New skull remains of the cynodont Irajatherium hernandezi Martinelli et al. 2005 (Probainognathia: Trithelodontidae) from the Late Triassic of southern Brazil.

KERBER, LEONARDO; FLAVIO PRETTO; MÜLLER, RODRIGO T.; MARTINELLI, AGUSTÍN G.

Porto Alegre

Simposio; PaleoRS; 2020

UFRGS

A new specimen of the non-mammaliaform cynodont Irajatherium hernandezi (Ictidosauria: Trithelodontidae) from the Late Triassic of southern Brazil is reported. The specimen CAPPA/UFSM 0275 was collected at the Linha São Luiz site (29º33?45??S; 53º26?48??W), which is part of the Caturrita Formation, upper portion of the Candelária Sequence, Riograndia Assemblage Zone. It comprises a partial skull preserving the left canine, two left postcanines, and two right postcanines; left dentary with canine and postcanines and a fragment of the right dentary. The specimen furnishes new information on the skull anatomy of this taxon, indicating the presence of a broad and short rostrum as long as the temporal region. The zygomatic arches are dorsoventrally low, and its widest point is slightly posterior to the mid-length of the orbito-temporal fenestra. This fenestra is large and ovoid, lacking a postorbital bar. Although damaged, the specimen indicates that the palate of I. hernandezi was long, slightly surpassing the level of the last postcanine. A digital endocast demonstrates that I. hernandezi shows both cerebral hemispheres divided by a median sulcus, similar to other probainognathians. A cladistic analysis including the new data supports previous interpretations where I. hernandezi is the basalmost Trithelodontidae, and also the oldest one. Finally, a morphological disparity analysis employing dental traits demonstrated a wide occupation of the morphospace by the probainognathian cynodonts from the Riograndia Assemblage Zone and a low competitive scenario for these prozostrodonts.