Vetusodon

Vetusodon is known from four specimens, all of which have been collected in South Africa from rocks belonging to the Daptocephalus Assemblage Zone. The holotype, BP/1/7971, consists of a well-preserved skull missing the lower jaw. It was discovered in 2010 in a mudstone bed in the province of KwaZulu-Natal by a team led by the palaeontologist Bruce S. Rubidge. The referred specimen CGP GHG141 was found in 1985 by Gideon Groenewald in the Thaba Nchu Mountain of the Free State province, and like the holotype, it consists of a partial skull missing the lower jaw. The specimen SAM-PK-K10596 is the least complete of the specimens, consisting only of a snout. It was found in 1958 or 1959 by the South African palaeontologist Alfred W. Crompton. The specimen SAM-PK-K10702 is the most complete of the four specimens, and comprises a complete skull with an intact lower jaw. It was discovered in 2009 or 2010 by Derik Wolvaardt at the Ripplemead Farm, close to the town of Nieu-Bethesda, Eastern Cape province.[1]

Vetusodon elikhulu was first described in 2019 by the palaeontologists Fernando Abdala, Leandro C. Gaetano, Roger M. H. Smith and Bruce S. Rubidge. Its generic name is derived from the Latin word vetus, meaning "old", and the Greek word οδοντος (odontos), meaning "tooth". The specific name elikhulu means "large" in the Zulu language, and alludes to the animal's size.[1]

Vetusodon is the largest known Permian cynodont, based on its skull length of around 18 centimetres (7.1 in). The temporal region (area behind the eye sockets) made up around 60 percent of the skull length. The jaws were quite robust, and the snout was unusually broad compared to other cynodonts of its time. The lower jaw was formed primarily by the dentary bones, whereas the other (postdentary) bones were reduced in size to an extent not otherwise seen in cynodonts this primitive. Vetusodon had a tall symphysis (joint) between the two halves of the lower jaw. Abdala et al. (2019) interpreted the symphysis as being fused, but a 2020 study by Huttenlocker and Sidor reinterpreted it as unfused.[2][3] The back portion of the dentary had a projection called the coronoid process, which was quite tall in Vetusodon. The secondary palate (the structure making up the roof of the mouth in most cynodonts) was formed by the premaxillary, maxillary and palatine bones. In Vetusodon, the two halves of the secondary palate were not joined together in the middle, rendering the vomer prominently visible from below. Depending on the phylogenetic position of Vetusodon, this incomplete secondary palate may either be an autapomorphy (derived trait) of this taxon, or a plesiomorphy (ancestral trait) shared with other primitive cynodonts.[1][3]

The teeth were conical, single-rooted and had no serrations. Vetusodon possessed four pairs of upper incisors, which were long and recurved. There was a small gap (diastema) between the last incisor and the canine. The upper canines were large and had a round, somewhat elongated cross-section. Behind the canines were a set of postcanine teeth. The upper postcanines varied in number between specimens, ranging from 7 pairs in the holotype, to 11 in the specimen SAM-PK-K10596. Unlike most cynodonts, which have postcanines with multiple cusps, the postcanines of Vetusodon only bore one cusp each.[1]

When Vetusodon was first described in 2019, it was included in a phylogenetic analysis to test its relationships to other cynodonts. It was found to be a derived member of the clade Epicynodontia, being more closely related to Eucynodontia than several Triassic genera such as Thrinaxodon. As Thrinaxodon and its relatives had relatively well-developed secondary palates, this would imply that the incomplete secondary palate of Vetusodon is secondarily reduced from a more complete one.[1][3] However, a 2020 study by Huttenlocker and Sidor,[2] followed by a 2021 study by Pusch and colleagues, recovered Vetusodon in a more early-diverging position within Epicynodontia, as the sister taxon of the Permian genus Cynosaurus. If this placement is correct, the incomplete secondary palate of Vetusodon may be an ancestral trait inherited from earlier cynodonts.[3]

Below are two cladograms from Abdala et al. (2019) and Pusch et al. (2021), which illustrate the varying phylogenetic placements of Vetusodon:

Vetusodon belongs to the Daptocephalus Assemblage Zone, a biozone that is part of the Beaufort Group, which itself is part of the larger Karoo Supergroup. Vetusodon belongs to a subzone of the Daptocephalus AZ known as the Lystrosaurus maccaigi-Moschorhinus Subzone.[4]

In addition to Vetusodon, several other synapsids are known from this subzone. These include the fellow cynodonts Cynosaurus and Nanictosaurus, the biarmosuchian Ictidorhinus, the dicynodonts Aulacephalodon, Daptocephalus, Dicynodon, Dicynodontoides, Diictodon, Dinanomodon, Emydops, Emydorhinus, Kwazulusaurus, Lystrosaurus, Oudenodon, Pelanomodon and Thliptosaurus, the gorgonopsians Arctognathus, Cyonosaurus, Lycaenops and Rubidgea and the therocephalians Ictidosuchoides, Lycideops, Moschorhinus, Nanictidops, Promoschorhynchus, Polycynodon and Scaloporhinus. Other animals include the bivalve Palaeanodonta, the ray-finned fish Atherstonia and Namaichthys, the temnospondyls Rhinesuchus and Uranocentrodon and the reptiles Owenetta, Pareiasaurus and Spondylolestes.[4]