Monotreme
Encyclopedia
Monotremes are mammal
s that lay eggs (Prototheria
) instead of giving birth to live young like marsupials (Metatheria
) and placental mammals (Eutheria
). The only surviving examples of monotremes are all indigenous to Australia
and New Guinea
, although there is evidence that they were once more widespread. Among living mammals they include the platypus
and four species of echidna
s (or spiny anteaters); there is debate regarding monotreme taxonomy (see below).
on their bodies; produce milk
through mammary glands to feed their young; have a single bone in their lower jaw; and have three middle-ear bones.
Monotremes were very poorly understood for many years, and to this day, some of the 19th century myths that grew up around them endure. It is still sometimes thought, for example, that the monotremes are "inferior" or quasi-reptilian, and that they are a distant ancestor of the "superior" placental mammals. It now seems clear that modern monotremes are the survivors of an early branching of the mammal tree; a later branching is thought to have led to the marsupial
and placental groups.
In common with reptiles and marsupial
s, monotremes lack the connective structure (corpus callosum
) which in placental mammals is the primary communication route between the right and left brain hemispheres. The anterior commissure
does provide an alternate communication route between the two hemispheres, though, and in monotremes and marsupials it carries all the commissural fibers arising from the neocortex
, whereas in placental mammals the anterior commissure carries only some of these fibers.
The key anatomical difference between monotremes and other mammals is the one that gave them their name; monotreme means 'single opening' in Greek and comes from the fact that their urinary, defecatory, and reproductive systems all open into a single duct, the cloaca
. This structure is very similar to the one found in reptiles. Monotremes and marsupials have a single cloaca (though marsupials also have a separate genital tract), while placental mammal females have separate openings for reproduction, urination, and defecation: the vagina
, the urethra
, and the anus
.
Monotremes lay egg
s. However, the egg is retained for some time within the mother, who actively provides the egg with nutrients. Monotremes also lactate, but have no defined nipple
s, excreting the milk from their mammary gland
s via openings in their skin. All species are long-lived, with low rates of reproduction and relatively prolonged parental care of infants. Infant echidnas are sometimes known as puggles, referencing their similarity in appearance to the Australian children's toy designed by Tony Barber. The same term, though not generally accepted, is popularly applied to young platypuses as well.
Extant monotremes lack teeth as adults. Fossil forms and modern platypus young have "tribosphenic" molar
s (with the occlusal
surface formed by three cusp
s arranged in a triangle), which are one of the hallmarks of extant mammals. Some recent work suggests that monotremes acquired this form of molar independently of placental mammals and marsupials, although this is not well established. The jaw of monotremes is constructed somewhat differently from that of other mammals, and the jaw opening muscle is different. As in all true mammals, the tiny bones that conduct sound to the inner ear are fully incorporated into the skull, rather than lying in the jaw as in cynodont
s and other pre-mammalian synapsids; this feature, too, is now claimed to have evolved independently in monotremes and theria
ns, although, as with the analogous evolution of the tribosphenic molar, this is disputed. The external opening of the ear still lies at the base of the jaw. The sequencing of the Platypus genome has also provided insight into the evolution of a number of monotreme traits such as venom and electroreception
, as well as showing some new unique features, such as the fact that monotremes possess 10 sex chromosomes and that their X chromosome resembles the sex chromosome of birds, suggesting that the two sex chromosomes of marsupial and placental mammals evolved more recently than the split from the monotreme lineage. This feature, along with some other genetic similarities with birds such as shared genes related to egg-laying, is thought to provide some insight into the most recent common ancestor
of the synapsid
lineage leading to mammals and the sauropsid
lineage leading to birds and modern reptiles, which are believed to have split about 315 million years ago.
The monotremes also have extra bones in the shoulder girdle, including an interclavicle
and coracoid, which are not found in other mammals. Monotremes retain a reptile-like gait, with legs that are on the sides of rather than underneath the body. The monotreme leg bears a spur in the ankle region; the spur is non-functional in echidnas, but contains a powerful venom
in the male Platypus.
It is still sometimes said that monotremes have less developed internal temperature control
mechanisms than other mammals, but recent research shows that monotremes maintain a constant body temperature in a wide variety of circumstances without difficulty (for example, the Platypus while living in an icy mountain stream). Early researchers were misled by two factors: firstly, monotremes maintain a lower average temperature than most mammals (around 32 °C (89.6 °F), compared to about 35 °C (95 °F) for marsupials, and 37 °C (98.6 °F) for most placentals); secondly, the Short-beaked Echidna
(which is much easier to study than the reclusive Platypus) maintains normal temperature only when it is active: during cold weather, it conserves energy by "switching off" its temperature regulation. Additional perspective came when reduced thermal regulation was observed in the hyrax
es, which are placental mammals
.
Contrary to previous research, the Echidna does indeed enter REM sleep, albeit only when the ambient temperature of its environment is around 25 °C (77 °F). At temperatures between 15 °C (59 °F) and 28 °C (82.4 °F), REMS is suppressed.
(the platypus
along with its fossil relatives) and Tachyglossa (the echidna
s, or spiny anteaters). The entire grouping is also traditionally placed into a subclass Prototheria
, which was extended to include several fossil
orders, but these are no longer seen as constituting a natural group allied to monotreme ancestry. A controversial hypothesis now relates the monotremes to a different assemblage of fossil mammals in a clade
termed Australosphenida
.
The traditional "theria
hypothesis" states that the divergence of the monotreme lineage from the Metatheria
(marsupial
) and Eutheria
(placental mammal) lineages happened prior to the divergence between marsupials and placental mammals, and that this explains why monotremes retain a number of "primitive" (or basal
) traits presumed to have been present in the Synapsid
ancestors of later mammals, such as egg-laying (this does not mean they are more 'primitive' in any absolute sense; just as placental mammals have 'derived
' features which would not have been present in the monotreme/placental common ancestor, such as live birth, monotremes have their own 'derived' features such as electroreception
and snouts modified into 'beaks'). Most morphological evidence supports the theria hypothesis, but one possible exception is a similar pattern of tooth replacement seen in monotremes and marsupials, which originally provided the basis for the competing "marsupionata hypothesis" in which the divergence between monotremes and marsupials happened later than the divergence between these lineages and the placental mammals. An analysis by Van Rheede in 2005 concluded that the genetic evidence favors the theria hypothesis, and this hypothesis continues to be the more widely-accepted one.
The time at which the monotreme line diverged from other mammalian lines is uncertain, but one survey of genetic studies gives an estimate of about 220 million years ago. Fossils of a jaw fragment 110 million years old were found at Lightning Ridge, New South Wales
. These fragments, from species Steropodon galmani, are the oldest known fossils of monotremes. Fossils from the genera Kollikodon
, Teinolophos
, and Obdurodon
have also been discovered. In 1991, a fossil tooth of a 61-million-year-old platypus was found in southern Argentina
(since named Monotrematum, though it is now considered to be an Obdurodon species). (See fossil monotremes below.) Molecular clock
and fossil dating gives a wide range of dates for the split between echidnas and platypuses, one survey putting the split at 19–48 million years ago, another putting it at 17–89 million years ago. All these dates are more recent than the oldest known platypus fossils, suggesting that both the short-beaked and long-beaked echidna species are derived from a platypus-like ancestor.
The precise relationships between extinct groups of mammals and modern groups such as monotremes are somewhat uncertain, but cladistic
analyses usually put the last common ancestor (LCA) of placentals and monotremes close to the LCA of placentals and multituberculates
, with a number of analyses giving a more recent LCA for placentals and monotremes, but some also suggesting that the LCA of placentals and multituberculates was more recent.
epoch. The known Mesozoic
monotremes are Steropodon
, Kollikodon
, and Teinolophos
, all from Australian deposits in the Cretaceous
, suggesting that monotremes had already diversified by that time. A platypus tooth has been found in the Palaeocene of Argentina, so Michael Benton
suggests in Vertebrate Palaeontology
that monotremes arose in Australia in the Late Jurassic
or Early Cretaceous
, and that some subsequently migrated across Antarctica to reach South America
, both of which were still united with Australia at that time. However, a number of genetic studies suggest a much earlier origin in the Triassic
.
Excepting Ornithorhynchus anatinus, all the animals listed in this section are known only from fossils.
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 that lay eggs (Prototheria
Prototheria
Prototheria is a taxonomic group, or taxon, to which the order Monotremata belongs. It is conventionally ranked as a subclass within the mammals.Most of the animals in this group are extinct...
) instead of giving birth to live young like marsupials (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...
) and placental mammals (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...
). The only surviving examples of monotremes are all indigenous to 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...
and New Guinea
New Guinea
New Guinea is the world's second largest island, after Greenland, covering a land area of 786,000 km2. Located in the southwest Pacific Ocean, it lies geographically to the east of the Malay Archipelago, with which it is sometimes included as part of a greater Indo-Australian Archipelago...
, although there is evidence that they were once more widespread. Among living mammals they include the 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 four species of 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...
s (or spiny anteaters); there is debate regarding monotreme taxonomy (see below).
General characteristics
Like other mammals, monotremes are warm-blooded with a high metabolic rate (though not as high as other mammals; see below); have hairHair
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....
on their bodies; produce milk
Milk
Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...
through mammary glands to feed their young; have a single bone in their lower jaw; and have three middle-ear bones.
Monotremes were very poorly understood for many years, and to this day, some of the 19th century myths that grew up around them endure. It is still sometimes thought, for example, that the monotremes are "inferior" or quasi-reptilian, and that they are a distant ancestor of the "superior" placental mammals. It now seems clear that modern monotremes are the survivors of an early branching of the mammal tree; a later branching is thought to have led to the 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...
and placental groups.
In common with reptiles and 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...
s, monotremes lack the connective structure (corpus callosum
Corpus callosum
The corpus callosum , also known as the colossal commissure, is a wide, flat bundle of neural fibers beneath the cortex in the eutherian brain at the longitudinal fissure. It connects the left and right cerebral hemispheres and facilitates interhemispheric communication...
) which in placental mammals is the primary communication route between the right and left brain hemispheres. The anterior commissure
Anterior commissure
The anterior commissure is a bundle of nerve fibers , connecting the two cerebral hemispheres across the midline, and placed in front of the columns of the fornix...
does provide an alternate communication route between the two hemispheres, though, and in monotremes and marsupials it carries all the commissural fibers arising from 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...
, whereas in placental mammals the anterior commissure carries only some of these fibers.
Cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...
. This structure is very similar to the one found in reptiles. Monotremes and marsupials have a single cloaca (though marsupials also have a separate genital tract), while placental mammal females have separate openings for reproduction, urination, and defecation: the vagina
Vagina
The vagina is a fibromuscular tubular tract leading from the uterus to the exterior of the body in female placental mammals and marsupials, or to the cloaca in female birds, monotremes, and some reptiles. Female insects and other invertebrates also have a vagina, which is the terminal part of the...
, the urethra
Urethra
In anatomy, the urethra is a tube that connects the urinary bladder to the genitals for the removal of fluids out of the body. In males, the urethra travels through the penis, and carries semen as well as urine...
, and the anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
.
Monotremes lay egg
Egg (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...
s. However, the egg is retained for some time within the mother, who actively provides the egg with nutrients. Monotremes also lactate, but have no defined nipple
Nipple
In 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...
s, excreting the milk from their mammary gland
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 via openings in their skin. All species are long-lived, with low rates of reproduction and relatively prolonged parental care of infants. Infant echidnas are sometimes known as puggles, referencing their similarity in appearance to the Australian children's toy designed by Tony Barber. The same term, though not generally accepted, is popularly applied to young platypuses as well.
Extant monotremes lack teeth as adults. Fossil forms and modern platypus young have "tribosphenic" molar
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"....
s (with the occlusal
Commonly used terms of relationship and comparison in dentistry
There are numerous commonly used terms of relationship and comparison that refer to different aspects of teeth and are frequently utilized in articles about dentistry...
surface formed by three 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 arranged in a triangle), which are one of the hallmarks of extant mammals. Some recent work suggests that monotremes acquired this form of molar independently of placental mammals and marsupials, although this is not well established. The jaw of monotremes is constructed somewhat differently from that of other mammals, and the jaw opening muscle is different. As in all true mammals, the tiny bones that conduct sound to the inner ear are fully incorporated into the skull, rather than lying in the jaw as in 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 and other pre-mammalian synapsids; this feature, too, is now claimed to have evolved independently in monotremes and 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....
ns, although, as with the analogous evolution of the tribosphenic molar, this is disputed. The external opening of the ear still lies at the base of the jaw. The sequencing of the Platypus genome has also provided insight into the evolution of a number of monotreme traits such as venom and electroreception
Electroreception
Electroreception is the biological ability to perceive natural electrical stimuli. It has been observed only in aquatic or amphibious animals, since water is a much better conductor than air. Electroreception is used in electrolocation and for electrocommunication.- Overview :Electroreception is...
, as well as showing some new unique features, such as the fact that monotremes possess 10 sex chromosomes and that their X chromosome resembles the sex chromosome of birds, suggesting that the two sex chromosomes of marsupial and placental mammals evolved more recently than the split from the monotreme lineage. This feature, along with some other genetic similarities with birds such as shared genes related to egg-laying, is thought to provide some insight into the most recent common ancestor
Most recent common ancestor
In genetics, the most recent common ancestor of any set of organisms is the most recent individual from which all organisms in the group are directly descended...
of 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 leading to mammals and the sauropsid
Sauropsida
Sauropsida is a group of amniotes that includes all existing reptiles and birds and their fossil ancestors, including the dinosaurs, the immediate ancestors of birds...
lineage leading to birds and modern reptiles, which are believed to have split about 315 million years ago.
The monotremes also have extra bones in the shoulder girdle, including an interclavicle
Interclavicle
An 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...
and coracoid, which are not found in other mammals. Monotremes retain a reptile-like gait, with legs that are on the sides of rather than underneath the body. The monotreme leg bears a spur in the ankle region; the spur is non-functional in echidnas, but contains a powerful venom
Platypus venom
The platypus is one of the few mammals to produce venom. Males have a pair of spurs on their hind limbs. The male's pair of spurs spits out a cocktail of poisons that, while excruciatingly painful, is not lethal to most animals.-Spur and crural gland:...
in the male Platypus.
Physiology
Monotreme metabolic rate is remarkably low by mammalian standards. The Platypus has an average body temperature of about 32 °C (89.6 °F) rather than the 37 °C (98.6 °F) typical of placental mammals. Research suggests this has been a gradual adaptation to harsh environmental conditions on the part of the small number of surviving monotreme species rather than a historical characteristic of monotremes.It is still sometimes said that monotremes have less developed internal temperature control
Thermoregulation
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...
mechanisms than other mammals, but recent research shows that monotremes maintain a constant body temperature in a wide variety of circumstances without difficulty (for example, the Platypus while living in an icy mountain stream). Early researchers were misled by two factors: firstly, monotremes maintain a lower average temperature than most mammals (around 32 °C (89.6 °F), compared to about 35 °C (95 °F) for marsupials, and 37 °C (98.6 °F) for most placentals); secondly, the Short-beaked Echidna
Short-beaked Echidna
The short-beaked echidna , also known as the spiny anteater because of its diet of ants and termites, is one of four living species of echidna and the only member of the genus Tachyglossus...
(which is much easier to study than the reclusive Platypus) maintains normal temperature only when it is active: during cold weather, it conserves energy by "switching off" its temperature regulation. Additional perspective came when reduced thermal regulation was observed in the hyrax
Hyrax
A hyrax is any of four species of fairly small, thickset, herbivorous mammals in the order Hyracoidea. The rock hyrax Procavia capensis, the yellow-spotted rock hyrax Heterohyrax brucei, the western tree hyrax Dendrohyrax dorsalis, and the southern tree hyrax, Dendrohyrax arboreus live in Africa...
es, which are placental mammals
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...
.
Contrary to previous research, the Echidna does indeed enter REM sleep, albeit only when the ambient temperature of its environment is around 25 °C (77 °F). At temperatures between 15 °C (59 °F) and 28 °C (82.4 °F), REMS is suppressed.
Taxonomy
They are conventionally treated as comprising a single order Monotremata, though a recent classification proposes to divide them into the orders PlatypodaPlatypoda
Platypoda is a suborder of the monotremes; it includes three families and a single living species, the Platypus. All others are extinct....
(the 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...
along with its fossil relatives) and Tachyglossa (the 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...
s, or spiny anteaters). The entire grouping is also traditionally placed into a subclass Prototheria
Prototheria
Prototheria is a taxonomic group, or taxon, to which the order Monotremata belongs. It is conventionally ranked as a subclass within the mammals.Most of the animals in this group are extinct...
, which was extended to include several fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
orders, but these are no longer seen as constituting a natural group allied to monotreme ancestry. A controversial hypothesis now relates the monotremes to a different assemblage of fossil mammals in a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
termed 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...
.
The traditional "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....
hypothesis" states that the divergence of the monotreme lineage from the 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...
(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...
) 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...
(placental mammal) lineages happened prior to the divergence between marsupials and placental mammals, and that this explains why monotremes retain a number of "primitive" (or basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
) traits presumed to have been present in 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...
ancestors of later mammals, such as egg-laying (this does not mean they are more 'primitive' in any absolute sense; just as placental mammals have 'derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
' features which would not have been present in the monotreme/placental common ancestor, such as live birth, monotremes have their own 'derived' features such as electroreception
Electroreception
Electroreception is the biological ability to perceive natural electrical stimuli. It has been observed only in aquatic or amphibious animals, since water is a much better conductor than air. Electroreception is used in electrolocation and for electrocommunication.- Overview :Electroreception is...
and snouts modified into 'beaks'). Most morphological evidence supports the theria hypothesis, but one possible exception is a similar pattern of tooth replacement seen in monotremes and marsupials, which originally provided the basis for the competing "marsupionata hypothesis" in which the divergence between monotremes and marsupials happened later than the divergence between these lineages and the placental mammals. An analysis by Van Rheede in 2005 concluded that the genetic evidence favors the theria hypothesis, and this hypothesis continues to be the more widely-accepted one.
The time at which the monotreme line diverged from other mammalian lines is uncertain, but one survey of genetic studies gives an estimate of about 220 million years ago. Fossils of a jaw fragment 110 million years old were found at Lightning Ridge, New South Wales
New South Wales
New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...
. These fragments, from species Steropodon galmani, are the oldest known fossils of monotremes. Fossils from the genera Kollikodon
Kollikodon
Kollikodon ritchiei is a fossil monotreme species. It is known only from an opalised dentary fragment, with one premolar and two molars in situ...
, 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...
, and Obdurodon
Obdurodon
Obdurodon is an extinct monotreme genus containing three species. Obdurodon differed from modern platypuses in that it had molar teeth .-Obdurodon dicksoni:...
have also been discovered. In 1991, a fossil tooth of a 61-million-year-old platypus was found in southern Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
(since named Monotrematum, though it is now considered to be an Obdurodon species). (See fossil monotremes below.) 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...
and fossil dating gives a wide range of dates for the split between echidnas and platypuses, one survey putting the split at 19–48 million years ago, another putting it at 17–89 million years ago. All these dates are more recent than the oldest known platypus fossils, suggesting that both the short-beaked and long-beaked echidna species are derived from a platypus-like ancestor.
The precise relationships between extinct groups of mammals and modern groups such as monotremes are somewhat uncertain, but cladistic
Cladistics
Cladistics 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...
analyses usually put the last common ancestor (LCA) of placentals and monotremes close to the LCA of placentals and multituberculates
Multituberculata
The Multituberculata were a group of rodent-like mammals that existed for approximately one hundred and twenty million years—the longest fossil history of any mammal lineage—but were eventually outcompeted by rodents, becoming extinct during the early Oligocene. At least 200 species are...
, with a number of analyses giving a more recent LCA for placentals and monotremes, but some also suggesting that the LCA of placentals and multituberculates was more recent.
- ORDER MONOTREMATA
- Suborder PlatypodaPlatypodaPlatypoda is a suborder of the monotremes; it includes three families and a single living species, the Platypus. All others are extinct....
- Family OrnithorhynchidaeOrnithorhynchidaeOrnithorhynchidae is one of the two extant families in the order Monotremata, and contains the Platypus and its extinct relatives. The other family is the Tachyglossidae, or echidnas...
: platypus- Genus Ornithorhynchus
- 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...
, Ornithorhynchus anatinus
- Platypus
- Genus Ornithorhynchus
- Family Ornithorhynchidae
- Suborder Tachyglossa
- Family Tachyglossidae: echidnas
- Genus Tachyglossus
- Short-beaked EchidnaShort-beaked EchidnaThe short-beaked echidna , also known as the spiny anteater because of its diet of ants and termites, is one of four living species of echidna and the only member of the genus Tachyglossus...
, Tachyglossus aculeatus- Tachyglossus aculeatus aculeatus
- Tachyglossus aculeatus acanthion
- Tachyglossus aculeatus lawesii
- Tachyglossus aculeatus multiaculeatus
- Tachyglossus aculeatus setosus
- Short-beaked Echidna
- Genus Zaglossus
- Sir David's Long-beaked EchidnaSir David's Long-beaked EchidnaSir David's long-beaked echidna , also known as Attenborough's long-beaked echidna or the Cyclops long-beaked echidna, is one of three species from the genus Zaglossus to occur in New Guinea. It is named in honour of Sir David Attenborough, the eminent naturalist...
, Zaglossus attenboroughi - Eastern Long-beaked EchidnaEastern Long-beaked EchidnaThe eastern long-beaked echidna , also known as Barton's long-beaked echidna, is one of three species from the genus Zaglossus to occur in New Guinea...
, Zaglossus bartoni- Zaglossus bartoni bartoni
- Zaglossus bartoni clunius
- Zaglossus bartoni diamondi
- Zaglossus bartoni smeenki
- Western Long-beaked EchidnaWestern Long-beaked EchidnaThe western long-beaked echidna is one of the four extant echidnas and one of three species of Zaglossus that occur in New Guinea. As Tachyglossus bruijni, this is the type species of Zaglossus....
, Zaglossus bruijni
- Sir David's Long-beaked Echidna
- Genus Tachyglossus
- Family Tachyglossidae: echidnas
- Suborder Platypoda
Fossil monotremes
The fossil record of monotremes is relatively sparse. Although biochemical and anatomical evidence suggests that monotremes diverged from the mammalian lineage before the marsupials and placental mammals arose, only a handful of monotreme fossils are known from before the MioceneMiocene
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...
epoch. The known 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...
monotremes are 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...
, Kollikodon
Kollikodon
Kollikodon ritchiei is a fossil monotreme species. It is known only from an opalised dentary fragment, with one premolar and two molars in situ...
, and 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...
, all from Australian deposits in 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...
, suggesting that monotremes had already diversified by that time. A platypus tooth has been found in the Palaeocene of Argentina, so Michael Benton
Michael Benton
Michael J. Benton is a British paleontologist, a Fellow of the Royal Society of Edinburgh, and professor of vertebrate palaeontology in the Department of Earth Sciences at the University of Bristol....
suggests in Vertebrate Palaeontology
Vertebrate Palaeontology (Benton)
Vertebrate Palaeontology is a basic textbook on vertebrate paleontology by Michael J. Benton, published by Blackwell's. It has so far appeared in three editions, published in 1990, 1997, and 2005...
that monotremes arose in Australia in the Late Jurassic
Late Jurassic
The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.2 ± 4.0 to 145.5 ± 4.0 million years ago , which is preserved in Upper Jurassic strata. In European lithostratigraphy, the name "Malm" indicates rocks of Late Jurassic age...
or Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
, and that some subsequently migrated across Antarctica to reach South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, both of which were still united with Australia at that time. However, a number of genetic studies suggest a much earlier origin 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...
.
Fossil species
- Family Kollikodontidae
- Genus KollikodonKollikodonKollikodon ritchiei is a fossil monotreme species. It is known only from an opalised dentary fragment, with one premolar and two molars in situ...
- Species Kollikodon ritchiei Ancient monotreme, 100–105 million years old.
- Genus Kollikodon
- Family SteropodontidaeSteropodontidaeThe Steropodontidae was a family of monotremes that are known from fossils from the Early Cretaceous in Australia.There are two genera placed in this family; Steropodon, and Teinolophos which has been tentatively placed in the family due to the similarity of the lower molars in these two...
May be part of Ornithorhynchidae; closely related to modern platypus.- Genus SteropodonSteropodonSteropodon galmani was a prehistoric species of monotreme, or egg-laying mammal, that lived during the middle Albian stage, in the Lower Cretaceous period...
- Species Steropodon galmani
- Genus TeinolophosTeinolophosTeinolophos 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...
- Species Teinolophos trusleri 123 million years old – oldest monotreme specimen.
- Genus Steropodon
- Family OrnithorhynchidaeOrnithorhynchidaeOrnithorhynchidae is one of the two extant families in the order Monotremata, and contains the Platypus and its extinct relatives. The other family is the Tachyglossidae, or echidnas...
- Genus Ornithorhynchus Oldest Ornithorhynchus specimen 9 million years old.
- Species Ornithorhynchus anatinus (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...
) Oldest specimen 10,000 years old.
- Species Ornithorhynchus anatinus (Platypus
- Genus ObdurodonObdurodonObdurodon is an extinct monotreme genus containing three species. Obdurodon differed from modern platypuses in that it had molar teeth .-Obdurodon dicksoni:...
Includes a number of MioceneMioceneThe 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...
(5–24 million years ago) platypuses.- Species Obdurodon dicksoni (Riversleigh PlatypusRiversleigh PlatypusThe Riversleigh Platypus is an ancient, semi-aquatic Monotreme from Australia during the lower and middle Miocene. Native to Queensland, The Riversleigh Platypus was discovered by Michael Archer, F. A. Jenkins, S. J. Hand, P. Murray, and H...
) - Species Obdurodon insignis
- Species Monotrematum sudamericanum 61 million years old. (Originally placed in separate genus, now thought an Obdurodon)
- Species Obdurodon dicksoni (Riversleigh Platypus
- Genus Ornithorhynchus Oldest Ornithorhynchus specimen 9 million years old.
- Family Tachyglossidae
- Genus Zaglossus Upper PleistocenePleistoceneThe Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
(0.1–1.8 million years ago).- Species Zaglossus hackettiZaglossus hackettiZaglossus hacketti is an extinct species of long-beaked echidna from Western Australia that is dated from the Pleistocene. It is known only from a few bones found in Western Australia. It was the size of a sheep, weighing probably up to 100 kg . This makes it the largest monotreme to have ever...
- Species Zaglossus robustusZaglossus robustusZaglossus robustus is an extinct species of long-beaked echidna known from the middle Miocene of Gulgong, New South Wales, Australia. It may belong in the genus Megalibgwilia. The supposed fossil platypus Ornithorhynchus maximus was based on a humerus of this species.-Literature cited:*Hall, B.K....
- Species Zaglossus hacketti
- Genus MegalibgwiliaMegalibgwiliaMegalibgwilia is a genus of echidna known only from Australian fossils that incorporates the oldest known echidna species. It lived during the Pleistocene, becoming extinct about 50,000 years ago....
- Megalibgwilia ramsayi Late PleistoceneLate PleistoceneThe Late Pleistocene is a stage of the Pleistocene Epoch. The beginning of the stage is defined by the base of the Eemian interglacial phase before the final glacial episode of the Pleistocene 126,000 ± 5,000 years ago. The end of the stage is defined exactly at 10,000 Carbon-14 years BP...
- Megalibgwilia robusta MioceneMioceneThe 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...
- Megalibgwilia ramsayi Late Pleistocene
- Genus Zaglossus Upper Pleistocene