Fernando Abdala

Abstract:  Platycraniellus elegans is an enigmatic Triassic cynodont from South Africa that has only been briefly described previously. New preparation of the holotype and additional unpublished material allows a detailed redescription and comparison with different cynodonts. Platycraniellus elegans is recognized as a valid species of basal cynodont. The distinct suborbital angulation of the zygomatic process, previously considered as a diagnostic character in chiniquodontid cynodonts, and more recently observed in some galesaurids, is also present in P. elegans. A larger, second specimen was initially referred to P. elegans, but most recently considered to belong to Galesaurus planiceps. Close comparison of this specimen with the holotype of P. elegans and with galesaurid specimens allows a tentative allocation to G. planiceps. A cladistic analysis of 32 taxa (two gorgonopsians, seven therocephalians and 23 cynodonts) and 96 craniodental characters places P. elegans as sister taxon of Eucynodontia. Results from the analysis favour a dichotomy between (1) Cynognathia, including the sectorial‐toothed cynodonts Ecteninion, Cynognathus and the gomphodont cynodonts, and (2) Probainognathia, including most sectorial‐toothed eucynodonts (e.g. Lumkuia, Probainognathus, Chiniquodon), tritheledontids, tritylodontids and mammaliaforms. The Late Triassic sectorial‐toothed Ecteninion is the most basal member of Cynognathia, whereas the Middle Triassic Lumkuia is the basal representative of Probainognathia. Tritylodontids (Oligokyphus and Kayentatherium) are placed among Probainognathia, forming a monophyletic group with the tritheledontid Pachygenelus, whereas Brasilitherium is the sister taxon of Mammaliaformes. The cladistic analysis also indicates paraphyly for Therocephalia, with the whaitsiid Theriognathus identified as sister taxon of Cynodontia.

Systematic and morphometric reappraisal of most specimens previously assigned to the cynodont clade Chiniquodontidae reveal that the group is diagnosed by two autapomorphies: (1) the distinctive angulation between the posterior portion of the maxillary and the anterior portion of the zygomatic arch and (2) very extended pterygoid flanges, ending in a thin projection; and a combination of features including: posterior postcanines sectorial with principal cusps backwardly recurved, and a long osseous palate. The species Chiniquodon theotonicus (including Belesodon magnificus, Probelesodon kitchingi, P. lewisi and P. minor) and C. sanjuanensis (comb. nov.) were the only recognised members of the family on the basis of qualitative characters. The South American species Cromptodon mamiferoides, Probainognathus jenseni and Thrinaxodon brasiliensis are thus excluded from the group, as are the African taxa Aleodon brachyrhamphus and Cistecynodon parvus. Allometric analysis of chiniquodontids sensu stricto reveals that: (1) all specimens can be confidently arranged in a single growth series, irrespective of their original species assignments; and (2) most of the measurements change isometrically with respect to skull length. Adults are virtually scaled–up juveniles, showing a deepened zygoma, a longer osseous palate, and a wider anterior muzzle in the canine region. Thus, allometric data reinforce the proposed synonymy of all forms, with the specific status of C. sanjuanensis supported by qualitative diagnostic traits.