The oldest ictidosaur cynodont (Therapsida) from the late Carnian of southern Brazil and its implication in probainognathian evolution

AGUSTIN MARTINELLI; MARINA BENTO SOARES; RODRIGUES PABLO; CESAR SCHULTZ

Simposio; X Simpósio Brasileiro de Paleontologia de Vertebrados; 2016

Ictidosauria, defined in 2007 as the clade including the most recent common ancestor of Riograndia guaibensis and Pachygenelus monus, and all its descendants, is a diverse group of probainognathian cynodonts that was traditionally nested as the sister group of Mammaliaformes, disputing its position with tritylodontids. After the discovery of Brasilodontidae, Ictidosauria was positioned in a more basal position within the Prozostrodontia clade. Up to now, the record of ictidosaurs spans from the early Norian (Late Triassic) of the southern Brazil and western Argentina to the Hettangian (Early Jurassic) of South Africa and North America. Seven species are hitherto known including the basal Riograndia guaibaensis plus the tritheledontids Irajatherium hernandezi, Chaliminia musteloides, Elliotherium kersteni, Pachygenelus monus, Diarthrognathus broomi, and Tritheledon riconoi. In this contribution we present the oldest ictidosaur from the Late Carnian of southern Brazil, coming from the Hyperodapedon Assemblage Zone (AZ, Candelária Sequence, Santa Maria Supersequence, Paraná Basin) of the Janner site (Agudo Municipality, RS, Brazil). In this site there is a predominance of Exaeretodon (Traversodontidae) over Hyperodapedon (Rhynchosauria) specimens as occur in the Exaeretodon Biozone of the upper half of the Ischigualasto Formation in Argentina. The new specimens include two partial skulls with jaws. The postcanine dentition is conspicuously similar to that of Riograndia guaibensis, from the younger Riograndia AZ. It consists of leaf-shape, sectorial postcanines with up to 10 small cusps distributed in the mesial (4 or 5) and distal (4 or 5) edges. Both taxa also share the lack of postorbital bar, a short and wide snout, and interpterygoid vacuities. Differing from Riograndia, the new taxon possesses a larger upper/lower canine, the postcanines have faint mesio-lingual and disto-lingual cingula, with small, discrete cusps, and its absolute size is about 3 times larger. Evidence on incisor morphology is still unavailable. Its inclusion in a phylogenetic analysis, positioned the new taxon as an ictidosaur, sister-taxon of Riograndia, both as sister taxon of Tritheledontidae. This topology reinforces the original proposal that considers Riograndia into its own Family Riograndidae. The relative large size of the new taxon at the base of the ictidosaur clade and within small sized prozostrodontians opens new questions into the complex and still poorly explored macroevolutionary aspects prior to the mammaliaform condition.