Eucynodontia
Encyclopedia
Eucynodontia is a grouping of animals that includes both mammal
s, such as dog
s, and mammal-like non-mammalian therapsids ("mammal-like reptiles") such as cynodont
s ("dog teeth"). Its membership was and is made up of both carnivore
s and herbivore
s. The chronological range extends from at least the Lower Triassic
, possibly the Upper Permian
, until the present day. This overview is concentrated on the proto-mammals, which are known from the Lower Triassic until the Lower Cretaceous
, both divisions of the Mesozoic
era ("the age of the Dinosaur
s").
. This wolf-sized predator had a nearly worldwide distribution. About 90% of its lower jaw
was accounted for by a single tooth-bearing bone called the dentary. Its teeth were differentiated, which enabled them to perform several functions, including tearing and chewing. A crocodile
tears at its prey, but it can't chew. It's an effective hunter, but a wasteful and messy eater. The ear of Cynognathus contained a solitary small bone for hearing, (the stapes). The jaw was attached to the skull
by a joint called the articular-quadrate. The significance of these features, and what happened next, is illustrated in a bit more detail below.
Cynognathus is the only known representative of a family called Cynognathidae. However, various further derived relatives are also known.
Chiniquodontids are best known from the lower part of the Upper Triassic of South America
. Some more enigmatic material (mainly teeth) has been recovered from Mid – Upper Triassic Europe
an strata
. A Middle Triassic genus from Africa
, Aleodon, has also been referred to this family, though this is seen as questionable by others. They ranged in size from tiny Gaumia (should it be a chiniquodontid) to the dog sized Chiniquodon
.
Dromatheriidae is a possible family based mainly on teeny teeth from the Upper Triassic of Europe, North America
, and possibly India
. Although remains are sparse, these fossil
s are very mammal-like. A hypothesized ancestor of mammals could convincingly have been equipped with teeth like this, and some of this material may or may not represent human ancestors. It would require better fossils to test the validity of that possibility. It could be an informal grade rather than a natural taxon
.
Another proposed family within Chiniquodontoidea is Therioherpetidae
from the Upper Triassic of South America, and perhaps Europe (Meurthodon). The status of this family has been differently interpreted by various researchers. Therioherpeton
has alternatively been referred to Dromatheriidae. Another possibility is at least in part affinities with Tritheledontidae. This is a matter which will require further finds and study. The South American members were mouse or rat-sized, while Meurthodon was even smaller.
Includes: Therioherpeton
, Charruodon
, Meurthodon
Includes: Eoraetia, Kunminia
The only chiniquodontoids known to have survived beyond the Triassic were the extremely mammal-like members of Tritheledontidae, (also known as Tritheledontidae). These were small insectivore
s of up to 20 cm in length, and their lifestyle was presumably extremely similar to that of the first mammals. This may well explain their disappearance. Remains are known from the Upper Triassic of South America, and the Lower Jurassic
of South Africa and North America.
Includes: Trithelodon
, Diarthrognathus
, Pachygenelus
, Pattsia, and Riograndia
. They’re also referred to as Tritylodontoidea. It’s very likely that this arrangement is more a matter of convenience than systematics. However, as it is a convenient structure, it will be followed here.
The most basal representatives are found within a family called Diademodontidae. Most fossils come from the Lower Triassic of South Africa. Other reports stem from Asia and perhaps Antarctica.
Includes: Diademodon
, Hazhenia (?), Ordosiodon (?), and Titanogomphodon
Somewhat more derived are the trirachodontids of Africa, Asia
, Russia
and possibly North America. Some were contemporaries of the diademodontids and the lineage seems to have survived until the Middle Triassic. Trirachodon
lived communally in warrens. This is known from several fossilized burrows preserved in South Africa, along with their inhabitants.
Includes: Trirachodon
, Cricodon, Neotrirachodon
, and Sinognathus
The most diverse of the Triassic gomphodonts are the members of Traversodontidae
. This family emerged during the Lower Triassic and continued until the end of that age. The original representatives were small, though later types reached lengths of 50cm or so. The most recent known remains come from near the end of the European Triassic. These are teeth from shrew
-sized animals. Fossils have been found in all continent
s, (excepting for Australasia and Antarctica), though the best remains are from the lower Upper Triassic strata of Argentina
and Brazil
, which seems to have been the heyday of the traversodonts.
Includes: Traversodon
, Andescynodon
, Arctotraversodon
, Boreogomphodon
, Colbertosaurus
, Dadadon
, Exaeretodon
, Gomphodontosuchus
, Habayia
, Ischignathus, Luangwa, Massetognathus
, Maubeugia
, Menadon
, Microscalenodon, Pascualgnathus
, Plinthogomphodon
, Rosieria
, Rusconiodon, Scalenodon
, Scalenodontoides, and Theropsodon
Possibly descended from the traversodonts is a family known as Tritylodontidae
. It's fairly often assumed that non-mammal
ian cynodonts went extinct at the end of the Triassic. This is incorrect, as already demonstrated by members of Tritheledontidae (the insectivores mentioned above). Tritylodonts were their plant-eating counterparts. They were generally larger (up to about 50 cm in length) and survived longer; until at least the Lower Cretaceous. Where preserved, the anatomy suggests burrowing animals, and suitably sized fossilized burrows have been found at one location in Colorado
, along with tracks and anatomical remains. A post-Cretaceous representative has some limited support, (Chronoperates
), but this is more generally seen as some kind of mammal or other. Tritylodonts were mammal-like in the extreme, and were usually classed as such until the 1920s. However, their anatomy maintained significant "reptilian" features, especially in the jaw and the ear
s.
The earliest fossils recovered to date stem from the Upper Triassic of Argentina. Most representatives are known from the Lower Jurassic, when this family had a more or less worldwide distribution. (One genus, Oligokyphus
, has been found in Europe, China
and North America. Fragmentary tritylodont remains have also been recovered from Antarctica.) The most recent undisputed material comes from Siberia
and Japan
. The demise of the tritylodonts may be connected with the rise of multituberculate
mammals, especially with regards to the Northern Hemisphere
.
Includes: Tritylodon
, Bienotherium
, Bienotheroides, Bocatherium, Dianzhongia, Dinnebitodon
, Kayentatherium
, Lufengia, Oligokyphus
, Stereognathus, Xenocretosuchus, Yunnanodon
. There are a few genera dealt with here as mammals, which should possibly or probably be labelled as non-mammals. This is certainly the case for Sinoconodon
. Its dental replacement and growth habits were not mammalian. It probably also applies for Adelobasileus and Gondwanadon, which are both from earliest Upper Triassic rock, (Carnian). It could also be the case for the members of Haramiyida
. However, other than for one exception, haramiyids are presently known only from tiny teeth. Until more substantial remains turn up, the affinities of haramiyids is a matter beyond resolution.
Transitional fossil
In general, non-mammalian, terrestrial vertebrates, (land-living critters with backbones), have a less dominant dentary in the lower jaw than the aforementioned Cynognathus and, when equipped with gnashers, have only one kind of tooth. However, as with Cynognathus, the inner ear contains a single small bone and the jaw joint is the same.
In mammals, (with the exceptions of the most basal representatives), the dentary is the only lower jaw bone; where present, the teeth are strongly differentiated; the inner ear has three bones for processing sound (incus
, malleus
and stapes); the jaw joint is the articular-quadrate (it’s at least overwhelmingly dominant among the basal representatives).
All these features as well as others can be seen in transition in the fossil record of the Triassic and Jurassic. The non-mammalian eucynodonts became progressively more mammal-like, and the anatomical distinctions between the more derived forms and the earliest mammals are best described as matters of degree.
Among the Triassic proto-mammals, the tooth-bearing dentary became ever more dominant, until the lower jaw bones were tiny. The teeth grew in complexity and efficiency. The mammalian jaw-cranium joint (dentary-squamosal) grew up alongside of, and eventually, (in mammals), entirely replaced the non-mammalian one. While the inner ear still worked with only one small bone, other important structural changes were underway; e.g. the cochlear canal appeared (e.g. Yunnanodon).
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, such as dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
s, and mammal-like non-mammalian therapsids ("mammal-like reptiles") such as 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 ("dog teeth"). Its membership was and is made up of both carnivore
Carnivore
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...
s and 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. The chronological range extends from at least the Lower 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...
, possibly the Upper 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...
, until the present day. This overview is concentrated on the proto-mammals, which are known from the Lower Triassic until the Lower 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...
, both divisions of the 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...
era ("the age 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").
Cynognathidae
Among the first and most basal of the eucynodonts was CynognathusCynognathus
Cynognathus crateronotus was a meter-long predator of the Early to Middle Triassic. It was among the more mammal-like of the Synapsids, a member of a grouping called Eucynodontia. The genus Cynognathus had an almost worldwide distribution...
. This wolf-sized predator had a nearly worldwide distribution. About 90% of its lower jaw
Jaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...
was accounted for by a single tooth-bearing bone called the dentary. Its teeth were differentiated, which enabled them to perform several functions, including tearing and chewing. A crocodile
Crocodile
A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...
tears at its prey, but it can't chew. It's an effective hunter, but a wasteful and messy eater. The ear of Cynognathus contained a solitary small bone for hearing, (the stapes). The jaw was attached to the skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
by a joint called the articular-quadrate. The significance of these features, and what happened next, is illustrated in a bit more detail below.
Cynognathus is the only known representative of a family called Cynognathidae. However, various further derived relatives are also known.
- FamilyFamily (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...
: Cynognathidae- CynognathusCynognathusCynognathus crateronotus was a meter-long predator of the Early to Middle Triassic. It was among the more mammal-like of the Synapsids, a member of a grouping called Eucynodontia. The genus Cynognathus had an almost worldwide distribution...
- Cynognathus
- Family: Probainognathidae
- ProbainognathusProbainognathusProbainognathus is a genus of meat-eating mammal-like reptile that lived during the lower Upper Triassic of South America. This creature had an incipient squamosal-dentary jaw-cranium joint, which is a clearly mammalian anatomical feature. It was at the very least closely related to the family of...
- LepagiaLepagiaLepagia is a poorly known genus of meat-eating mammal-like reptile, , which lived during the Upper Triassic in Europe. Partly due to the paucity of remains the precise affinities of this genus are unclear. It seems reasonably closely related to Probainognathus, a somewhat earlier inhabitant of...
(?)
- Probainognathus
- Unclear affinities
- EcteninionEcteninionEcteninion is a genus of meat-eating mammal-like reptiles that lived during the Triassic of South America. Its affinities are unclear, but it seems to be related to Probainognathus. The genus Ecteninion was named by Martinez R.N., May C.L. and Forster C.A...
- "HahniaHahniaHahnia is a poorly known genus of meat-eating mammal-like reptiles that lived during the Upper Triassic in Europe. This genus is based on tiny, isolated teeth, and its affinities with other cynodonts are unclear. The generic name is invalid, thus the inverted commas. The name Hahnia has already...
"
- Ecteninion
Chiniquodontoidea
Further non-mammalian eucynodonts are known, though the structure followed here is probably in need of review. The main lineage of meat-eaters is the superfamily of Chiniquodontoidea. Some schemes divide this into two families, a further pair of possible families and some odd bits and pieces.Chiniquodontids are best known from the lower part of the Upper Triassic of 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...
. Some more enigmatic material (mainly teeth) has been recovered from Mid – Upper Triassic Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
an strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
. A Middle Triassic genus from Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, Aleodon, has also been referred to this family, though this is seen as questionable by others. They ranged in size from tiny Gaumia (should it be a chiniquodontid) to the dog sized Chiniquodon
Chiniquodon
Chiniquodon is a genus of carnivorous "mammal-like reptile", , which lived during the lower Upper Triassic in South America. These animals were up to the size of a dog. This genus seems to be most closely related to a contemporary genus, Probelesodon...
.
- Family: ChiniquodontidaeChiniquodontidaeChiniquodontidae is a family of meat-eating advanced mammal-like reptiles that lived during the Upper Triassic of South America and perhaps Europe. A further possible representative, Aleodon, has been identified from the Middle Triassic of Africa...
- Aleodon
- BelesodonBelesodonBelesodon is a genus that was described by the species Belesodon magnificus.- Species:Belesodon magnificus, was the first cynodont found in the Santa Maria Formation in Paleorrota, Brazil. It was described by Friedrich von Huene in...
- ChiniquodonChiniquodonChiniquodon is a genus of carnivorous "mammal-like reptile", , which lived during the lower Upper Triassic in South America. These animals were up to the size of a dog. This genus seems to be most closely related to a contemporary genus, Probelesodon...
- Gaumia
- ProbelesodonProbelesodonProbelesodon is an extinct genus of chiniquodontid cynodont. Fossils have been found from Argentina and Brazil of Triassic age. Specimens were first discovered from the Chañares Formation in La Rioja Provence, Argentina that date back to the Anisian stage of the early Middle Triassic. A new...
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, and possibly India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
. Although remains are sparse, these fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s are very mammal-like. A hypothesized ancestor of mammals could convincingly have been equipped with teeth like this, and some of this material may or may not represent human ancestors. It would require better fossils to test the validity of that possibility. It could be an informal grade rather than a natural taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
.
- Family: Dromatheriidae
- Dromatherium
- Microconodon
- Pseudotriconodon
- TricuspesTricuspesTricuspes is an extinct genus of cynodont that lived in what would be Europe during the Triassic from 203.6—199.6 mya, existing for approximately...
Another proposed family within Chiniquodontoidea is Therioherpetidae
Therioherpetidae
Therioherpetidae are based on Therioherpeton Cargnin, a small cynodont who lived in the Triassic and was collected in Santa Maria, Santa Maria Formation, the Geopark of Paleorrota, Brazil. This family consists of small carnivores...
from the Upper Triassic of South America, and perhaps Europe (Meurthodon). The status of this family has been differently interpreted by various researchers. Therioherpeton
Therioherpeton
Therioherpeton is a genus based on Therioherpeton cargnini, a small cynodont who lived in the Triassic and was collected in Santa Maria, Santa Maria Formation, the Geopark of Paleorrota, Brazil. It was a small carnivore. For his characteristic , was touted as a likely ancestor of mammals.-...
has alternatively been referred to Dromatheriidae. Another possibility is at least in part affinities with Tritheledontidae. This is a matter which will require further finds and study. The South American members were mouse or rat-sized, while Meurthodon was even smaller.
- Family: Therioherpetidae
Includes: Therioherpeton
Therioherpeton
Therioherpeton is a genus based on Therioherpeton cargnini, a small cynodont who lived in the Triassic and was collected in Santa Maria, Santa Maria Formation, the Geopark of Paleorrota, Brazil. It was a small carnivore. For his characteristic , was touted as a likely ancestor of mammals.-...
, Charruodon
Charruodon
Charruodon tetracuspidatus is an extinct species of synapsid which existed in Geopark of Paleorrota, Brazil during the middle–late Triassic period. It is placed in its own genus, Charruodon. Only one specimen is known, and is of uncertain provenance....
, Meurthodon
- Unclear affinities
Includes: Eoraetia, Kunminia
The only chiniquodontoids known to have survived beyond the Triassic were the extremely mammal-like members of Tritheledontidae, (also known as Tritheledontidae). These were small 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 of up to 20 cm in length, and their lifestyle was presumably extremely similar to that of the first mammals. This may well explain their disappearance. Remains are known from the Upper Triassic of South America, and the Lower 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...
of South Africa and North America.
- Family: Tritheledontidae
Includes: Trithelodon
Trithelodon
Trithelodon was an extinct genus of a group of mammal-like reptiles called cynodonts. Like cynodonts, Trithelodon had many traits shared by mammals. Trithelodon was probably an insectivore, and nocturnal....
, Diarthrognathus
Diarthrognathus
Diarthrognathus was a genus of mammal-like reptiles, known from fossil evidence found in South Africa. The Diarthrognathus is of interest as it possesses a jaw structure that is similar to both mammals and reptiles, with a reptilian joint between the quadrate and articular in addition to the...
, Pachygenelus
Pachygenelus
Pachygenelus is an extinct genus of tritheledontid cynodont. Fossils have been found from the Karoo basin in South Africa and date back to the Early Jurassic....
, Pattsia, and Riograndia
Riograndia
Riograndia is a genus of tritheledontid cynodont from the Late Triassic of South America. The type species is R. guaibensis. Riograndia guaibensis was estimated to have been about 15 cm long and 30 g in weight. Remains have been found in the Caturrita Formation of the geopark of Paleorrota...
Gomphodonts and Tritylodonts
Traditionally, the herbivorous counterparts of the chiniquodontoids were grouped together under the term gomphodonts, ("peg teeth"). Their chronological range extended from the Lower Triassic until the Lower Cretaceous, and remains have been found on every continent in the world, with the exception of AustralasiaAustralasia
Australasia is a region of Oceania comprising Australia, New Zealand, the island of New Guinea, and neighbouring islands in the Pacific Ocean. The term was coined by Charles de Brosses in Histoire des navigations aux terres australes...
. They’re also referred to as Tritylodontoidea. It’s very likely that this arrangement is more a matter of convenience than systematics. However, as it is a convenient structure, it will be followed here.
The most basal representatives are found within a family called Diademodontidae. Most fossils come from the Lower Triassic of South Africa. Other reports stem from Asia and perhaps Antarctica.
- Family: Diademodontidae
Includes: Diademodon
Diademodon
Diademodon is an vegetarian extinct genus of therapsid. It was about the size of a small cow. Fossils of the Diademodon tetragonus were found in levels of the Rio Seco de la Quebrada geological formation in Mendoza Province, Argentina....
, Hazhenia (?), Ordosiodon (?), and Titanogomphodon
Somewhat more derived are the trirachodontids of Africa, Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
, Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
and possibly North America. Some were contemporaries of the diademodontids and the lineage seems to have survived until the Middle Triassic. 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:...
lived communally in warrens. This is known from several fossilized burrows preserved in South Africa, along with their inhabitants.
- Family: TrirachodontidaeTrirachodontidaeTrirachodontidae is an extinct family of cynognathian cynodonts from the Triassic of China and southern Africa. Trirachodontids appeared during the Early Triassic soon after the Permian-Triassic extinction event and quickly spread over a wide geographic area in a comparatively brief amount of time...
Includes: 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:...
, Cricodon, Neotrirachodon
Neotrirachodon
Neotrirachodon is an extinct genus of synapsid which existed in Russia during the Middle Triassic period. It contains the species Neotrirachodon expectatus....
, and Sinognathus
The most diverse of the Triassic gomphodonts are the members of Traversodontidae
Traversodontidae
Traversodontidae is a family of herbivorous cynodonts. Traversodonts were primarily Gondwanan, with many species known from Africa and South America. Recently, traversodonts have also been found from Europe and eastern North America. Traversodonts first appeared in the Middle Triassic, diversified...
. This family emerged during the Lower Triassic and continued until the end of that age. The original representatives were small, though later types reached lengths of 50cm or so. The most recent known remains come from near the end of the European Triassic. These are teeth from shrew
Shrew
A shrew or shrew mouse is a small molelike mammal classified in the order Soricomorpha. True shrews are also not to be confused with West Indies shrews, treeshrews, otter shrews, or elephant shrews, which belong to different families or orders.Although its external appearance is generally that of...
-sized animals. Fossils have been found in all continent
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
s, (excepting for Australasia and Antarctica), though the best remains are from the lower Upper Triassic strata of 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...
and Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
, which seems to have been the heyday of the traversodonts.
- Family: TraversodontidaeTraversodontidaeTraversodontidae is a family of herbivorous cynodonts. Traversodonts were primarily Gondwanan, with many species known from Africa and South America. Recently, traversodonts have also been found from Europe and eastern North America. Traversodonts first appeared in the Middle Triassic, diversified...
Includes: Traversodon
Traversodon
Traversodon is a genus of therapsid. Being a cynodont, it was a relative of the direct ancestor to modern mammals. Traversodon lived in what is now the Northeastern part of the United States and South America.-Species:...
, Andescynodon
Andescynodon
Andescynodon is a genus of traversodontid cynodont from the Middle Triassic of Argentina. Fossils are known from the Cerro de las Cabras Formation. Andescynodon is one of the most basal traversodontids...
, Arctotraversodon
Arctotraversodon
Arctotraversodon is a genus of traversodontid cynodont from the Late Triassic of Canada. Fossils first described from the Wolfville Formation in Nova Scotia in 1984 and represented the first known traversodontid from North America. The type and only species is A...
, Boreogomphodon
Boreogomphodon
Boreogomphodon is a genus of traversodontid cynodont from the Late Triassic of the eastern United States. Fossils have been found from the Turkey Branch Formation in Virginia.-Description and history:...
, Colbertosaurus
Colbertosaurus
Colbertosaurus is a genus of traversodontid cynodont from the Middle Triassic of Argentina. A single species C. muralis was named in 1954 from a fragmentary jaw. Colbertosaurus was originally placed in Ictidosauria, an outdated name for a group of cynodonts that includes tritheledontids...
, Dadadon
Dadadon
Dadadon is a traversodontid cynodont which existed in Madagascar during the late Middle Triassic. The only species in the genus is Dadadon isaloi....
, Exaeretodon
Exaeretodon
Exaeretodon is a genus of traversodontid cynodont; several species are known, from various formations. E. argentinus, E. frenguelli, and E. vincei are from the Carnian-age Ischigualasto Formation of Argentina. E. major and E. riograndensis are from the Ladinian-age portion of the Santa Maria...
, Gomphodontosuchus
Gomphodontosuchus
Gomphodontosuchus is a genus created to have described the species Gomphodontosuchus arasiliensis.-Species:Gomphodontosuchus brasiliensis, is a species first collected in 1928 by Friedrich von Huene in Santa Maria Formation, the Geopark of Paleorrota, Brazil.-References:* * *...
, Habayia
Habayia
Habayia is a genus of traversodontid cynodont from the Late Triassic of Belgium. A single postcanine tooth was found in Habay-la-Vieille in southern Belgium. Based on the size of the tooth, Habayia was very small. Habayia lived during the Rhaetian stage of the Late Triassic at a time when western...
, Ischignathus, Luangwa, Massetognathus
Massetognathus
A relative of Cynognathus, Massetognathus was a plant-eating cynodont belonging to the Traversodontid family. This cynodont lived in what is now South America, in Brazil and Argentina during the Middle Triassic period .Massetognathus was about long. It had cheek teeth specially adapted to...
, Maubeugia
Maubeugia
Maubeugia is a genus of traversodontid cynodont from the Late Triassic of France. Isolated postcanine teeth are known from Saint-Nicolas-de-Port in northeastern France. The type species M. lotharingica was named in 1997. Many other cynodont teeth were found alongside those of Maubeugia, including...
, Menadon
Menadon
Menadon is an extinct genus of synapsid which existed in Madagascar during the middle to late Triassic period. The type species is Menadon besairiei....
, Microscalenodon, Pascualgnathus
Pascualgnathus
Pascualgnathus is a genus of traversodontid cynodont from the Middle Triassic of Argentina. Fossils have been found from the Río Seco de la Quebrada Formation. The type species P. polanskii was named in 1966.-Description:...
, Plinthogomphodon
Plinthogomphodon
Plinthogomphodon is a genus of traversodontid cynodont from the Late Triassic of the eastern United States. Fossils have been found from the Deep River basin of North Carolina, part of the larger Newark Supergroup. It is known from a single eroded snout. The type and only species is P....
, Rosieria
Rosieria
Rosieria is a genus of dwarf cynodont from the Late Triassic of France. It belongs to the family Traversodontidae, a herbivorous group known mainly from Gondwana. The type species R. delsatei was named in 1997 on the basis of a few isolated postcanine teeth found in Saint-Nicolas-de-Port in...
, Rusconiodon, Scalenodon
Scalenodon
Scalenodon is a genus of traversodontid cynodont from the Middle Triassic of Africa and possibly Russia. The type species S. angustifrons was named in 1946 and several other species were named in the following years...
, Scalenodontoides, and Theropsodon
Theropsodon
Theropsodon is a genus of traversodontid cynodont from the Middle Triassic of Tanzania. Fossils have been found from the Manda Formation. A single holotype skull of the type species T. njaliliris was named by German paleontologist Friedrich von Huene in 1950....
Possibly descended from the traversodonts is a family known as Tritylodontidae
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...
. It's fairly often assumed that non-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...
ian cynodonts went extinct at the end of the Triassic. This is incorrect, as already demonstrated by members of Tritheledontidae (the insectivores mentioned above). Tritylodonts were their plant-eating counterparts. They were generally larger (up to about 50 cm in length) and survived longer; until at least the Lower Cretaceous. Where preserved, the anatomy suggests burrowing animals, and suitably sized fossilized burrows have been found at one location in Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
, along with tracks and anatomical remains. A post-Cretaceous representative has some limited support, (Chronoperates
Chronoperates
Chronoperates paradoxus is a possible therapsid whose remains have been found in Alberta, Canada. It has also been suggested that it may be a symmetrodont.-Sources:http://darrennaish.blogspot.com/2006/05/time-wandering-cynodonts-and-docodonts.html...
), but this is more generally seen as some kind of mammal or other. Tritylodonts were mammal-like in the extreme, and were usually classed as such until the 1920s. However, their anatomy maintained significant "reptilian" features, especially in the jaw and the ear
Ear
The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....
s.
The earliest fossils recovered to date stem from the Upper Triassic of Argentina. Most representatives are known from the Lower Jurassic, when this family had a more or less worldwide distribution. (One genus, Oligokyphus
Oligokyphus
Oligokyphus was an advanced herbivorous cynodont of the late Triassic to early Jurassic periods. Originally considered to be an early mammal, it is now classified as a synapsid because Oligokyphus does not have the mammalian jaw attachments and it retains a vestigial joint between the quadrate...
, has been found in Europe, 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 North America. Fragmentary tritylodont remains have also been recovered from Antarctica.) The most recent undisputed material comes from Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...
and Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
. The demise of the tritylodonts may be connected with the rise of multituberculate
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...
mammals, especially with regards to the Northern Hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
.
- Family: TritylodontidaeTritylodontidaeTritylodontids 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...
Includes: Tritylodon
Tritylodon
Trityldon was a genus of tritylodont, one of the most advanced group of cynodont therapsids. They lived in the Early Jurassic and possibly Late Triassic periods along with dinosaurs. They also shared a lot of characteristics with mammals, and were once considered mammals because of overall...
, Bienotherium
Bienotherium
Bienotherium was a therapsid from the Early Jurassic of China. Despite its size, it is closely related to Lufengia, and is the largest tritylodont from the Lufeng Formation in China....
, Bienotheroides, Bocatherium, Dianzhongia, Dinnebitodon
Dinnebitodon
Dinnebitodon was an advanced herbivorous cynodont of the early Jurassic period. It has only been found in the Kayenta Formation in northeastern Arizona. It closely resembles the related genus Kayentatherium from the same formation...
, Kayentatherium
Kayentatherium
Kayentatherium is a genus of tritylodont, historically referred to as "mammal-like" reptiles. It is one of three tritylodonts from the Kayenta Formation of northern Arizona, United States.-Description:...
, Lufengia, Oligokyphus
Oligokyphus
Oligokyphus was an advanced herbivorous cynodont of the late Triassic to early Jurassic periods. Originally considered to be an early mammal, it is now classified as a synapsid because Oligokyphus does not have the mammalian jaw attachments and it retains a vestigial joint between the quadrate...
, Stereognathus, Xenocretosuchus, Yunnanodon
Possible non-mammals
A rather loose definition of Mammalia could start with creatures of roughly the anatomical grade of 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...
. There are a few genera dealt with here as mammals, which should possibly or probably be labelled as non-mammals. This is certainly the case for Sinoconodon
Sinoconodon
Sinoconodon rigneyi is an ancient proto-mammal that appears in the fossil record in the late Triassic period, about 208 million years ago. Although the animal seems more related to Morganucodon than anything else, it differed substantially from other Mammaliaformes in its dental and growth habits...
. Its dental replacement and growth habits were not mammalian. It probably also applies for Adelobasileus and Gondwanadon, which are both from earliest Upper Triassic rock, (Carnian). It could also be the case for the members of Haramiyida
Haramiyida
Haramiyidans seem to be the earliest known herbivores amongst basal mammals, assuming they are mammals. Their teeth, which are by far the most common remains, resemble those of the multituberculates...
. However, other than for one exception, haramiyids are presently known only from tiny teeth. Until more substantial remains turn up, the affinities of haramiyids is a matter beyond resolution.
Transitional fossilTransitional fossilA transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...
s
In general, non-mammalian, terrestrial vertebrates, (land-living critters with backbones), have a less dominant dentary in the lower jaw than the aforementioned Cynognathus and, when equipped with gnashers, have only one kind of tooth. However, as with Cynognathus, the inner ear contains a single small bone and the jaw joint is the same.In mammals, (with the exceptions of the most basal representatives), the dentary is the only lower jaw bone; where present, the teeth are strongly differentiated; the inner ear has three bones for processing sound (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....
, 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 stapes); the jaw joint is the articular-quadrate (it’s at least overwhelmingly dominant among the basal representatives).
All these features as well as others can be seen in transition in the fossil record of the Triassic and Jurassic. The non-mammalian eucynodonts became progressively more mammal-like, and the anatomical distinctions between the more derived forms and the earliest mammals are best described as matters of degree.
Among the Triassic proto-mammals, the tooth-bearing dentary became ever more dominant, until the lower jaw bones were tiny. The teeth grew in complexity and efficiency. The mammalian jaw-cranium joint (dentary-squamosal) grew up alongside of, and eventually, (in mammals), entirely replaced the non-mammalian one. While the inner ear still worked with only one small bone, other important structural changes were underway; e.g. the cochlear canal appeared (e.g. Yunnanodon).