The affinities of the early cynodont reptile, Nanictosaurus

This investigation into the anatomy of the four extant specimens of Nanictosaurus has revealed that there is one valid species, viz. N. kitchingi Broom 1936 which has two junior synonyms, viz. N. robustus Broom 1940 and N. rubidgei Broom 1940. The closest known relative of Nanictosaurus is the well-known cynodont Thrinaxodon liorhinus. The differences from Thrinaxodon and other early cynodonts are discussed and illustrated. INTRODUCTION The cynodonts made their first appearance in the South African fossil record in strata of theAulacephalodonCistecephalus Assemblage Zone (Kitching, 1977; Keyser and Smith, 1978). They became more abundant in the overlyingDicynodon lacerticeps-Whaitsia Assemblage Zone (formerly called the Daptocephalus Zone), where Procynosuchus, Cynosaurus andNanictosaurus are found. At the beginning of the Triassic one cynodont, Thrinaxodon, is sufficiently abundant to be regarded as a zone marker (the zone now called the LystrosaurusThrinaxodon Assemblage Zone), which is a rare distinction for a non-herbivore; it speaks volumes for its adaptation to the environment. Thrinaxodon is represented by a large number of specimens and is well-known. Lately our knowledge of the cranial anatomy of the earlier cynodont Procynosuchus has been substantially improved by the work of Kemp (1979). Nanictosaurus and Cynosaurus, on the other hand, are known only on the basis of a few imperfect skulls and the majority of these have not been adequately described to date. MATERIAL Four specimens have been assigned to the genus Nanictosaurus, and these were referred to three different species: TM 279 Small skull from Wapadsberg, Bethesda Road, Graaff-Reinet district; Dicynodon lacerticepsWhaitsia Assemblage Zone (Late Permian). Holotype of Nanictosaurus kitchingi Broom 1936. RC47. Small skull (the smallest of the four) from Well wood, Graaff-Reinet district; Dicynodon lacerticepsWhaitsia Assemblage Zone. Holotype of Nanictosaurus rubidgei Broom 1940 (p. 190). RC48 Incomplete skull from Hoeksplaas, Murraysburg district: Dicynodon lacerticeps-Whaitsia Assemblage Zone. Ho1otype of Nanictosaurus robustus Broom 1940 (p. 192). RC 133 Fairly small skull from Wellwood, GraaffReinet district: Dicynodon lacerticeps-Whaitsia Assemblage Zone. This specimen was provisionally referred to Nanictosaurus rubidgei by Brink & Kitching (1953) and Van Heerden (1976). RC47 and RC 133 were prepared prior to 1976. RC 133 was prepared further for the present study; it proved to be rather disappointing because the palate was not preserved. RC 48 and TM 279 were also prepared and have thereby yielded very useful information. The snout of TM 279 was weathered and therefore it was decided not to prepare the dorsal surface, but rather the palate and basicranium. All preparation work was carried out by the technical staff of the Palaeontology Department of the National Museum, Bloemfontein. Earlier Work: Broom ( 1936; 1940) did not assign Nanictosaurus to a particular cynodont family. He focussed attention on the long main cusps of the postcanine teeth and also noted a similarity with Thrinaxodon ( 1940: 192). Brink & Kitching (1953) briefly described a second specimen of N. rubidgei (RC 133, notRC 307 as in their paper) and assigned the genus to the Galesauridae. Many authors have followed this classification (cf. Haughton & Brink 1955; Romer 1966; Van Heerden 1976; Brink 1983). Hopson & Kitching (1972) included one species of Nanictosaurus in the Procynosuchidae (N. robustus = Procynosuchus delaharpeae) another in the Galesauridae (N. kitchingi = Cynosaurus suppostus); no mention was made of N. rubidgei but Kitching (pers. comm. 1971) was of the opinion that it is a junior synonym of Thrinaxodon liorhinus. The only detailed description of the Nanictosaurus material was given by Van Heerden (1976) who described