Therapsida is a major group of eupelycosaurian synapsids that includes mammals and their ancestors. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented more underneath the body, as opposed to the sprawling posture of many reptiles and salamanders. The earliest fossil attributed to Therapsida used to be Tetraceratops insignis from the Lower Permian. However in 2020, a new study has found that Tetraceratops is not actually a true Therapsid, but should be considered to be a member of the more ancient Sphenacodontia from which the therapsids evolved.
Therocephalia is an extinct suborder 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. The phylogeny of therocephalians has been disputed, as the monophyly of the group and the relationships of its members are unclear.
Theriognathus is an extinct genus of therocephalian therapsid belonging to the family Whaitsiidae, from South Africa and Tanzania. Theriognathus has been dated as existing during the Late Permian. Although Theriognathus means mammal jaw, the lower jaw is actually made up of several bones as seen in modern reptiles, in contrast to mammals. Theriognathus displayed many different reptilian and mammalian characteristics. For example, Theriognathus had canine teeth like mammals, and a secondary palate, multiple bones in the mandible, and a typical reptilian jaw joint, all characteristics of reptiles. It is speculated that Theriognathus was either carnivorous or omnivorous based on its teeth, and was suited to hunting small prey in undergrowth. This synapsid adopted a sleek profile of a mammalian predator, with a narrow snout and around 1 meter long. Theriognathus is represented by 56 specimens in the fossil record.
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 it is to reptiles, its braincase is very primitive and shares more in common with modern amphibians and reptiles than it does with mammals.
Akidnognathidae is an extinct family of therocephalian therapsids from the Late Permian and Early Triassic of South Africa and Russia. 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.
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 Bauriodea.
Nanictidopidae is an extinct family of therocephalian therapsids from the Late Permian. Two genera are currently included in the family, Nanictidops from South Africa and Purlovia from Russia. Nanictidopids have short skulls and were probably herbivorous.
Blattoidealestes is an extinct genus of therocephalian therapsid from the Middle Permian of South Africa. The type species Blattoidealestes gracilis was named by South African paleontologist Lieuwe Dirk Boonstra from the Tapinocephalus Assemblage Zone in 1954. Dating back to the Middle Permian, Blattoidealestes is one of the oldest therocephalians. It is similar in appearance to the small therocephalian Perplexisaurus from Russia, and may be closely related.
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.
Ictidodon is an extinct genus of therocephalian therapsids from the Late Permian of South Africa. The type species Ictidodon agilis was named by South African paleontologist Robert Broom in 1925. Broom classified Ictidodon in the Scaloposauridae, a group of small-bodied therocephalians that are now thought to be juvenile forms of larger therocephalians. Ictidodon and many other scaloposaurids are now classified as basal members of the clade Baurioidea.
Silpholestes is an extinct genus of therocephalian therapsids from the Late Permian of South Africa. The type species Silpholestes jackae was named by South African paleontologist Robert Broom in 1948 from the Cistecephalus Assemblage Zone.
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.
Scalopocephalus is an extinct genus of therocephalian therapsids from the Late Permian of South Africa. German paleontologist Friedrich von Huene discovered the holotype skull of Scalopocephalus from the Cistecephalus Assemblage Zone in 1924 and named the type species Scalopocephalus watsonianus in 1937. Scalopocephalus is similar in appearance to Scaloposaurus and was first classified in the family Scaloposauridae. Scaloposauridae is no longer recognized as a valid group, and Scalopocephalus is now classified as a basal member of Baurioidea.
Scalopodon is an extinct genus of therocephalian therapsids from the Late Permian of Russia. The type species Scalopodon tenuisfrons was named in 1999 from the Kotelnichsky District of Kirov Oblast. Scalopodon is known from a single fragmentary holotype specimen including the back of the skull, the left side of the lower jaw and isolated postorbital and prefrontal bones. The skull was found in the Deltavjatia Assemblage Zone, which dates back to the early Wuchiapingian about 260 million years ago. Distinguishing features of Scalopodon include narrow frontal bones and a distinctive sagittal crest along the parietal region at the back of the skull. Scalopodon was originally classified in the family Scaloposauridae, and was the first scaloposaurid found in Russia. More recent studies of therocephalians have found scaloposaurids like Scalopodon to be juvenile forms of larger therocephalians and do not consider Scaloposauridae to be a valid group. Scalopodon and most other scaloposaurids are now classified as basal members of Baurioidea.
Macroscelesaurus is an extinct genus of therocephalian therapsid from the Late Permian of South Africa. The type species Macroscelesaurus janseni was named by Sidney H. Haughton in 1918 from the Cistecephalus Assemblage Zone. It is one of the few therocephalians known from postcranial remains.
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.
Yikezhaogia is an extinct genus of therocephalian therapsids from the Early Triassic of Mongolia. It is known from a single fragmentary skull and associated postcranial bones representing the species Yikezhaogia megafenestrala. It is identifiable as a therocephalian by its thin postorbital bar behind the eye socket, its elongated temporal opening behind the bar, and a thin lower jaw with a low coronoid process. Large tooth sockets in the upper jaw indicate that Yikezhaogia had large caniniform teeth. The teeth of the lower jaw are blunt-tipped and cylindrical. Although its exact position among therocephalians is uncertain, Yikezhaogia is probably a basal member of the group Baurioidea.
Simorhinella is an extinct genus of therocephalian therapsids from the Late Permian of South Africa. It is known from a single species, Simorhinella baini, named by South African paleontologist Robert Broom in 1915. Broom named it on the basis of a single fossil collected by the British Museum of Natural History in 1878 that included the skull and jaws forward from the eye sockets. The skull is unusual in that it has an extremely short and deep snout, unlike the longer and lower snouts of most other therocephalians. Because of the skull's distinctiveness, the classification of Simorhinella within Therocephalia is uncertain. However, a 2014 study proposed that it was closely related to the basal therocephalian Lycosuchus, placing it in the family Lycosuchidae.
