Tetracynodon

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Tetracynodon
Temporal range: Late Permian - Early Triassic
Tetracynodon1.jpg
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade: Therapsida
Clade: Therocephalia
Family: Lycideopidae
Genus: Tetracynodon
Broom and Robinson, 1948
Species
  • T. tenuisBroom and Robinson, 1948
  • T. dartiSigogneau, 1963

Tetracynodon is an extinct genus of therocephalian. Fossils of Tetracynodon have been found in the Karoo Basin of South Africa. Two species are known: the type species T. tenuis from the Late Permian and the species T. darti from the Early Triassic. Both species were small-bodied and probably fed on insects and small vertebrates. Although Tetracynodon is more closely related to mammals than to reptiles, its braincase is very primitive and more resembles that of modern amphibians and reptiles than of mammals. [1]

Contents

Permo-Triassic survivorship

Tetracynodon was one of the few therapsid genera known to have survived the Permo-Triassic extinction event. Aside from Tetracynodon, the only therocephalian genera known from both sides of the Permo-Triassic boundary are Moschorhinus , Ictidosuchoides and Promoschorhynchus . The Triassic species Tetracynodon darti would have been part of the extinction's survivor fauna, a low-diversity community of therapsids and other land vertebrates. The Triassic species Tetracynodon darti is relatively small compared to Permian baurioids. This may be a possible example of the Lilliput effect in which small species are more common immediately after a mass extinction. [1]

Phylogeny

Tetracynodon was once classified among a group of therocephalians called scaloposaurids, all characterized by their very small size. Scaloposauridae is no longer recognized as a valid group because many scaloposaurid features are now thought to be characteristics of juvenile individuals rather than evidence of close evolutionary relationships. [1]

In 2008, Tetracynodon tenuis was proposed to be a juvenile of the larger therocephalian Lycideops , which is known from the same Late Permian strata. Since T. tenuis is the type species of Tetracynodon and the type species of Lycideops, L. longiceps, was named earlier than T. tenuis, the species would be a synonym of Lycideops longiceps and the genus Tetracynodon would be invalid. Moreover, since the study found that Lycideops was not directly related to the Early Triassic Tetracynodon darti, it suggested that this lineage of therocephalians did not survive uninterrupted past the Permo-Triassic extinction event. [2]

Tetracynodon was included in a phylogenetic analysis of therocephalians in 2012 and was placed in clade Baurioidea along with many other prior scaloposaurids. It was found to nest in a smaller clade with Lycideops and Choerosaurus , designated as the family Lycideopidae. Tetracynodon tenuis and T. darti were found to be sister taxa, meaning that they were a valid phylogenetic grouping and T. tenuis could not be a juvenile of Lycideops. Below is a cladogram from the study: [1]

Therapsida

Biarmosuchus tener

Titanophoneus potens

Gorgonopsia

Anomodontia

Eutheriodontia
Cynodontia

Charassognathus gracilis

Dvinia prima

Procynosuchus delaharpeae

Therocephalia

Lycosuchus vanderrieti

Glanosuchus macrops

Ictidosaurus angusticeps

Pristerognathus polyodon

Eutherocephalia

Euchambersia mirabilis

Promoschorhynchus platyrhinus

Olivierosuchus parringtoni

Cerdops burgheri

Moschorhinus kitchingi

Ictidostoma hemburyi

Hofmeyria atavus

Mirotenthes digitipes

Viatkosuchus sumini

Moschowhaitsia vjuschkovi

Theriognathus microps

Baurioidea

Ictidosuchus primaevus

Ictidosuchops rubidgei

Ictidosuchoides longiceps

Regisaurus jacobi

Scaloposaurus constrictus

Ericiolacerta parva

Bauriidae

Lycideopidae

Lycideops longiceps

Choerosaurus dejageri

Tetracynodon tenuis

Tetracynodon darti

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<span class="mw-page-title-main">Dinocephalia</span> Extinct clade of stem-mammals

Dinocephalians are a clade of large-bodied early therapsids that flourished in the Early and Middle Permian between 279.5 and 260 million years ago (Ma), but became extinct during the Capitanian mass extinction event. Dinocephalians included herbivorous, carnivorous, and omnivorous forms. Many species had thickened skulls with many knobs and bony projections. Dinocephalians were the first non-mammalian therapsids to be scientifically described and their fossils are known from Russia, China, Brazil, South Africa, Zimbabwe, and Tanzania.

<i>Lystrosaurus</i> Genus of Late Permian and Early Triassic dicynodont therapsids

Lystrosaurus is an extinct genus of herbivorous dicynodont therapsids from the late Permian and Early Triassic epochs. It lived in what is now Antarctica, India, China, Mongolia, European Russia and South Africa. Four to six species are currently recognized, although from the 1930s to 1970s the number of species was thought to be much higher. They ranged in size from that of a small dog to 8 feet long.

<i>Ericiolacerta</i> Extinct genus of therapsid from the #REDIRECT early Triassic

Ericiolacerta is an extinct genus of small therocephalian therapsids from the early Triassic of South Africa and Antarctica. Ericiolacerta, meaning "hedgehog lizard", was named by D.M.S. Watson in 1931. The species E. parva is known from the holotype specimen which consists of a nearly complete skeleton found in the Lystrosaurus Assemblage Zone within the Katberg Formation of the Beaufort Group in South Africa, and from a partial jaw found in the Lower Triassic Fremouw Formation in Antarctica. Ericiolacerta was around 20 centimetres (7.9 in) in length, with long limbs and relatively small teeth. It probably ate insects and other small invertebrates. The therocephalians – therapsids with mammal-like heads – were abundant in Permian times, but only a few made it into the Triassic. Ericiolacerta was one of those. It is possible that they gave rise to the cynodonts, the only therapsid group to survive into post-Triassic times. Cynodonts gave rise to mammals.

<span class="mw-page-title-main">Therocephalia</span> Extinct order of therapsids

Therocephalia is an extinct clade of eutheriodont therapsids from the Permian and Triassic. The therocephalians ("beast-heads") are named after their large skulls, which, along with the structure of their teeth, suggest that they were carnivores. Like other non-mammalian synapsids, therocephalians were once described as "mammal-like reptiles". Therocephalia is the group most closely related to the cynodonts, which gave rise to the mammals. This relationship takes evidence in a variety of skeletal features.

<span class="mw-page-title-main">Eutherocephalia</span> Extinct clade of therapsids

Eutherocephalia is an extinct clade of advanced therocephalian therapsids. Eutherocephalians are distinguished from the lycosuchids and scylacosaurids, two early therocephalian families. While lycosuchids and scyalosaurids became extinct by the end of the Permian period, eutherocephalians survived the Permian–Triassic extinction event. The group eventually became extinct in the Middle Triassic.

<i>Euchambersia</i> Extinct genus of therapsid from Late Permian South Africa

Euchambersia is an extinct genus of therocephalian therapsids that lived during the Late Permian in what is now South Africa and China. The genus contains two species. The type species E. mirabilis was named by paleontologist Robert Broom in 1931 from a skull missing the lower jaw. A second skull, belonging to a probably immature individual, was later described. In 2022, a second species, E. liuyudongi, was named by Jun Liu and Fernando Abdala from a well-preserved skull. It is a member of the family Akidnognathidae, which historically has also been referred by as the synonymous Euchambersiidae.

<i>Moschorhinus</i> Genus of synapsid from late Permian and early Triassic South Africa

Moschorhinus is an extinct genus of therocephalian in the family Akidnognathidae with only one species: M. kitchingi. It was a carnivorous synapsid which has been found in the Late Permian to Early Triassic of the South African Karoo Supergroup. It was a large carnivore, reaching 1.5 m (4.9 ft) in total body length with the largest skull comparable to that of a lion in size. It had a broad, blunt snout which bore long, straight canines. It appears to have replaced the gorgonopsids ecologically, and hunted much like a big cat. While most abundant in the Late Permian, it survived a little after the Permian Extinction, though these Triassic individuals had stunted growth.

<i>Raranimus</i> Extinct genus of therapsids

Raranimus is an extinct genus of therapsids of the Middle Permian. It was described in 2009 from a partial skull found in 1998 from the Dashankou locality of the Qingtoushan Formation, outcropping in the Qilian Mountains of Gansu, China. The genus is the most basal known member of the clade Therapsida, to which the later Mammalia belong.

<span class="mw-page-title-main">Akidnognathidae</span> Extinct family of therapsids

Akidnognathidae is an extinct family of therocephalian therapsids from the Late Permian and Early Triassic of South Africa, Russia and China. The family includes many large-bodied therocephalians that were probably carnivorous, including Moschorhinus and Olivierosuchus. One akidnognathid, Euchambersia, may even have been venomous. Akidnognathids have robust skulls with a pair of large caniniform teeth in their upper jaws. The family is morphologically intermediate between the more basal therocephalian group Scylacosauridae and the more derived group Baurioidea.

<span class="mw-page-title-main">Baurioidea</span> Extinct superfamily of therapsids

Baurioidea is a superfamily of therocephalian therapsids. It includes advanced therocephalians such as Regisaurus and Bauria. The superfamily was named by South African paleontologist Robert Broom in 1911. Bauriamorpha, named by D. M. S. Watson and Alfred Romer in 1956, is a junior synonym of Baurioidea.

<span class="mw-page-title-main">Bauriidae</span> Extinct family of therapsids

Bauriidae is an extinct family of therocephalian therapsids. Bauriids were the latest-surviving group of therocephalians after the Permian–Triassic extinction event, going extinct in the Middle Triassic. They are among the most advanced eutherocephalians and possess several mammal-like features such as a secondary palate and wide postcanine teeth at the back of the jaws. Unlike other therocephalians, bauriids were herbivorous. They were also smaller than earlier members of the group. Two subfamilies are classified within Bauriidae: Nothogomphodontinae and Bauriinae.

Homodontosaurus is an extinct genus of therocephalian therapsids from the Late Permian of South Africa. The type species Homodontosaurus kitchingi was named by South African paleontologist Robert Broom in 1949. Broom based his description on a small skull found in the Cistecephalus Assemblage Zone near Graaff-Reinet. The skull is very small, at about 55 millimetres (2.2 in) long and 20 millimetres (0.79 in) wide. Homodontosaurus has large eye sockets and an elongated snout. The lower jaw is long, thin, and curved. Numerous small teeth line the upper jaw and are long, pointed, and round in cross-section.

Icticephalus is an extinct genus of therocephalian therapsids from the Middle and Late Permian of South Africa. The type species Icticephalus polycynodon was named from the Tapinocephalus Assemblage Zone by South African paleontologist Robert Broom in 1915. Specimens of Icticephalus have also been described from the Cistecephalus Assemblage Zone. Broom originally placed Icticephalus in the Scaloposauridae, a group of very small therocephalians. Most scaloposaurids are now thought to be juvenile forms of other therocephalians, and Scaloposauridae is no longer recognized as a valid grouping. Icticephalus and other former scaloposaurids are now classified as basal members of Baurioidea.

Ictidodraco is an extinct genus of therocephalian therapsids from the Late Permian of South Africa. The type species Ictidodraco longiceps was named by South African paleontologists Robert Broom and John T. Robinson in 1948 from the Cistecephalus Assemblage Zone. Ictidodraco was once classified as a scaloposaurian in the family Silpholestidae. Scaloposauria and Silpholestidae are no longer regarded as valid groups, and Ictidodraco is now classified as a basal member of the clade Baurioidea.

Lycideopidae is an extinct family of therocephalians from the Late Permian and Early Triassic of South Africa.

Lycideops is an extinct genus of therocephalians from the Late Permian of South Africa. The type species is Lycideops longiceps, named in 1931 by South African paleontologist Robert Broom. Fossils of Lycideops come from the Dicynodon Assemblage Zone of the Beaufort Group. Lycideops is a member of the family Lycideopidae. Like other lycideopids, Lycideops has a long snout.

<i>Hazhenia</i> Genus of from the Early Triassic of China

Hazhenia is an extinct genus of therocephalian therapsids from the Early Triassic of China, of which Hazhenia concava is the only species. Hazhenia was named in 1981 from the Heshanggou Formation in the Ordos Desert of Inner Mongolia. It lived during the Olenekian Age of the Early Triassic, about 247 million years ago. Hazhenia belongs to a group of therocephalians called Baurioidea and possesses many mammal-like features such as cusped teeth and a secondary palate, both of which evolved independently in baurioids. Within Baurioidea it is most closely related to the genus Ordosiodon, which is also known from Inner Mongolia but comes from the slightly younger Ermaying Formation. Both genera were once placed in the family Ordosiidae, but as the name is preoccupied by a family of Cambrian trilobites, it is no longer valid.

<i>Thliptosaurus</i> Extinct genus of dicynodonts

Thliptosaurus is an extinct genus of small kingoriid dicynodont from the latest Permian period of the Karoo Basin in KwaZulu-Natal, South Africa. It contains the type and only known species T. imperforatus. Thliptosaurus is from the upper Daptocephalus Assemblage Zone, making it one of the youngest Permian dicynodonts known, living just prior to the Permian mass extinction. It also represents one of the few small bodied dicynodonts to exist at this time, when most other dicynodonts had large body sizes and many small dicynodonts had gone extinct. The unexpected discovery of Thliptosaurus in a region of the Karoo outside of the historically sampled localities suggests that it may have been part of an endemic local fauna not found in these historic sites. Such under-sampled localities may contain 'hidden diversities' of Permian faunas that are unknown from traditional samples. Thliptosaurus is also unusual for dicynodonts as it lacks a pineal foramen, suggesting that it played a much less important role in thermoregulation than it did for other dicynodonts.

<i>Counillonia</i> Extinct genus of dicynodonts

Counillonia is an extinct genus of dicynodont therapsid from the area of Luang Prabang in Laos, Southeast Asia that lived at around the time of the Permian-Triassic boundary and possibly dates to the earliest Early Triassic. Its type and only known species is C. superoculis. Counillonia was related to the Triassic dicynodonts such as Lystrosaurus and the Kannemeyeriiformes that survived the Permian mass extinction, but it was more closely related to the Permian genus Dicynodon than to either of these lineages. Counillonia may then possibly represent another line of dicynodonts that survived the Permian mass extinction into the Triassic period, depending on its age. The discovery of Counillonia in Laos and its unexpected evolutionary relationships hint at the less well understood geographies of dicynodont diversity across the Permo-Triassic boundary outside of well explored regions like the Karoo Basin in South Africa.

References

  1. 1 2 3 4 Sigurdsen, T.; Huttenlocker, A. K.; Modesto, S. P.; Rowe, T. B.; Damiani, R. (2012). "Reassessment of the morphology and paleobiology of the therocephalian Tetracynodon darti (Therapsida), and the phylogenetic relationships of Baurioidea". Journal of Vertebrate Paleontology. 32 (5): 1113. doi:10.1080/02724634.2012.688693.
  2. Kammerer, C. (2008). "A new therocephalian from the Cistecephalus Assemblage Zone of South Africa and new information on therocephalian systematics". Journal of Vertebrate Paleontology. 28 (3, suppl.): 98A–99A. doi:10.1080/02724634.2008.10010459.