eutherians are mainly distinguished by various features of their teeth, ankles and feet.
, show that some mammaliaforms and true mammals were larger and had a variety of lifestyles. For example:
) and that the earliest members of each group were anatomically very similar to early members of other groups and differed only in their genetics
. These debates extend to the definition of and relationships between the major groups of placentals — the controversy about Afrotheria
is a good example.
For the sake of brevity and simplicity the diagram omits some extinct groups in order to focus on the ancestry of well-known modern groups of placentals — X marks extinct groups. The diagram also shows the following:
This family tree contains some surprises and puzzles. For example:
s to work out family trees in much the same way as paleontologists do with features of fossils — if two organisms' genes are more similar to each other than to those of a third organism, the two organisms are more closely related to each other than to the third.
Molecular phylogeneticists have proposed a family tree that is very different from the one with which paleontologists are familiar. Like paleontologists, molecular phylogeneticists have different ideas about various details, but here is a typical family tree according to molecular phylogenetics: Note that the diagram shown here omits extinct groups, as one cannot extract DNA from fossils.
Here are the most significant of the many differences between this family tree and the one familiar to paleontologists:
The grouping together of the Afrotheria has some geological justification. All surviving members of the Afrotheria originate from South American or (mainly) African lineages — even the Indian elephant
, which diverged from an African lineage about . As Pangaea
broke up Africa and South America separated from the other continents less than 150M years ago, and from each other between 100M and 80M years ago. The earliest known eutherian mammal is Eomaia, from about 125M years ago. So it would not be surprising if the earliest eutherian immigrants into Africa and South America were isolated there and radiated
into all the available ecological niche
s.
Nevertheless these proposals have been controversial. Paleontologists naturally insist that fossil evidence must take priority over deductions from samples of the DNA of modern animals. More surprisingly, these new family trees have been criticised by other molecular phylogeneticists, sometimes quite harshly:
diverged about 100M to 85M years ago, but that modern families
first appeared in the late Eocene
and early Miocene
.
Some paleontologists object that no placental fossils have been found from before the end of the Cretaceous — for example Maelestes gobiensis, from about 75M years ago, is a eutherian but not a true placental. Many Cretaceous fossil sites contain well-preserved lizards, salamanders, birds, and mammals, but not the modern forms of mammals. It is likely that they simply did not exist, and that the molecular clock
runs fast during major evolutionary radiation
s. On the other hand there is fossil evidence from of hoof
ed animals that may be ancestors of modern ungulate
s.
Fossils of the earliest members of most modern groups date from the Paleocene
, a few date from later and very few from the Cretaceous
, before the extinction of the dinosaur
s. But some paleontologists, influenced by molecular phylogenetic studies, have used statistical methods to extrapolate
backwards from fossils of members of modern groups and concluded that primates arose in the late Cretaceous. However statistical studies of the fossil record confirm that mammals were restricted in size and diversity right to the end of the Cretaceous, and rapidly grew in size and diversity during the Early Paleocene
.
Hadrocodium
, whose fossils date from the early Jurassic
, provides the first clear evidence of fully mammalian jaw joints and middle ears, in which the jaw joint is formed by the dentary and squamosal
bones while the articular
and quadrate
move to the middle ear, where they are known as the incus
and malleus
. Traditionally, separate ear bones have been used as the diagnostic cut-off point from mammals, making Hadrocodium the oldest known mammal. It does however fall outside the mammalian crown group
of extant species, and is thus classified as a member of the mammaliaformes
rather than as a true mammal by the crown group phylogenetic
definition.
One analysis of the monotreme Teinolophos
suggested that this animal had a pre-mammalian jaw joint formed by the angular and quadrate bones and that the typical mammalian middle ear evolved twice independently, in monotremes and in theria
n mammals, but this idea has been disputed. In fact 2 of the suggestion's authors co-authored a later paper that reinterpreted the same features as evidence that Teinolophos was a full-fledged platypus
, which means it would have had a mammalian jaw joint and middle ear.
Later research demonstrated that caseins already appeared in the common mammalian ancestor approximately 200-310 Millions of years ago.
.
From 1955 onwards some scientists have interpreted the foramina (passages) in the maxilla
e (upper jaws) and premaxilla
e (small bones in front of the maxillae) of cynodont
s as channels that supplied blood vessels and nerves to vibrissae (whiskers), and suggested that this was evidence of hair or fur. But foramina do not necessarily show that an animal had vibrissae — for example the modern lizard Tupinambis has foramina that are almost identical to those found in the non-mammalian cynodont Thrinaxodon
.
(distinguishing characteristic) of the Boreosphenida
, a group that contains the Theria
and therefore includes the last common ancestor of modern marsupials and placentals — and therefore that all earlier mammals had sprawling limbs.
Sinodelphys
(the earliest known marsupial) and Eomaia
(the earliest known eutheria
n) lived about 125M years ago, so erect limbs must have evolved before then.
ness" is a complex and rather ambiguous term, because it includes some or all of the following:
Since scientists cannot know much about the internal mechanisms of extinct creatures, most discussion focuses on homeothermy and tachymetabolism.
Modern monotremes have a lower body temperature and more variable metabolic rate than marsupials and placentals. So the main question is when a monotreme-like metabolism evolved in mammals. The evidence found so far suggests Triassic
cynodont
s may have had fairly high metabolic rates, but is not conclusive.
s that warm and moisten inhaled air and extract heat and moisture from exhaled air. An animal with respiratory turbinates can maintain a high rate of breathing without the danger of drying its lungs out, and therefore may have a fast metabolism. Unfortunately these bones are very delicate and therefore have not yet been found in fossils. But rudimentary ridges like those that support respiratory turbinates have been found in Triassic therapsids such as Thrinaxodon
and Diademodon
, which suggests that they may have had fairly high metabolic rates.
helps mammals to breathe, especially during strenuous activity. For a diaphragm to work, the ribs must not restrict the abdomen, so that expansion of the chest can be compensated for by reduction in the volume of the abdomen and vice versa. The advanced cynodonts have very mammal-like rib cages, with greatly reduced lumbar ribs. This suggests that these animals had diaphragms, were capable of strenuous activity for fairly long periods and therefore had high metabolic rates. On the other hand these mammal-like rib cages may have evolved to increase agility. The movement of even advanced therapsids was however "like a wheelbarrow", with the hindlimbs providing all the thrust while the forelimbs only steered the animal, in other words advanced therapsids were not as agile as either modern mammals or the early dinosaur
s. So the idea that the main function of these mammal-like rib cages was to increase agility is doubtful.
would have made it rather difficult for them to move and breathe at the same time, but not as difficult as it is for animals such as lizards, which have completely sprawling limbs. The cynodonts (advanced therapsids) had costal plates that stiffened the rib cage and therefore may have reduced sideways flexing of the trunk while moving, which would have made it a little easier for them to breathe while moving . These facts suggest that advanced therapsids were significantly less active than modern mammals of similar size and therefore may have had slower metabolisms.
The first clear evidence of hair or fur is in fossils of Castorocauda, from 164M years ago in the mid Jurassic
; arguments that advanced therapsids had hair are unconvincing.
The evolution of mammals within the synapsid
Synapsid
Synapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...
lineage (sometimes called "mammal-like reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s") was a gradual process that took approximately 70 million years, beginning in the mid-Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
. By the mid-Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
, there were many species that looked like mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, and the first true mammals appeared in the early Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
. The earliest known marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
, Sinodelphys
Sinodelphys
Sinodelphys is an extinct mammal from the Early Cretaceous. To date, it is the oldest metatherian fossil known, estimated to be 125 million years old...
, appeared 125 million years ago in the early Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
, around the same time as Eomaia
Eomaia
Eomaia is an extinct fossil mammal, discovered in rocks that were found in the Yixian Formation, Liaoning Province, China, and dated to the Barremian Age of the Lower Cretaceous about . The fossil is in length and virtually complete. An estimate of the body weight is between . It is exceptionally...
, the first known eutheria
Eutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
n (member of placentals' "parent" group); and the earliest known monotreme
Monotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
, Teinolophos
Teinolophos
Teinolophos trusleri was a prehistoric species of monotreme, or egg-laying mammal. It is known from a lower jawbone found in Flat Rocks, Victoria, Australia. It lived during the Aptian age of the Lower Cretaceous. It is the earliest known relative of the Platypus.The species name honours the...
, appeared two million years later. After the Cretaceous-Tertiary extinction wiped out the non-avian dinosaurs (birds are generally regarded as the surviving dinosaurs) and several other mammalian groups, placental and marsupial mammals diversified into many new forms and ecological niches throughout the Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
, by the end of which all modern orders
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
had appeared.
From the point of view of phylogenetic nomenclature
Phylogenetic nomenclature
Phylogenetic nomenclature or phylogenetic taxonomy is an alternative to rank-based nomenclature, applying definitions from cladistics . Its two defining features are the use of phylogenetic definitions of biological taxon names, and the lack of obligatory ranks...
, mammals are the only surviving synapsid
Synapsid
Synapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...
s. The synapsid lineage became distinct from the sauropsid ("reptile") lineage in the late Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
period, between 320 and 315 million years ago, and were the most common and largest land vertebrates of the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
period. But in the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
period a previously obscure group of sauropsids, the archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
s, became the dominant vertebrates and one archosaur group, the dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s, dominated the rest of the Mesozoic era. These changes forced the Mesozoic mammalaiforms
Mammaliaformes
Mammaliaformes is a clade that contains the mammals and their closest extinct relatives. Phylogenetically, it is defined as a clade including the most recent common ancestor of Sinoconodon, morganuconodonts, docodonts, Monotremata, Marsupialia, Placentalia, extinct members of this clade, and all...
("nearly mammals") into nocturnal niches, and may have contributed greatly to the development of mammalian traits such as endothermy, hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
and a large brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
. Later in the Mesozoic mammals spread into other ecological niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
s, for example aquatic, gliding and even preying on dinosaurs
Repenomamus
Repenomamus is the largest mammal known from the Cretaceous period of Manchuria, and it is the mammal for which there is the best evidence that it fed on dinosaurs. It is not possible to determine if Repenomamus actively hunted live dinosaurs or scavenged dead dinosaurs.-Paleobiology:Repenomamus...
. Non-mammalian synapsids are often called mammal-like reptiles despite diverging very early from the other reptilian lines.
Most of the evidence consists of fossils. For many years fossils of Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
mammals and their immediate ancestors were very rare and fragmentary, but since the mid 1990s there have been many important new finds, especially in China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
. The relatively new techniques of molecular phylogenetics
Molecular phylogeny
Molecular phylogenetics is the analysis of hereditary molecular differences, mainly in DNA sequences, to gain information on an organism's evolutionary relationships. The result of a molecular phylogenetic analysis is expressed in a phylogenetic tree...
have also shed light on some aspects of mammalian evolution by estimating the timing of important divergence points for modern species. When used carefully, these techniques often, but not always, agree with the fossil record.
Although mammary glands are a signature feature of modern mammals, little is known about the evolution of lactation
Lactation
Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, however it predates mammals. In humans the process of feeding milk is called breastfeeding or nursing...
, and virtually nothing is known about the evolution of another distinctive feature, the neocortex
Neocortex
The neocortex , also called the neopallium and isocortex , is a part of the brain of mammals. It is the outer layer of the cerebral hemispheres, and made up of six layers, labelled I to VI...
region of the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
. This is because these soft tissues are not often preserved in the fossil record. Hence, most study of the evolution of mammals centers around the development of the middle ear bones
Evolution of mammalian auditory ossicles
The evolution of mammalian auditory ossicles is one of the most well-documented and important evolutionary events, demonstrating both numerous transitional forms as well as an excellent example of exaptation, the re-purposing of existing structures during evolution.In reptiles, the eardrum is...
from components of the ancestral amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...
jaw joint. Other much-studied aspects include the evolution of erect limb posture, a bony secondary palate
Palate
The palate is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but, in most other tetrapods, the oral and nasal cavities are not truly separate. The palate is divided into two parts, the anterior...
, fur
Fur
Fur is a synonym for hair, used more in reference to non-human animals, usually mammals; particularly those with extensives body hair coverage. The term is sometimes used to refer to the body hair of an animal as a complete coat, also known as the "pelage". Fur is also used to refer to animal...
and hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
, and warm-bloodedness.
Definition of "mammal"
Mammary gland
A mammary gland is an organ in mammals that produces milk to feed young offspring. Mammals get their name from the word "mammary". In ruminants such as cows, goats, and deer, the mammary glands are contained in their udders...
s in females. Other features are required when classifying fossils, since mammary glands and other soft-tissue features are not visible in fossils.
Paleontologists
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
therefore use a distinguishing feature that is shared by all living mammals (including monotremes) but is not present in any of the early Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
therapsids: Mammals use two bones
Ossicles
The ossicles are the three smallest bones in the human body. They are contained within the middle ear space and serve to transmit sounds from the air to the fluid-filled labyrinth . The absence of the auditory ossicles would constitute a moderate-to-severe hearing loss...
for hearing that all other amniotes use for eating. The earliest amniotes had a jaw joint composed of the articular
Articular
The articular bone is part of the lower jaw of most tetrapods, including amphibians, sauropsids and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular...
(a small bone at the back of the lower jaw) and the quadrate
Quadrate bone
The quadrate bone is part of a skull in most tetrapods, including amphibians, sauropsids , and early synapsids. In these animals it connects to the quadratojugal and squamosal in the skull, and forms part of the jaw joint .- Evolutionary variation :In snakes, the quadrate bone has become elongated...
(a small bone at the back of the upper jaw). All non-mammalian amniotes use this system including lizards, crocodilians, dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s (and their descendants the birds), and therapsids. But mammals have a different jaw joint, composed only of the dentary (the lower jaw bone, which carries the teeth) and the squamosal
Squamosal
The squamosal is a bone of the head of higher vertebrates. It is the principal component of the cheek region in the skull, lying below the temporal series and otic notch and bounded anteriorly by postorbital. Posteriorly, the squamosal articulates with the posterior elements of the palatal complex,...
(another small skull bone). In mammals, the quadrate and articular bones have become the incus
Incus
The incus or anvil is the anvil-shaped small bone or ossicle in themiddle ear. It connects the malleus to the stapes. It was first described by Alessandro Achillini of Bologna.The incus transmits sound vibrations from the malleus to the stapes....
and malleus
Malleus
The malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum...
bones in the middle ear
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
.
Mammals also have a double occipital condyle
Occipital condyle
The occipital condyles are undersurface facets of the occipital bone in vertebrates, which function in articulation with the superior facets of the atlas vertebra....
; they have two knobs at the base of the skull that fit into the topmost neck vertebra, and other vertebrates have a single occipital condyle. But paleontologists use only the jaw joint and middle ear as criteria for identifying fossil mammals, as it would be confusing if they found a fossil that had one feature but not the other (e.g. a mammalian jaw and ear but a non-mammalian single occipital condyle).
Due to the incremental changes in transitional fossils, it has been said
We may again ask the question, What is a mammal? Where we draw the line between reptile and mammal has no biological significance. It is purely a matter of convenience. There are two obvious choices, both immediately following a period of rapid evolution that make as definite a break as we can hope to find.
The ancestry of mammals
Amniotes
The first fully terrestrial vertebrateVertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s were amniotes — their eggs had internal membranes that allowed the developing embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
to breathe but kept water in. This allowed amniotes to lay eggs on dry land, while amphibians generally need to lay their eggs in water (a few amphibians, such as the Surinam toad
Surinam toad
Surinam toads, also called star-fingered toads, are members of the frog genus Pipa, within the family Pipidae. They are native to northern South America...
, have evolved
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
other ways of getting around this limitation). The first amniotes apparently arose in the late Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
from the ancestral reptiliomorph
Reptiliomorpha
Reptiliomorpha refers to an order or subclass of reptile-like amphibians, which gave rise to the amniotes in the Carboniferous. Under phylogenetic nomenclature, the Reptiliomorpha includes their amniote descendants though, even in phylogenetic nomenclature, the name is mostly used when referring to...
s.
Within a few million years two important amniote lineages became distinct: mammals' synapsid
Synapsid
Synapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...
ancestors and the sauropsids, from which lizard
Lizard
Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...
s, snake
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...
s, crocodilians, dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s and bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s are descended. The earliest known fossils of synapsids and sauropsids (such as Archaeothyris
Archaeothyris
Archaeothyris was a very early mammal-like reptile, which lived in the late Carboniferous period. Dated to 306 million years ago, it is the oldest undisputed synapsid known....
and Hylonomus
Hylonomus
Hylonomus was a very early reptile. It lived 312 million years ago during the Late Carboniferous period.It is the earliest unquestionable reptile ....
resp.) date from about 320 to 315 million years ago. Unfortunately it is difficult to be sure about when each of them evolved, since vertebrate fossils from the late Carboniferous are very rare, and therefore the actual first occurrences of each of these types of animal might have been considerably earlier
Signor-Lipps effect
The Signor–Lipps effect is a paleontological principle proposed by Philip W. Signor and Jere H. Lipps which states that, since the fossil record of organisms is never complete, neither the first nor the last organism in a given taxon will be recorded as a fossil.One famous example is the...
.
Synapsids
Synapsid
Synapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...
skulls are identified by the distinctive pattern of the holes behind each eye, which served the following purposes:
- made the skull lighter without sacrificing strength.
- saved energy by using less bone.
- probably provided attachment points for jaw muscles. Having attachment points further away from the jaw made it possible for the muscles to be longer and therefore to exert a strong pull over a wide range of jaw movement without being stretched or contracted beyond their optimum range.
Early Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
terrestrial fossils indicate that one synapsid group, the pelycosaur
Pelycosaur
The pelycosaurs are an informal grouping composed of basal or primitive Late Paleozoic synapsid amniotes. Some species were quite large and could grow up to 3 meters or more, although most species were much smaller...
s, were the most common land vertebrates of their time and included the largest land animals of the time.
Therapsids
Therapsids descended from pelycosaurs in the middle PermianPermian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
and took over their position as the dominant land vertebrates. They differ from pelycosaurs in several features of the skull and jaws, including larger temporal fenestrae and incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s that are equal in size.
The therapsids went through a series of stages, beginning with animals that were very like their pelycosaur ancestors and ending with some that could easily be mistaken for mammals:
- gradual development of a bony secondary palateHard palateThe hard palate is a thin horizontal bony plate of the skull, located in the roof of the mouth. It spans the arch formed by the upper teeth.It is formed by the palatine process of the maxilla and horizontal plate of palatine bone....
. Most books and articles interpret this as a prequisite for the evolution of mammals' high metabolic rate, because it enabled these animals to eat and breathe at the same time. But some scientists point out that some modern ectothermEctothermAn ectotherm, from the Greek εκτός "outside" and θερμός "hot", refers to organisms that control body temperature through external means. As a result, organisms are dependent on environmental heat sources and have relatively low metabolic rates. For example, many reptiles regulate their body...
s use a fleshy secondary palate to separate the mouth from the airway, and that a bony palate provides a surface on which the tongue can manipulate food, facilitating chewing rather than breathing. The interpretation of the bony secondary palate as an aid to chewing also suggests the development of a faster metabolism, since chewing makes it possible to digest food more quickly. In mammals the palate is formed by two specific bones, but various Permian therapsids had other combinations of bones in the right places to function as a palate. - the dentary gradually becomes the main bone of the lower jaw.
- progress towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraintCarrier's constraintCarrier's constraint is the observation that air-breathing vertebrates which have two lungs and flex their bodies sideways during locomotion find it very difficult to move and breathe at the same time, because:...
. But this process was erratic and very slow — for example: all herbivorous therapsids retained sprawling limbs (some late forms may have had semi-erect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semi-sprawling hindlimbs. In fact, modern monotremeMonotremeMonotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
s still have semi-sprawling limbs. - in the TriassicTriassicThe Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
, progress towards the mammalian jaw and middle ear. - there is plausible evidence of hair in Triassic therapsids, but none for Permian therapsids (see below).
- some scientists have argued that some Triassic therapsids show signs of lactationLactationLactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, however it predates mammals. In humans the process of feeding milk is called breastfeeding or nursing...
(see below).
Therapsid family tree
(simplified from; only those that are most relevant to the evolution of mammals are described below)Only the dicynodonts, therocephalians and cynodonts survived into the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
.
Biarmosuchia
The BiarmosuchiaBiarmosuchia
Biarmosuchia, also known as Eotitanosuchia and Phthinosuchia, is an assemblage of primitive Permian therapsids that represent either a paraphyletic stem group or a very early off-shoot of the main therapsid tree....
were the most primitive and pelycosaur-like of the therapsids.
Dinocephalians
DinocephaliaDinocephalia
Dinocephalia are a clade of large early therapsids that flourished during the Middle Permian, but became extinct leaving no descendants.-Description:...
ns ("terrible heads") were large, some as large as a rhinoceros, and included both carnivores and herbivores. Some of the carnivores had semi-erect hindlimbs, but all dinocephalians had sprawling forelimbs. In many ways they were very primitive therapsids, for example they had no secondary palate and their jaws were rather "reptilian".
Anomodonts
The anomodonts ("anomalous teeth") were the most successful of the herbivorous therapsids — one sub-group, the dicynodontDicynodont
Dicynodontia is a taxon of anomodont therapsids or mammal-like reptiles. Dicynodonts were small to large herbivorous animals with two tusks, hence their name, which means 'two dog tooth'...
s, survived almost to the end of the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
. But anomodonts were very different from modern herbivorous mammals, as their only teeth were a pair of fangs in the upper jaw and it is generally agreed that they had beaks like those of birds or ceratopsians.
Theriodonts
The theriodonts ("beast teeth") and their descendants had jaw joints in which the lower jaw's articularArticular
The articular bone is part of the lower jaw of most tetrapods, including amphibians, sauropsids and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular...
bone tightly gripped the skull's very small quadrate bone
Quadrate bone
The quadrate bone is part of a skull in most tetrapods, including amphibians, sauropsids , and early synapsids. In these animals it connects to the quadratojugal and squamosal in the skull, and forms part of the jaw joint .- Evolutionary variation :In snakes, the quadrate bone has become elongated...
. This allowed a much wider gape, and one group, the carnivorous gorgonopsia
Gorgonopsia
Gorgonopsia is a suborder of therapsid synapsids. Their name is a reference to the Gorgons of Greek mythology. Like other therapsids, gorgonopsians were at one time called "mammal-like reptiles"...
ns ("gorgon faces"), took advantage of this to develop "sabre teeth". But the theriodont's jaw hinge had a longer term significance — the much reduced size of the quadrate bone was an important step in the development of the mammalian jaw joint and middle ear.
The gorgonopsians still had some primitive features: no bony secondary palate (but other bones in the right places to perform the same functions); sprawling forelimbs; hindlimbs that could operate in both sprawling and erect postures. But the therocephalia
Therocephalia
Therocephalians are an extinct suborder of carnivorous eutheriodont therapsids that lived from the middle and late Permian into the Triassic 265.0—245.0 Ma existing for approximately ....
ns ("beast heads"), which appear to have arisen at about the same time as the gorgonopsians, had additional mammal-like features, e.g. their finger and toe bones had the same number of phalanges (segments) as in early mammals (and the same number that primates have, including humans).
Cynodonts
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
, include the ancestors of all mammals — one sub-group, the tritheledonts, is widely regarded as the most likely to contain mammals' ancestor. Cynodonts' mammal-like features include further reduction in the number of bones in the lower jaw, a secondary bony palate, cheek teeth with a complex pattern in the crowns, and a brain which filled the endocranial cavity.
Multi-chambered burrows have been found, containing as many as 20 skeletons of the Early Triassic cynodont Trirachodon
Trirachodon
Trirachodon is an extinct genus of tritylodontoid cynodont. Fossils have been found from the Cynognathus Assemblage Zone of the Beaufort Group in South Africa, dating back to the Early and Middle Triassic.-Description:...
; the animals are thought to have been drowned by a flash flood
Flash flood
A flash flood is a rapid flooding of geomorphic low-lying areas—washes, rivers, dry lakes and basins. It may be caused by heavy rain associated with a storm, hurricane, or tropical storm or meltwater from ice or snow flowing over ice sheets or snowfields...
. The extensive shared burrows indicate that these animals were capable of complex social behaviors.
Triassic takeover
The catastrophic Permian-Triassic mass extinctionPermian-Triassic extinction event
The Permian–Triassic extinction event, informally known as the Great Dying, was an extinction event that occurred 252.28 Ma ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras...
slightly more than 250 million years ago killed off about 70 percent of terrestrial vertebrate
Chordate
Chordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...
species and the majority of land plants.
As a result, ecosystems and food chains collapsed, and the establishment of new stable ecosystems took about 30 million years. With the disappearance of the gorgonopsians, who were dominant predators in the late Permian, the way was open for the cynodonts to compete with another previously obscure group, the archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
s, for dominance of the carnivorous niches. The archosaurs include the ancestors of crocodilians, dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s and bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s.
The archosaurs quickly became the dominant carnivores, a development often called the "Triassic takeover." Their success may have been due to the fact that the early Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
was predominantly arid and therefore archosaurs' superior water conservation gave them a decisive advantage. All known archosaurs have glandless skins and eliminate nitrogenous waste in a uric acid
Uric acid
Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates such as ammonium acid urate. Uric acid is created when the body breaks down purine nucleotides. High blood concentrations of uric acid...
paste containing little water, while the cynodonts probably excreted most such waste in a solution of urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
, as mammals do today; considerable water is required to keep urea dissolved.
But the Triassic takeover may have been a vital factor in the evolution of the mammals. Two groups stemming from the early cynodonts were successful in niches with minimal competition from the archosaurs: the tritylodonts
Tritylodontidae
Tritylodontids were small to medium-sized, highly specialized and extremely mammal-like cynodonts. They were the last family of the non-mammalian synapsids. One of the last cynodont lines to appear, the Tritylodontidae descended from a Cynognathus-like cynodont...
, who were herbivore
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
s, and the mammals, who were small nocturnal insectivore
Insectivore
An insectivore is a type of carnivore with a diet that consists chiefly of insects and similar small creatures. An alternate term is entomophage, which also refers to the human practice of eating insects....
s. As a result:
- The therapsid trend towards differentiated teeth with precise occlusionOcclusion (dentistry)Occlusion, in a dental context, means simply the contact between teeth. More technically, it is the relationship between the maxillary and mandibular teeth when they approach each other, as occurs during chewing or at rest....
accelerated, because of the need to hold captured arthropodArthropodAn arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s and crush their exoskeletonExoskeletonAn exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
s. - As the body length of the mammals' ancestors fell below 50 mm (2 inches), advances in thermal insulationThermal insulationThermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
and temperature regulation became necessary for nocturnal life. - Acute senses of hearing and smell became vital.
- This accelerated the development of the mammalian middle ear, and therefore of the mammalian jaw since bones that had been part of the jaw joint became part of the middle ear.
- The increase in the size of the olfactory lobes of the brain increased brain weight as a percentage of total body weight. Brain tissue requires a disproportionate amount of energy. The need for more food to support the enlarged brains increased the pressures for improvements in insulation, temperature regulation and feeding.
- As a side-effect of the nocturnal life, discerning colors became less important (they lost two out of four opsins), and this is reflected in the fact that most mammals have poor color visionColor visionColor vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect, emit, or transmit...
, including the "lower primates"ProsimianProsimians are a grouping of mammals defined as being primates, but not monkeys or apes. They include, among others, lemurs, bushbabies, and tarsiers. They are considered to have characteristics that are more primitive than those of monkeys and apes. Prosimians are the only primates native to...
such as lemurLemurLemurs are a clade of strepsirrhine primates endemic to the island of Madagascar. They are named after the lemures of Roman mythology due to the ghostly vocalizations, reflective eyes, and the nocturnal habits of some species...
s.
Many uncertainties
The Triassic takeover probably accelerated the evolution of mammals. The nearly-mammals are preserved in few good fossils, mainly because they were mostly smaller than rats:- They were largely restricted to environments that are less likely to provide good fossils. Floodplains as the best terrestrial environments for fossilization provide little fossils of nearly-mammals, because they are dominated by medium to large animals, and the Triassic therapsids and near-mammals could not compete with archosaurs in the medium to large size range.
- Their delicate bones were vulnerable to being destroyed before they could be fossilized — by scavengers (including fungi and bacteriaBacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
) and by being trodden on. - Small fossils are harder to spot and more vulnerable to being destroyed by weathering and other natural stresses before they are discovered.
The problem of the lack of good fossils of nearly-mammals is a part of the general problem that Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
mammals were, until recently, known almost exclusively by teeth and jaws. Since the 1980s, the number of Mesozoic fossil mammals has increased decisively, from 116 genera known in 1979 to about 310 in 2007, with an increase in quality such that "at least 18 Mesozoic mammals are represented by nearly complete skeletons".
As a result:
- In many cases it is difficult to assign a Mesozoic mammal or near-mammal fossil to a genusGenusIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
. - All the available fossils of a genus seldom add up to a complete skeleton, and hence it is difficult to decide which genera are most like each other and therefore most likely to be closely related. In other words, it becomes very difficult to classify them by means of cladisticsCladisticsCladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
, which is the most reliable and least subjective method currently available.
So the evolution of mammals in the Mesozoic is full of uncertainties, although there is no room for doubt that true mammals did first appear in the Mesozoic.
Mammals or mammaliaformes?
One result of these uncertainties has been a change in the paleontologists' definition of "mammal". For a long time a fossil was considered a mammal if it met the jaw-ear criterion (the jaw joint consists only of the squamosal and dentary; and the articularArticular
The articular bone is part of the lower jaw of most tetrapods, including amphibians, sauropsids and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular...
and the quadrate
Quadrate bone
The quadrate bone is part of a skull in most tetrapods, including amphibians, sauropsids , and early synapsids. In these animals it connects to the quadratojugal and squamosal in the skull, and forms part of the jaw joint .- Evolutionary variation :In snakes, the quadrate bone has become elongated...
bones have become the middle ear's malleus
Malleus
The malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum...
and incus
Incus
The incus or anvil is the anvil-shaped small bone or ossicle in themiddle ear. It connects the malleus to the stapes. It was first described by Alessandro Achillini of Bologna.The incus transmits sound vibrations from the malleus to the stapes....
). But more recently some paleontologists have usually defined "mammal" as the crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
mammals, i.e. the last common ancestor of monotremes, marsupials and placentals and all of its descendants. The need to address the animals that are more mammal-like than cynodonts, but less closely related to monotremes, marsupials and placentals, led to erecting the group mammaliaformes
Mammaliaformes
Mammaliaformes is a clade that contains the mammals and their closest extinct relatives. Phylogenetically, it is defined as a clade including the most recent common ancestor of Sinoconodon, morganuconodonts, docodonts, Monotremata, Marsupialia, Placentalia, extinct members of this clade, and all...
to accommodate these primitive forms. Mammaliaformes is a paraphyletic taxon, representing the early radiation of mammals after the jaw-ear criterion. Although this now appears to be the majority approach, some paleontologists have resisted it because it simply moves most of the problems into the new taxon (a paraphyletic one at that) without solving the original problem; the Mammaliaformes includes some animals with "mammalian" jaw joints and some with "reptilian" (articular-to-quadrate) jaw joints; and the newer definition of "mammal" and "mammaliaformes" depends on last common ancestors of both groups, which have not yet been found. Despite these objections, this article follows the majority approach and treats most of the cynodonts' Mesozoic descendants as mammaliaformes.
Family tree — cynodonts to mammals
(based on Mammaliformes - Palaeos)Multituberculates
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
") are often called the "rodents of the Mesozoic" but this is an example of convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
rather than meaning that they are closely related to the Rodent
Rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
ia. At first sight they look like mammals: their jaw joints consists of only the dentary and squamosal
Squamosal
The squamosal is a bone of the head of higher vertebrates. It is the principal component of the cheek region in the skull, lying below the temporal series and otic notch and bounded anteriorly by postorbital. Posteriorly, the squamosal articulates with the posterior elements of the palatal complex,...
bones, and the quadrate
Quadrate bone
The quadrate bone is part of a skull in most tetrapods, including amphibians, sauropsids , and early synapsids. In these animals it connects to the quadratojugal and squamosal in the skull, and forms part of the jaw joint .- Evolutionary variation :In snakes, the quadrate bone has become elongated...
and articular
Articular
The articular bone is part of the lower jaw of most tetrapods, including amphibians, sauropsids and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular...
bones are part of the middle ear; their teeth are differentiated, occlude and have mammal-like cusp
Cusp (dentistry)
A cusp is an occlusal or incisal eminence on a tooth.Canine teeth, otherwise known as cuspids, each possess a single cusp, while premolars, otherwise known as bicuspids, possess two each. Molars normally possess either four or five cusps...
s; they have a zygomatic arch
Zygomatic arch
The zygomatic arch or cheek bone is formed by the zygomatic process of temporal bone and the temporal process of the zygomatic bone , the two being united by an oblique suture; the tendon of the Temporalis passes medial to the arch to gain insertion into the coronoid process...
; the structure of the pelvis
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
suggests that they gave birth to tiny helpless young, like modern marsupials. And they lived for over 120 million years (from mid Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
, about 160M years ago, to early Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
, about 35M years ago), which in terms of clade longevity would make them the most successful mammaliaformes ever. But a closer look shows that they are very different from modern mammals:
- Their "molars" have two parallel rows of tubercles, unlike the tribosphenic (three-peaked) molarsMolar (tooth)Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
of early mammals. - The chewing action is completely different. Mammals chew with a side-to-side grinding action, which means that usually the molars occlude on only one side at a time. Multituberculates' jaws were incapable of side-to-side movement and chewed by dragging the lower teeth backwards against the upper ones as the jaw closed.
- The anterior (forward) part of the zygomatic arch mostly consists of the maxillaMaxillaThe maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...
(upper jawbone) rather than the jugalJugalThe jugal is a skull bone found in most reptiles, amphibians, and birds. In mammals, the jugal is often called the malar or Zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species....
, and the jugal is a small bone in a little slot in the maxillary process (extension). - The squamosalSquamosalThe squamosal is a bone of the head of higher vertebrates. It is the principal component of the cheek region in the skull, lying below the temporal series and otic notch and bounded anteriorly by postorbital. Posteriorly, the squamosal articulates with the posterior elements of the palatal complex,...
does not form part of the braincase. - The rostrumRostrum (anatomy)The term rostrum is used for a number of unrelated structures in different groups of animals:*In crustaceans, the rostrum is the forward extension of the carapace in front of the eyes....
(snout) is unlike that of mammals, in fact it looks more like that of a pelycosaurPelycosaurThe pelycosaurs are an informal grouping composed of basal or primitive Late Paleozoic synapsid amniotes. Some species were quite large and could grow up to 3 meters or more, although most species were much smaller...
such as DimetrodonDimetrodonDimetrodon was a predatory synapsid genus that flourished during the Permian period, living between 280–265 million years ago ....
. The multituberculate rostrum is box-like, with the large flat maxillae forming the sides, the nasalNasal boneThe nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face, and form, by their junction, "the bridge" of the nose.Each has two surfaces and four borders....
the top, and the tall premaxillaPremaxillaThe incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....
at the front.
Morganucodontidae
The MorganucodonMorganucodon
Morganucodon is an early mammalian genus which lived during the Late Triassic. It first appeared about 205 million years ago. This has also been identified with Eozostrodon. Unlike many other early mammals, Morganucodon is well represented by abundant and well preserved, though in the vast...
tidae first appeared in the late Triassic, about 205M years ago. They are an excellent example of transitional fossils, since they have both the dentary-squamosal and articular-quadrate jaw joints. They were also one of the first discovered and most thoroughly studied of the mammaliaformes
Mammaliaformes
Mammaliaformes is a clade that contains the mammals and their closest extinct relatives. Phylogenetically, it is defined as a clade including the most recent common ancestor of Sinoconodon, morganuconodonts, docodonts, Monotremata, Marsupialia, Placentalia, extinct members of this clade, and all...
, since an unusually large number of morganucodont fossils have been found.
Docodonts
Docodonta
Docodonta is an order of extinct proto-mammals that lived during the mid- to late-Mesozoic era. Their most distinguishing physical features were their relatively sophisticated set of molars, from which the order gets its name. In the fossil record, Docodonta is represented primarily by isolated...
is Castorocauda ("beaver tail"), which lived in the mid Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
about 164M years ago and was first discovered in 2004 and described in 2006. Castorocauda was not a typical docodont (most were omnivores) and not a true mammal, but it is extremely important in the study of the evolution of mammals because the first find was an almost complete skeleton (a real luxury in paleontology) and it breaks the "small nocturnal insectivore" stereotype:
- It was noticeably larger than most Mesozoic mammal-like fossils — about 17 in (43.2 cm) from its nose to the tip of its 5 inches (12.7 cm) tail, and may have weighed 500 gram.
- It provides the earliest absolutely certain evidence of hair and fur. Previously the earliest was EomaiaEomaiaEomaia is an extinct fossil mammal, discovered in rocks that were found in the Yixian Formation, Liaoning Province, China, and dated to the Barremian Age of the Lower Cretaceous about . The fossil is in length and virtually complete. An estimate of the body weight is between . It is exceptionally...
, a true mammal from about 125M years ago. - It had aquatic adaptations including flattened tail bones and remnants of soft tissue between the toes of the back feet, suggesting that they were webbed. Previously the earliest known semi-aquatic mammal-like animals were from the EoceneEoceneThe Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
, about 110M years later. - Castorocauda's powerful forelimbs look adapted for digging. This feature and the spurs on its ankles make it resemble the platypusPlatypusThe platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
, which also swims and digs. - Its teeth look adapted for eating fish: the first two molars had cusps in a straight row, which made them more suitable for gripping and slicing than for grinding; and these molars are curved backwards, to help in grasping slippery prey.
Hadrocodium
Hadrocodium
Hadrocodium wui is an extinct basal mammal species that lived during the Lower Jurassic in what is now the Yunnan province of China...
as an "aunt" of crown mammals, while symmetrodonts
Symmetrodonta
Symmetrodonta is a basal group of Mesozoic mammals characterized by the triangular aspect of the molars when viewed from above and the absence of a well-developed talonid. The traditional group of symmetrodonts ranges in age from the latest Triassic to the Late Cretaceous. One species,...
and kuehneotheriids
Kuehneotheriidae
Kuehneotheriidae is a clade within Symmetrodonta and was created to embrace Kuehneotherium and Woutersia, which lived in Europe in the late Triassic and early Jurassic...
are more closely related to true mammals. But fossils of symmetrodonts and kuehneotheriids are so few and fragmentary that they are poorly understood and may be paraphyletic. On the other hand there are good fossils of Hadrocodium (about 195M years ago in the very early Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
) and they have some important features:
- The jaw joint consists only of the squamosal and dentary bones, and the jaw contains no smaller bones to the rear of the dentary, unlike the therapsid design.
- In therapsids and most mammaliaformesMammaliaformesMammaliaformes is a clade that contains the mammals and their closest extinct relatives. Phylogenetically, it is defined as a clade including the most recent common ancestor of Sinoconodon, morganuconodonts, docodonts, Monotremata, Marsupialia, Placentalia, extinct members of this clade, and all...
the eardrumEardrumThe eardrum, or tympanic membrane, is a thin membrane that separates the external ear from the middle ear in humans and other tetrapods. Its function is to transmit sound from the air to the ossicles inside the middle ear. The malleus bone bridges the gap between the eardrum and the other ossicles...
stretched over a trough at the rear of the lower jaw. But Hadrocodium had no such trough, which suggests its ear was part of the cranium, as it is in mammals — and hence that the former articularArticularThe articular bone is part of the lower jaw of most tetrapods, including amphibians, sauropsids and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular...
and quadrateQuadrate boneThe quadrate bone is part of a skull in most tetrapods, including amphibians, sauropsids , and early synapsids. In these animals it connects to the quadratojugal and squamosal in the skull, and forms part of the jaw joint .- Evolutionary variation :In snakes, the quadrate bone has become elongated...
had migrated to the middle ear and become the malleusMalleusThe malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum...
and incusIncusThe incus or anvil is the anvil-shaped small bone or ossicle in themiddle ear. It connects the malleus to the stapes. It was first described by Alessandro Achillini of Bologna.The incus transmits sound vibrations from the malleus to the stapes....
. On the other hand the dentary has a "bay" at the rear that mammals lack. This suggests that Hadrocodium's dentary bone retained the same shape that it would have had if the articular and quadrate had remained part of the jaw joint, and therefore that Hadroconium or a very close ancestor may have been the first to have a fully mammalian middle ear. - Therapsids and earlier mammaliaforms had their jaw joints very far back in the skull, partly because the ear was at the rear end of the jaw but also had to be close to the brain. This arrangement limited the size of the braincase, because it forced the jaw muscles to run round and over it. Hadrocodium's braincase and jaws were no longer bound to each other by the need to support the ear, and its jaw joint was further forward. In its descendants or those of animals with a similar arrangement, the brain case was free to expand without being constrained by the jaw and the jaw was free to change without being constrained by the need to keep the ear near the brain — in other words it now became possible for mammal-like animals both to develop large brains and to adapt their jaws and teeth in ways that were purely specialized for eating.
The earliest crown mammals
The crown groupCrown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
mammals are the extant mammals and their relatives back to their last common ancestor, variously called 'crown mammals' or 'true mammals'. This part of the story introduces new complications, since the crown group are the only group that still has living members, enabling both anatomical and DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
analysis:
- One has to distinguish between extinct groups and those that have living representatives.
- One often feels compelled to try to explain the evolution of features that do not appear in fossils. This endeavor often involves Molecular phylogenetics, a technique that has become popular since the mid-1980s but is still often controversial because of its assumptions, especially about the reliability of the molecular clockMolecular clockThe molecular clock is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. It is used to estimate the time of occurrence of events called speciation or radiation...
.
Family tree of early crown mammals
(based on Mammalia: Overview - Palaeos; X marks extinct groups)Australosphenida and Ausktribosphenidae
AusktribosphenidaeAusktribosphenidae
Ausktribosphenidae is a group name that has been given to some rather puzzling fossils—Ausktribosphenos nyktos and Bishops whitmorei—which*Appear to have tribosphenic molars, a type of tooth which is otherwise known only in Therians....
is a group name that has been given to some rather puzzling finds that:
- appear to have tribosphenic molarsMolar (tooth)Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
, a type of tooth that is otherwise known only in placentals and marsupials. - come from mid CretaceousCretaceousThe Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
deposits in Australia — but Australia was connected only to Antarctica, and placentals originated in the northern hemisphere and were confined to it until continental driftContinental driftContinental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912...
formed land connections from North America to South America, from Asia to Africa and from Asia to India (the late Cretaceous map at http://www.scotese.com/cretaceo.htm shows how the southern continents are separated). - are represented only by teeth and jaw fragments, which is not very helpful.
Australosphenida
Australosphenida
The Australosphenida are a clade of mammals. Today, living specimens exist only in Australia and New Guinea with only five surviving species, but fossils have been found in Madagascar and Argentina...
is a group that has been defined in order to include the Ausktribosphenidae and monotremes. Asfaltomylos (mid- to late Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
, from Patagonia
Patagonia
Patagonia is a region located in Argentina and Chile, integrating the southernmost section of the Andes mountains to the southwest towards the Pacific ocean and from the east of the cordillera to the valleys it follows south through Colorado River towards Carmen de Patagones in the Atlantic Ocean...
) has been interpreted as a basal australosphenid (animal that has features shared with both Ausktribosphenidae and monotremes; lacks features that are peculiar to Ausktribosphenidae or monotremes; also lacks features that are absent in Ausktribosphenidae and monotremes) and as showing that australosphenids were widespread throughout Gondwanaland (the old Southern hemisphere super-continent).
But recent analysis of Teinolophos
Teinolophos
Teinolophos trusleri was a prehistoric species of monotreme, or egg-laying mammal. It is known from a lower jawbone found in Flat Rocks, Victoria, Australia. It lived during the Aptian age of the Lower Cretaceous. It is the earliest known relative of the Platypus.The species name honours the...
suggests Teinolophos (about 115M years ago) was a "crown group" (advanced and relatively specialised) monotreme, so the basal (most primitive) monotremes must have appeared considerably earlier; that some alleged Australosphenids were also "crown group" monotremes (e.g. Steropodon
Steropodon
Steropodon galmani was a prehistoric species of monotreme, or egg-laying mammal, that lived during the middle Albian stage, in the Lower Cretaceous period...
); and that other alleged Australosphenids (e.g. Ausktribosphenos, Bishops, Ambondro, Asfaltomylos) are therefore more closely related to and possibly members of the Therian mammals (group that includes marsupials and placentals, see below).
Monotremes
The earliest known monotremeMonotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
is Teinolophos
Teinolophos
Teinolophos trusleri was a prehistoric species of monotreme, or egg-laying mammal. It is known from a lower jawbone found in Flat Rocks, Victoria, Australia. It lived during the Aptian age of the Lower Cretaceous. It is the earliest known relative of the Platypus.The species name honours the...
, which lived about 123M years ago in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. Recent (2007, published 2008) analysis suggest that it was not a basal (primitive, ancestral) monotreme but a full-fledged platypus
Platypus
The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
, and therefore that the platypus and echidna
Echidna
Echidnas , also known as spiny anteaters, belong to the family Tachyglossidae in the monotreme order of egg-laying mammals. There are four extant species, which, together with the platypus, are the only surviving members of that order and are the only extant mammals that lay eggs...
lineages diverged considerably earlier and that basal monotremes were even earlier.
A more recent study (2009), however, has suggested that while Teinolophis was a type of platypus, it also was a basal monotreme and predated the radiation of modern monotremes. The semi-aquatic lifestyle of platypuses prevented them from being outcompeted by the marsupials that migrated to Australia millions of years ago, since joeys need to keep attached to their mothers and would drown if their mothers ventured into water. Genetic evidence has determined that echidnas diverged from the platypus lineage as recently as 19-48M when they made their transition from semi-aquatic to terrestrial lifestyle.
Monotremes have some features that may be inherited from the cynodont
Cynodont
Cynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...
ancestors:
- they use the same orifice to urinate, defecate and reproduce ("monotreme" means "one hole") — as lizards and birds also do.
- they lay eggsEgg (biology)An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
that are leathery and uncalcified, like those of lizards, turtles and crocodilians.
Unlike in other mammals, female monotremes do not have nipples and feed their young by "sweating" milk from patches on their bellies.
Of course these features are not visible in fossils, and the main characteristics from paleontologists' point of view are:
- a slender dentary bone in which the coronoid processCoronoid process of the mandibleThe mandible's coronoid process is a thin, triangular eminence, which is flattened from side to side and varies in shape and size....
is small or non-existent. - the external opening of the ear lies at the posterior base of the jaw.
- the jugalJugalThe jugal is a skull bone found in most reptiles, amphibians, and birds. In mammals, the jugal is often called the malar or Zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species....
bone is small or non-existent. - a primitive pectoral girdle with strong ventral elements: coracoids, clavicles and interclavicleInterclavicleAn interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. Monotremes, although part of the mammalian class, do have interclavicles...
. Note: theriaTheriaTheria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians and metatherians . The only omitted extant mammal group is the egg-laying monotremes....
n mammals have no interclavicle. - sprawling or semi-sprawling forelimbs.
Theria
TheriaTheria
Theria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians and metatherians . The only omitted extant mammal group is the egg-laying monotremes....
("beasts") is a name applied to the hypothetical group from which both metatheria
Metatheria
Metatheria is a grouping within the animal class Mammalia. First proposed by Thomas Henry Huxley in 1880, it is nearly synonymous with the earlier taxon Marsupialia though it is slightly wider since it also contains the nearest fossil relatives of marsupial mammals.The earliest known...
(which include marsupials) and eutheria
Eutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
(which include placentals) descended. Although no convincing fossils of basal therians have been found (just a few teeth and jaw fragments), metatheria and eutheria share some features that one would expect to have been inherited from a common ancestral group:
- no interclavicleInterclavicleAn interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. Monotremes, although part of the mammalian class, do have interclavicles...
. - coracoid bones non-existent or fused with the shoulder bladesScapulaIn anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....
to form coracoid processes. - a type of crurotarsalCrurotarsalA crurotarsal joint is one that’s situated between the bones of crus, i.e. shin and the proximal tarsal bones, i.e. astragalus and calcaneum....
ankle joint in which: the main joint is between the tibiaTibiaThe tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....
and astragalusTalus bone-External links:* *...
; the calcaneum has no contact with the tibia but forms a heel to which muscles can attach. (The other well-known type of crurotarsal ankle is seen in crocodilians and works differently — most of the bending at the ankle is between the calcaneum and astragalus). - tribosphenic molars.
Metatheria
The living MetatheriaMetatheria
Metatheria is a grouping within the animal class Mammalia. First proposed by Thomas Henry Huxley in 1880, it is nearly synonymous with the earlier taxon Marsupialia though it is slightly wider since it also contains the nearest fossil relatives of marsupial mammals.The earliest known...
are all marsupials ("animals with pouches"). A few fossil genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
such as the Mongolian late Cretaceous Asiatherium may be marsupials or members of some other metatherian group(s).
The oldest known marsupial is Sinodelphys
Sinodelphys
Sinodelphys is an extinct mammal from the Early Cretaceous. To date, it is the oldest metatherian fossil known, estimated to be 125 million years old...
, found in 125M-year-old early Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
in China's northeastern Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.
Didelphimorphia
Didelphimorphia
Opossums make up the largest order of marsupials in the Western Hemisphere, including 103 or more species in 19 genera. They are also commonly called possums, though that term technically refers to Australian fauna of the suborder Phalangeriformes. The Virginia opossum was the first animal to be...
(common opossums of the Western Hemisphere
Western Hemisphere
The Western Hemisphere or western hemisphere is mainly used as a geographical term for the half of the Earth that lies west of the Prime Meridian and east of the Antimeridian , the other half being called the Eastern Hemisphere.In this sense, the western hemisphere consists of the western portions...
) first appeared in the late Cretaceous and still have living representatives, probably because they are mostly semi-arboreal unspecialized omnivores.
The best-known feature of marsupials is their method of reproduction:
- The mother develops a kind of yolk sack in her womb that delivers nutrients to the embryoEmbryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
. Embryos of bandicoots, koalas and wombats additionally form placentaPlacentaThe placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
-like organs that connect them to the uterineUterusThe uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
wall, although the placenta-like organs are smaller than in placental mammals and it is not certain that they transfer nutrients from the mother to the embryo. - Pregnancy is very short, typically four to five weeks. The embryo is born at a very young age of development, and is usually less than 2 in (5.1 cm) long at birth. It has been suggested that the short pregnancy is necessary to reduce the risk that the mother's immune systemImmune systemAn immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
will attack the embryo. - The newborn marsupial uses its forelimbs (with relatively strong hands) to climb to a nippleNippleIn its most general form, a nipple is a structure from which a fluid emanates. More specifically, it is the projection on the breasts or udder of a mammal by which breast milk is delivered to a mother's young. In this sense, it is often called a teat, especially when referring to non-humans, and...
, which is usually in a pouch on the mother's belly. The mother feeds the baby by contracting muscles over her mammary glands, as the baby is too weak to suck. The newborn marsupial's need to use its forelimbs in climbing to the nipple has prevented the forelimbs from evolving into paddles or wings and has therefore prevented the appearance of aquatic or truly flying marsupials (although there are several marsupial gliders).
Thylacine
The thylacine or ,also ;binomial name: Thylacinus cynocephalus, Greek for "dog-headed pouched one") was the largest known carnivorous marsupial of modern times. It is commonly known as the Tasmanian tiger or the Tasmanian wolf...
or "marsupial wolf" is a good example), marsupial skeletons have some features that distinguish them from placentals:
- Some, including the thylacine, have 4 molarsMolar (tooth)Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
. No placentals have more than 3. - All have a pair of palatal fenestraeFenestraeFenestræ is a Latin word that means "window".* In histology, fenestræ are small pores in endothelial cells that allow for rapid exchange of molecules between sinusoid blood vessels and surrounding tissue...
, window-like openings on the bottom of the skull (in addition to the smaller nostril openings).
Marsupials also have a pair of marsupial bones (sometimes called "epipubic bones
Epipubic bones
Epipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials and of some fossil mammals: multituberculates, monotremes, and even basal eutherians ....
"), which support the pouch in females. But these are not unique to marsupials, since they have been found in fossils of multituberculates, monotremes, and even eutherians — so they are probably a common ancestral feature that disappeared at some point after the ancestry of living placental mammals diverged from that of marsupials.
Some researchers think the epipubic bones' original function was to assist locomotion by supporting some of the muscles that pull the thigh forwards.
Eutheria
The living EutheriaEutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
("true beasts") are all placentals. But the earliest known eutherian, Eomaia
Eomaia
Eomaia is an extinct fossil mammal, discovered in rocks that were found in the Yixian Formation, Liaoning Province, China, and dated to the Barremian Age of the Lower Cretaceous about . The fossil is in length and virtually complete. An estimate of the body weight is between . It is exceptionally...
, found in China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
and dated to 125M years ago, has some features that are more like those of marsupials (the surviving metatherians):
- Epipubic bonesEpipubic bonesEpipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials and of some fossil mammals: multituberculates, monotremes, and even basal eutherians ....
extending forwards from the pelvis, which are not found in any modern placental, but are found in all other mammals — non-placental eutherians, marsupials, monotremes and mammaliaformesMammaliaformesMammaliaformes is a clade that contains the mammals and their closest extinct relatives. Phylogenetically, it is defined as a clade including the most recent common ancestor of Sinoconodon, morganuconodonts, docodonts, Monotremata, Marsupialia, Placentalia, extinct members of this clade, and all...
— and even in the cynodontCynodontCynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...
therapsids that are closest to mammals. Their function is to stiffen the body during locomotion. This stiffening would be harmful in pregnant placentals, whose abdomens need to expand. - A narrow pelvic outlet, which indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials. This suggests that the placenta was a later development.
- 5 incisors in each side of the upper jaw. This number is typical of metatherians, and the maximum numberDentitionDentition pertains to the development of teeth and their arrangement in the mouth. In particular, the characteristic arrangement, kind, and number of teeth in a given species at a given age...
in modern placentals is 3, except for homodonts such as the armadilloArmadilloArmadillos are New World placental mammals, known for having a leathery armor shell. Dasypodidae is the only surviving family in the order Cingulata, part of the superorder Xenarthra along with the anteaters and sloths. The word armadillo is Spanish for "little armored one"...
. But Eomaia's molarMolar (tooth)Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
to premolarPremolarThe premolar teeth or bicuspids are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant, making eight premolars total in the mouth. They have at least two cusps. Premolars can be considered as a 'transitional tooth' during chewing, or...
ratio (it has more pre-molars than molars) is typical of eutherians, including placentals, and not normal in marsupials.
Eomaia also has a Meckelian groove
Meckelian groove
The Meckelian groove is an opening in the medial surface of the mandible which exposes the Meckelian cartilage....
, a primitive feature of the lower jaw that is not found in modern placental mammals.
These intermediate features are consistent with molecular phylogenetics estimates that the placentals diversified about 110M years ago, 15M years after the date of the Eomaia fossil.
Eomaia also has many features that strongly suggest it was a climber, including several features of the feet and toes; well-developed attachment points for muscles that are used a lot in climbing; and a tail that is twice as long as the rest of the spine.
Placentals' best-known feature is their method of reproduction:
- The embryo attaches itself to the uterusUterusThe uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
via a large placentaPlacentaThe placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
via which the mother supplies food and oxygen and removes waste products. - Pregnancy is relatively long and the young are fairly well-developed at birth. In some species (especially herbivores living on plains) the young can walk and even run within an hour of birth.
It has been suggested that the evolution of placental reproduction was made possible by retrovirus
Retrovirus
A retrovirus is an RNA virus that is duplicated in a host cell using the reverse transcriptase enzyme to produce DNA from its RNA genome. The DNA is then incorporated into the host's genome by an integrase enzyme. The virus thereafter replicates as part of the host cell's DNA...
es that:
- make the interface between the placenta and uterus into a syncytiumSyncytiumIn biology, a syncytium is a large cell-like structure; filled with cytoplasm and containing many nuclei. Most cells in eukaryotic organisms have a single nucleus; syncytia are specialized forms used by various organisms.The term may also refer to cells that are connected by specialized membrane...
, i.e. a thin layer of cells with a shared external membrane. This allows the passage of oxygen, nutrients and waste products but prevents the passage of blood and other cells, which would cause the mother's immune systemImmune systemAn immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
to attack the fetusFetusA fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
. - reduce the aggressiveness of the mother's immune system (which is good for the foetus but makes the mother more vulnerable to infections).
From a paleontologist's point of view, eutherians are mainly distinguished by various features of their teeth, ankles and feet.
Expansion of ecological niches in the Mesozoic
There is still some truth in the "small, nocturnal insectivores" stereotype but recent finds, mainly in ChinaChina
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
, show that some mammaliaforms and true mammals were larger and had a variety of lifestyles. For example:
- Castorocauda, a member of Docodonta which lived in the middle JurassicJurassicThe Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
about 164 million years, was about 42.5 cm (16.7 in) long, weighed 500 gram, had limbs that were adapted for swimming and digging and teeth adapted for eating fish. - Multituberculates, are allotherians that survived for over 125 million years (from mid Jurassic, about 160M years ago, to early Oligocene, about 35M years ago) are often called the "rodents of the Mesozoic", because they had continuously growing incisorIncisorIncisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s like those of modern rodents.
- FruitafossorFruitafossorFruitafossor was a termite-eating mammal endemic to North America during the Late Jurassic epoch , existing for approximately ....
, from the late Jurassic period about 150 million years ago, was about the size of a chipmunkChipmunkChipmunks are small striped squirrels native to North America and Asia. They are usually classed either as a single genus with three subgenera, or as three genera.-Etymology and taxonomy:...
and its teeth, forelimbs and back suggest that it broke open the nest of social insects to prey on them (probably termites, as ants had not yet appeared). - Volaticotherium, allotherians from the boundary the early Cretaceous about 125M years ago, is the earliest-known gliding mammal and had a gliding membrane that stretched out between its limbs, rather like that of a modern flying squirrelFlying squirrelFlying squirrels, scientifically known as Pteromyini or Petauristini, are a tribe of 44 species of squirrels .- Description :...
. This also suggests it was active mainly during the day. - RepenomamusRepenomamusRepenomamus is the largest mammal known from the Cretaceous period of Manchuria, and it is the mammal for which there is the best evidence that it fed on dinosaurs. It is not possible to determine if Repenomamus actively hunted live dinosaurs or scavenged dead dinosaurs.-Paleobiology:Repenomamus...
, tricodonts from the early Cretaceous 130 million years ago, was a stocky, badgerBadgerBadgers are short-legged omnivores in the weasel family, Mustelidae. There are nine species of badger, in three subfamilies : Melinae , Mellivorinae , and Taxideinae...
-like predator that sometimes preyed on young dinosaurDinosaurDinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s. Two species have been recognized, one more than 1 m (39.4 in) long and weighing about 12 kilogram, the other less than 0.5 m (19.7 in) long and weighing 4 kilogram.
Evolution of major groups of living mammals
There are currently vigorous debates between traditional paleontologists ("fossil-hunters") and molecular phylogeneticists about how and when the modern groups of mammals diversified, especially the placentals. Generally the traditional paelontologists date the appearance of a particular group by the earliest known fossil whose features make it likely to be a member of that group, while the molecular phylogeneticists suggest that each lineage diverged earlier (usually in the CretaceousCretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
) and that the earliest members of each group were anatomically very similar to early members of other groups and differed only in their genetics
Gênes
Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
. These debates extend to the definition of and relationships between the major groups of placentals — the controversy about Afrotheria
Afrotheria
Afrotheria is a clade of mammals, the living members of which belong to groups from Africa or of African origin: golden moles, sengis , tenrecs, aardvarks, hyraxes, elephants and sea cows. The common ancestry of these animals was not recognized until the late 1990s...
is a good example.
Fossil-based family tree of placental mammals
Here is a very simplified version of a typical family tree based on fossils, based on Cladogram of Mammalia - Palaeos. It tries to show the nearest thing there is at present to a consensus view, but some paleontologists have very different views, for example:- The most common view is that placentals originated in the southern hemisphere, but some paleontologists argue that they first appeared in LaurasiaLaurasiaIn paleogeography, Laurasia was the northernmost of two supercontinents that formed part of the Pangaea supercontinent from approximately...
(old supercontinent containing modern Asia, N. America and Europe). - Paleontologists differ about when the first placentals appeared, with estimates ranging from 20M years before the end of the Cretaceous to just after the end of the Cretaceous. And molecular biologistsMolecular phylogenyMolecular phylogenetics is the analysis of hereditary molecular differences, mainly in DNA sequences, to gain information on an organism's evolutionary relationships. The result of a molecular phylogenetic analysis is expressed in a phylogenetic tree...
argue for a much earlier origin. - Most paleontologists suggest that placentals should be divided into XenarthraXenarthraThe superorder Xenarthra is a group of placental mammals , existent today only in the Americas and represented by anteaters, tree sloths, and armadillos. The origins of the order can be traced back as far as the Paleogene in South America...
and the rest, but a few think these animals diverged later.
For the sake of brevity and simplicity the diagram omits some extinct groups in order to focus on the ancestry of well-known modern groups of placentals — X marks extinct groups. The diagram also shows the following:
- the age of the oldest known fossils in many groups, since one of the major debates between traditional paleontologists and molecular phylogeneticists is about when various groups first became distinct.
- well-known modern members of most groups.
This family tree contains some surprises and puzzles. For example:
- The closest living relatives of cetaceans (whales, dolphins, porpoises) are artiodactyls, hoofed animals, which are almost all pure vegetarians.
- Bats are fairly close relatives of primates.
- The closest living relatives of elephants are the aquatic sirenians, while their next relatives are hyraxes, which look more like well-fed guinea pigs.
- There is little correspondence between the structure of the family (what was descended from what) and the dates of the earliest fossils of each group. For example the earliest fossils of perissodactyls (the living members of which are horses, rhinos and tapirs) date from the late Paleocene but the earliest fossils of their "sister group" the Tubulidentata date from the early Miocene, nearly 50M years later. Paleontologists are fairly confident about the family relationships, which are based on cladistic analyses, and believe that fossils of the ancestors of modern aardvarks have simply not been found yet.
Family tree of placental mammals according to molecular phylogenetics
Molecular phylogenetics uses features of organisms' geneGene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s to work out family trees in much the same way as paleontologists do with features of fossils — if two organisms' genes are more similar to each other than to those of a third organism, the two organisms are more closely related to each other than to the third.
Molecular phylogeneticists have proposed a family tree that is very different from the one with which paleontologists are familiar. Like paleontologists, molecular phylogeneticists have different ideas about various details, but here is a typical family tree according to molecular phylogenetics: Note that the diagram shown here omits extinct groups, as one cannot extract DNA from fossils.
Here are the most significant of the many differences between this family tree and the one familiar to paleontologists:
- The top-level division is between Atlantogenata and Boreoeutheria, instead of between Xenarthra and the rest. However, analysis of transposable element insertions supports a 3-way top-level split between Xenarthra, Afrotheria and Boreoeutheria and the Atlantogenata clade does not receive significant support in recent distance-based molecular phylogenetics.
- Afrotheria contains several groups that are only distantly related according to the paleontologists' version: Afroinsectiphilia ("African insectivores"), Tubulidentata (aardvarks, which paleontologists regard as much closer to odd-toed ungulates than to other members of Afrotheria), Macroscelidea (elephant shrews, usually regarded as close to rabbits and rodents). The only members of Afrotheria that paleontologists would regard as closely related are Hyracoidea (hyraxes), Proboscidea (elephants) and Sirenia (manatees, dugongs).
- Insectivores are split into 3 groups: one is part of Afrotheria and the other two are distinct sub-groups within Boreoeutheria.
- Bats are closer to Carnivora and odd-toed ungulates than to primates and Dermoptera (colugos).
- Perissodactyla (odd-toed ungulates) are closer to Carnivora and bats than to Artiodactyla (even-toed ungulates).
The grouping together of the Afrotheria has some geological justification. All surviving members of the Afrotheria originate from South American or (mainly) African lineages — even the Indian elephant
Indian Elephant
The Indian Elephant is one of three recognized subspecies of the Asian elephant, and native to mainland Asia. Since 1986, Elephas maximus has been listed as endangered by IUCN as the population has declined by at least 50% over the last three generations, estimated to be 60–75 years...
, which diverged from an African lineage about . As Pangaea
Pangaea
Pangaea, Pangæa, or Pangea is hypothesized as a supercontinent that existed during the Paleozoic and Mesozoic eras about 250 million years ago, before the component continents were separated into their current configuration....
broke up Africa and South America separated from the other continents less than 150M years ago, and from each other between 100M and 80M years ago. The earliest known eutherian mammal is Eomaia, from about 125M years ago. So it would not be surprising if the earliest eutherian immigrants into Africa and South America were isolated there and radiated
Adaptive radiation
In evolutionary biology, adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different...
into all the available ecological niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
s.
Nevertheless these proposals have been controversial. Paleontologists naturally insist that fossil evidence must take priority over deductions from samples of the DNA of modern animals. More surprisingly, these new family trees have been criticised by other molecular phylogeneticists, sometimes quite harshly:
- Mitochondrial DNAMitochondrial DNAMitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...
's mutation rate in mammals varies from region to region — some parts hardly ever change and some change extremely quickly and even show large variations between individuals within the same species. - Mammalian mitochondrial DNA mutates so fast that it causes a problem called "saturation", where random noise drowns out any information that may be present. If a particular piece of mitochondrial DNA mutates randomly every few million years, it will have changed several times in the 60 to 75M years since the major groups of placental mammals diverged.
Timing of placental evolution
Recent molecular phylogenetic studies suggest that most placental ordersOrder (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
diverged about 100M to 85M years ago, but that modern families
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
first appeared in the late Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
and early Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
.
Some paleontologists object that no placental fossils have been found from before the end of the Cretaceous — for example Maelestes gobiensis, from about 75M years ago, is a eutherian but not a true placental. Many Cretaceous fossil sites contain well-preserved lizards, salamanders, birds, and mammals, but not the modern forms of mammals. It is likely that they simply did not exist, and that the molecular clock
Molecular clock
The molecular clock is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. It is used to estimate the time of occurrence of events called speciation or radiation...
runs fast during major evolutionary radiation
Evolutionary radiation
An evolutionary radiation is an increase in taxonomic diversity or morphological disparity, due to adaptive change or the opening of ecospace. Radiations may affect one clade or many, and be rapid or gradual; where they are rapid, and driven by a single lineage's adaptation to their environment,...
s. On the other hand there is fossil evidence from of hoof
Hoof
A hoof , plural hooves or hoofs , is the tip of a toe of an ungulate mammal, strengthened by a thick horny covering. The hoof consists of a hard or rubbery sole, and a hard wall formed by a thick nail rolled around the tip of the toe. The weight of the animal is normally borne by both the sole...
ed animals that may be ancestors of modern ungulate
Ungulate
Ungulates are several groups of mammals, most of which use the tips of their toes, usually hoofed, to sustain their whole body weight while moving. They make up several orders of mammals, of which six to eight survive...
s.
Fossils of the earliest members of most modern groups date from the Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
, a few date from later and very few from the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
, before the extinction of the dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s. But some paleontologists, influenced by molecular phylogenetic studies, have used statistical methods to extrapolate
Extrapolation
In mathematics, extrapolation is the process of constructing new data points. It is similar to the process of interpolation, which constructs new points between known points, but the results of extrapolations are often less meaningful, and are subject to greater uncertainty. It may also mean...
backwards from fossils of members of modern groups and concluded that primates arose in the late Cretaceous. However statistical studies of the fossil record confirm that mammals were restricted in size and diversity right to the end of the Cretaceous, and rapidly grew in size and diversity during the Early Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
.
Jaws and middle ears
See also Evolution of mammalian auditory ossiclesEvolution of mammalian auditory ossicles
The evolution of mammalian auditory ossicles is one of the most well-documented and important evolutionary events, demonstrating both numerous transitional forms as well as an excellent example of exaptation, the re-purposing of existing structures during evolution.In reptiles, the eardrum is...
Hadrocodium
Hadrocodium
Hadrocodium wui is an extinct basal mammal species that lived during the Lower Jurassic in what is now the Yunnan province of China...
, whose fossils date from the early Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
, provides the first clear evidence of fully mammalian jaw joints and middle ears, in which the jaw joint is formed by the dentary and squamosal
Squamosal
The squamosal is a bone of the head of higher vertebrates. It is the principal component of the cheek region in the skull, lying below the temporal series and otic notch and bounded anteriorly by postorbital. Posteriorly, the squamosal articulates with the posterior elements of the palatal complex,...
bones while the articular
Articular
The articular bone is part of the lower jaw of most tetrapods, including amphibians, sauropsids and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular...
and quadrate
Quadrate bone
The quadrate bone is part of a skull in most tetrapods, including amphibians, sauropsids , and early synapsids. In these animals it connects to the quadratojugal and squamosal in the skull, and forms part of the jaw joint .- Evolutionary variation :In snakes, the quadrate bone has become elongated...
move to the middle ear, where they are known as the incus
Incus
The incus or anvil is the anvil-shaped small bone or ossicle in themiddle ear. It connects the malleus to the stapes. It was first described by Alessandro Achillini of Bologna.The incus transmits sound vibrations from the malleus to the stapes....
and malleus
Malleus
The malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum...
. Traditionally, separate ear bones have been used as the diagnostic cut-off point from mammals, making Hadrocodium the oldest known mammal. It does however fall outside the mammalian crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
of extant species, and is thus classified as a member of the mammaliaformes
Mammaliaformes
Mammaliaformes is a clade that contains the mammals and their closest extinct relatives. Phylogenetically, it is defined as a clade including the most recent common ancestor of Sinoconodon, morganuconodonts, docodonts, Monotremata, Marsupialia, Placentalia, extinct members of this clade, and all...
rather than as a true mammal by the crown group phylogenetic
Phylogenetic nomenclature
Phylogenetic nomenclature or phylogenetic taxonomy is an alternative to rank-based nomenclature, applying definitions from cladistics . Its two defining features are the use of phylogenetic definitions of biological taxon names, and the lack of obligatory ranks...
definition.
One analysis of the monotreme Teinolophos
Teinolophos
Teinolophos trusleri was a prehistoric species of monotreme, or egg-laying mammal. It is known from a lower jawbone found in Flat Rocks, Victoria, Australia. It lived during the Aptian age of the Lower Cretaceous. It is the earliest known relative of the Platypus.The species name honours the...
suggested that this animal had a pre-mammalian jaw joint formed by the angular and quadrate bones and that the typical mammalian middle ear evolved twice independently, in monotremes and in theria
Theria
Theria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians and metatherians . The only omitted extant mammal group is the egg-laying monotremes....
n mammals, but this idea has been disputed. In fact 2 of the suggestion's authors co-authored a later paper that reinterpreted the same features as evidence that Teinolophos was a full-fledged platypus
Platypus
The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
, which means it would have had a mammalian jaw joint and middle ear.
Milk production (lactation)
It has been suggested that lactation's original function was to keep eggs moist. Much of the argument is based on monotremes (egg-laying mammals):- Monotremes do not have nipples but secrete milk from a hairy patch on their bellies.
- During incubation, monotremes' eggs are covered in a sticky substance whose origin is not known. Before the eggs are laid, their shells have only three layers. Afterwards a fourth layer appears, and its composition is different from that of the original three. The sticky substance and the fourth layer may be produced by the mammary glands.
- If so, that may explain why the patches from which monotremes secrete milk are hairy — it is easier to spread moisture and other substances over the egg from a broad, hairy area than from a small, bare nipple.
Later research demonstrated that caseins already appeared in the common mammalian ancestor approximately 200-310 Millions of years ago.
Hair and fur
The first clear evidence of hair or fur is in fossils of Castorocauda, from 164M years ago in the mid JurassicJurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
.
From 1955 onwards some scientists have interpreted the foramina (passages) in the maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...
e (upper jaws) and premaxilla
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....
e (small bones in front of the maxillae) of cynodont
Cynodont
Cynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...
s as channels that supplied blood vessels and nerves to vibrissae (whiskers), and suggested that this was evidence of hair or fur. But foramina do not necessarily show that an animal had vibrissae — for example the modern lizard Tupinambis has foramina that are almost identical to those found in the non-mammalian cynodont Thrinaxodon
Thrinaxodon
Thrinaxodon was a cynodont, an ermine-sized therapsid. Pits on the skull suggest that Thrinaxodon may have had whiskers, and by extension a protective covering of fur. There are suggestions that it was warm-blooded...
.
Erect limbs
The evolution of erect limbs in mammals is incomplete — living and fossil monotremes have sprawling limbs. In fact some scientists think that the parasagittal (non-sprawling) limb posture is a synapomorphySynapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
(distinguishing characteristic) of the Boreosphenida
Mammal classification
Mammalia is a class of animal within the Phylum Chordata. Mammal classification has been through several iterations since Carolus Linnaeus initially defined the class. Many earlier ideas have been completely abandoned by modern taxonomists, among these are the idea that bats are related to birds...
, a group that contains the Theria
Theria
Theria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians and metatherians . The only omitted extant mammal group is the egg-laying monotremes....
and therefore includes the last common ancestor of modern marsupials and placentals — and therefore that all earlier mammals had sprawling limbs.
Sinodelphys
Sinodelphys
Sinodelphys is an extinct mammal from the Early Cretaceous. To date, it is the oldest metatherian fossil known, estimated to be 125 million years old...
(the earliest known marsupial) and Eomaia
Eomaia
Eomaia is an extinct fossil mammal, discovered in rocks that were found in the Yixian Formation, Liaoning Province, China, and dated to the Barremian Age of the Lower Cretaceous about . The fossil is in length and virtually complete. An estimate of the body weight is between . It is exceptionally...
(the earliest known eutheria
Eutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
n) lived about 125M years ago, so erect limbs must have evolved before then.
Warm-bloodedness
"Warm-bloodedWarm-blooded
The term warm-blooded is a colloquial term to describe animal species which have a relatively higher blood temperature, and maintain thermal homeostasis primarily through internal metabolic processes...
ness" is a complex and rather ambiguous term, because it includes some or all of the following:
- Endothermy, i.e. the ability to generate heat internally rather than via behaviors such as basking or muscular activity.
- Homeothermy, i.e. maintaining a fairly constant body temperature.
- Tachymetabolism, i.e. maintaining a high metabolic rate, particularly when at rest. This requires a fairly high and stable body temperature, since biochemical processes run about half as fast if an animal's temperature drops by 10°C; most enzymeEnzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s have an optimum operating temperatureOperating temperatureAn operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
and their efficiency drops rapidly outside the preferred range.
Since scientists cannot know much about the internal mechanisms of extinct creatures, most discussion focuses on homeothermy and tachymetabolism.
Modern monotremes have a lower body temperature and more variable metabolic rate than marsupials and placentals. So the main question is when a monotreme-like metabolism evolved in mammals. The evidence found so far suggests Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
cynodont
Cynodont
Cynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...
s may have had fairly high metabolic rates, but is not conclusive.
Respiratory turbinates
Modern mammals have respiratory turbinates, convoluted structures of thin bone in the nasal cavity. These are lined with mucous membraneMucous membrane
The mucous membranes are linings of mostly endodermal origin, covered in epithelium, which are involved in absorption and secretion. They line cavities that are exposed to the external environment and internal organs...
s that warm and moisten inhaled air and extract heat and moisture from exhaled air. An animal with respiratory turbinates can maintain a high rate of breathing without the danger of drying its lungs out, and therefore may have a fast metabolism. Unfortunately these bones are very delicate and therefore have not yet been found in fossils. But rudimentary ridges like those that support respiratory turbinates have been found in Triassic therapsids such as Thrinaxodon
Thrinaxodon
Thrinaxodon was a cynodont, an ermine-sized therapsid. Pits on the skull suggest that Thrinaxodon may have had whiskers, and by extension a protective covering of fur. There are suggestions that it was warm-blooded...
and Diademodon
Eucynodontia
Eucynodontia is a grouping of animals that includes both mammals, such as dogs, and mammal-like non-mammalian therapsids such as cynodonts . Its membership was and is made up of both carnivores and herbivores. The chronological range extends from at least the Lower Triassic, possibly the Upper...
, which suggests that they may have had fairly high metabolic rates.
Bony secondary palate
Mammals have a secondary bony palate, which separates the respiratory passage from the mouth, allowing them to eat and breathe at the same time. Secondary bony palates have been found in the more advanced cynodonts and have been used as evidence of high metabolic rates. But some cold-blooded vertebrates have secondary bony palates (crocodilians and some lizards), while birds, which are warm-blooded, do not have them.Diaphragm
A muscular diaphragmThoracic diaphragm
In the anatomy of mammals, the thoracic diaphragm, or simply the diaphragm , is a sheet of internal skeletal muscle that extends across the bottom of the rib cage. The diaphragm separates the thoracic cavity from the abdominal cavity and performs an important function in respiration...
helps mammals to breathe, especially during strenuous activity. For a diaphragm to work, the ribs must not restrict the abdomen, so that expansion of the chest can be compensated for by reduction in the volume of the abdomen and vice versa. The advanced cynodonts have very mammal-like rib cages, with greatly reduced lumbar ribs. This suggests that these animals had diaphragms, were capable of strenuous activity for fairly long periods and therefore had high metabolic rates. On the other hand these mammal-like rib cages may have evolved to increase agility. The movement of even advanced therapsids was however "like a wheelbarrow", with the hindlimbs providing all the thrust while the forelimbs only steered the animal, in other words advanced therapsids were not as agile as either modern mammals or the early dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s. So the idea that the main function of these mammal-like rib cages was to increase agility is doubtful.
Limb posture
The therapsids had sprawling forelimbs and semi-erect hindlimbs. This suggests that Carrier's constraintCarrier's constraint
Carrier's constraint is the observation that air-breathing vertebrates which have two lungs and flex their bodies sideways during locomotion find it very difficult to move and breathe at the same time, because:...
would have made it rather difficult for them to move and breathe at the same time, but not as difficult as it is for animals such as lizards, which have completely sprawling limbs. The cynodonts (advanced therapsids) had costal plates that stiffened the rib cage and therefore may have reduced sideways flexing of the trunk while moving, which would have made it a little easier for them to breathe while moving . These facts suggest that advanced therapsids were significantly less active than modern mammals of similar size and therefore may have had slower metabolisms.
Insulation (hair and fur)
Insulation is the "cheapest" way to maintain a fairly constant body temperature, without consuming energy to produce more body heat. Therefore, possession of hair or fur would be good evidence of homeothermy but would not be such strong evidence of a high metabolic rate.The first clear evidence of hair or fur is in fossils of Castorocauda, from 164M years ago in the mid Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
; arguments that advanced therapsids had hair are unconvincing.
Mammalian brain
Mammals are noted for their large brain size relative to body size, compared to other animal groups. Recent findings suggest that the first brain area to expand was the brain area involved in smell. Scientists scanned the skulls of early mammal species dating back to 190-200 million years ago and compared the brain case shapes to earlier pre-mammal species and found that the brain area involved in the sense of smell was the first to enlarge. This change may have allowed these early mammals to hunt insects at night when dinosaurs were not active.External links
- The Cynodontia covers several aspects of the evolution of cynodonts into mammals, with plenty of references.