Theriognathus

Theriognathus (from Greek therion: beast, mammal, Greek, gnathos, “jaw,” +us, pronounced THEH-ree-OG-nah-thuss) is an extinct genus of therocephalian therapsid belonging to the family Whaitsiidae, from South Africa, Tanzania, and Russia. 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.[1] It is speculated that Theriognathus was either carnivorous or omnivorous based on its teeth, and was suited to hunting small prey in undergrowth. This mammal-like reptile 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.

Geography
Theriognathus is represented by abundant occurrences in the Karoo Basin of South Africa, sequestered records in the Upper Permian Ruhuhu Basin of Tanzania, Zambia, and Russia.[3] Late Permian therapsids are best known from the Karoo Basin, which covers a massive area and provides Permian sediments that total 12 km in thickness. During the Late Permian, sediments were fed into the Karoo Basin from a ring of mountains that encircled Southern Gondwana (an ancient supercontinent, partly located on what is now South America and Antarctica) In the Karoo is the Beaufort Group, a subdivision of the basin. This division is further broken down into assemblage zones, intervals of geological strata that are defined based on the distributions of previously found tetrapod taxa.[4]

Theriognathus has been found in the Wuchiapingian, upper Tropidostoma, Cistecephalus, and lower Dicynodon assemblage zones in the Upper Permian, Karoo Basin, South Africa; Wuchiapingian, Cistecephalus assemblage zones in the Upper Permian, Ruhuhu Basin, Tanzania; Wuchiapingian Cistecephalus assemblage zones in the Upper Permian, Luangwa Basin, Zambia; the Wuchiapingian, Upper Permian, Kotelnich, Kirov region, Russia;, the Changxingian Archosaurus assemblage zone, Upper Permian, Vladimir region, Russia.[5]

Theriognathus was also found in the Tapinocephalus assemblage zone, which was an odd find since most specimen in that zone tend to be medium to large in size. Theriognathus was an exception to that norm. Diversification was shown by an increase in richness of eutherocephalian groups in the Cistecephalus and Dicynodon assemblage zones. During this time some of the more specialized genera, including Theriognathus, split from pre-existing families.

History of discovery
Theriognathus was first described by Sir Richard Owen in 1876 based on specimens that were discovered and donated by A.G. Bain from South Africa. The specimens (consisting of therocephalians, including Theriognathus) were described and illustrated in “Catalogue of the Fossil Reptiles of South Africa in the Collections of the British Museum,” by Owen. Theriognathus was considered a dicynodont at that time.[6] In 1950, Von Huen described two Theriognathus skulls (whatsiids Notosollasia and Notaelurops, now synonymous with Theriognathus), from the Upper Permian Ruhuhu Basin of Tanzania. In 1980, most whaisiid genera were combined with Theriognathus, creating two species. Theriognathus major was the new species name that included Whaitsia major, and Theriognathus laticeps was the new species that included Notosollasia laticeps. Whaitsia, Alopecopsis, Notosallasia, Hyenosaurus, Moshorhynchus, Notaelurops, and Aneugomphius have been synonymized with Theriognathus. These genera were linked due to lack of postcanine teeth.[1]

Two Russian specimens that were previously placed in Moschowhaitsiidae are Viatkosuchus from the Wuchiapinigan Kotelnich locality, and Mochowhaitsia from the Changxingian Vyazniki-1 locality in Archosaurus assemblage zone, were placed in Theriognathus in 1995 by Tararinov.[5] Results from the analysis of a basal cynodont, Charassognathus gacilis, from the Late Permian Tropidostoma Assemblage Zone, South Africa, supported an original hypothesis of T.S. Kemp’s concept of Therocephalia being paraphyletic, placing Cynodontia as the sister taxon to Theriognathus.[7] This was significant in the relationship of Theriognathus and cynodonts, as well as being the first computer assisted cladistics analysis to support the possible paraphyletic relationship of Therocephalia. A few authors have supported that Theriognathus and cynodonts have a sister group relationship, based on the existence of an enlarged epiterygoid and a quadrate notch in the squamosal.