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 study concluded that Tetraceratops is not a true therapsid, but should be considered 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.
Moschorhinus is an extinct genus of therocephalian in the family Akidnognathidae, with only one species: M. kitchingi. It was a carnivorous, lion-sized synapsid which has been found in the Late Permian to Early Triassic of the South African Karoo Supergroup. 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.
Pristerognathus is an extinct genus of therocephalian, known from the late Middle Permian (Capitanian) of South Africa. It lends its name to the Pristerognathus Assemblage Zone of the Beaufort Group of South African geological strata. Pristerognathus was a medium-sized therocephalian with a 25 cm (9.8 in) skull and a total length up to 1.5 m.
Theriognathus is an extinct genus of therocephalian therapsid belonging to the family Whaitsiidae, known from fossils from South Africa, Zambia, 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.
Ictidosuchoides is an extinct genus of ictidosuchid therocephalians. Fossils have been found from the Karoo Basin in South Africa. The genus is known to have been one of the few therocephalians to have survived the Permian-Triassic extinction event in this area, although its numbers were quite low after the extinction.
Chlynovia is an extinct genus of therocephalian therapsids from the Late Permian of Russia. The type species is C. serridentatus, named in 2000. Chlynovia was originally classified within Scaloposauria, a group of therocephalians characterized by their small size and lightly built skulls. Scaloposaurians are no longer recognized as a true grouping, but instead represent the juvenile forms of many types of therocephalians. Chlynovia was placed in the family Perplexisauridae along with Perplexisaurus, but both therocephalians are now placed in the family Ictidosuchidae.
Lycosuchidae is an extinct family of therocephalian therapsids from the Middle Permian Beaufort Group of South Africa. It currently contains two monotypic genera, Lycosuchus, represented by L. vanderrieti, which was named by paleontologist Robert Broom in 1903, and Simorhinella, represented by S. baini, which was named by Broom in 1915. Both species are characterized by their large body sizes and short, deep snouts. Two sets of enlarged canine teeth were once regarded as a defining feature of lycosuchids, but recent studies have proposed that the front-most pair are replacements for the ones behind them, which would have eventually fallen out as individuals aged. Lycosuchids are the earliest known therocephalians and are also thought to be the most basal.
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.
Choerosaurus is an extinct genus of therocephalian therapsids from the Late Permian of South Africa. The type species Choerosaurus dejageri was named by South African paleontologist Sidney H. Haughton from the Tropidostoma Assemblage Zone in 1929.
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.
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.
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.
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.
Pelictosuchus is an extinct genus of therocephalian therapsids from the Late Permian of South Africa. It is classified in the family Akidnognathidae. The type species Pelictosuchus paucidens was named by South African paleontologist Robert Broom in 1940 from the Dicynodon Assemblage Zone.
Trochosuchidae is an extinct family of therocephalian therapsids from the Permian period. It includes the genera Trochosaurus, Trochosuchus, Trochorhinus, and Hyaenasuchus. Trochosuchids are known only from the Tapinocephalus Assemblage Zone of the Beaufort Group in South Africa. Trochosuchids are basal therocephalians and are similar in appearance to another primitive group of therocephalians, the Scylacosauridae. Compared to scylacosuchids, trochosuchids have broader snouts and more flattened skulls lacking prominent sagittal crests. They have two enlarged canines on either side of the upper jaw, similar to lycosuchid therocephalians but unlike scylacosuchids, which only have one pair of canines.
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.
Phthinosuchia is an extinct group of therapsids including two poorly known species, Phthinosuchus discors and Phthinosaurus borrisiaki, from the Middle Permian of Russia. Phthinthosuchus is known a partial crushed skull and Phthinosaurus is known from an isolated lower jaw. The two species have traditionally been grouped together based on their shared primitive characteristics, but more recent studies have proposed that they are more distantly related. Phthinosuchus is either a carnivorous gorgonopsian relative or an anteosaurian dinocephalian while Phthinosaurus is either a herbivorous rhopalodont dinocephalian or a therocephalian.
