Redescription and taxonomic reinterpretation of cf. Probainognathus (Cynodontia, Probainognathia) from the Upper Triassic Ischigualasto Formation (San Juan, Argentina)

ELIANA FERNANDEZ; AGUSTIN MARTINELLI; RICARDO MARTINEZ; FERNANDO ABDALA

San Juan

Congreso; IV Congreso Latinoamericano de Paleontología de Vertebrados; 2011

Five non-mammalian cynodonts are known from the Late Triassic Ischigualasto Formation, two traversodontids and three faunivorous probainognathians. Within this latter clade the recognized taxa are: Chiniquodon sanjuanensis Martínez and Forster, Ecteninion lunensis Martínez, May and Forster, and cf. Probainognathus sp. Romer, all sharing sectorial postcanines and a skull average ranging from 30 to 130 mm. The specimen PVSJ 410 consists of a complete skull with articulated lower jaws with a total length of 31 mm and constitutes the only material from Ischigualasto Formation referred to cf. Probainognathus (Bonaparte and Crompton, 1994). It was identified as a juvenile individual of this genus based essentially on the upper postcanine morphology. The upper postcanines of PVSJ 410 are transversely narrow, without cingulum, with a main cusp A, one accessory mesial cusp B, and one to two distal accessory cusps (C and D). The cusps are sharp with well marked carnassial notch. The main cusp is the tallest and located slightly mesial on the crown; the accessory cusps B and C are almost at the same level whereas the cusp D is placed more dorsally. Clearly, this postcanine pattern differs from that of Probainognathus, which has upper postcanines with a prominent central cusp and small accessory mesial and distal cusps that decrease gradually in size. The dentition of PVSJ 410 also differs from that of Ecteninion and chiniquodontids (two to three cusps with the main central cusp strongly curved backwards) and brasilodontids (one vertical large cusp with and anterior and posterior smaller accesory cups). Juvenile features and the presence of some associated derived traits in PVSJ 410 hampered a precise taxonomic identification. However, the postcanine tooth pattern in association with other derived features (e.g. slender dentary and zygoma; absence of contact between the prefrontal and postorbital on the orbital margin; the alveolar level of the incisors positioned at a higher position than the level of the postcanines) permit us to reassess and revise its filogenetic position. In our results, PVSJ410 is a new probainognathian taxon, positioned as more derived than Probainognathus and Prozostrodon Bonaparte and Barberena, and as sister taxon of Therioherpeton cargnini Bonaparte and Barberena plus its inclusive clade (e.g. tritheledontids, tritylodontids, brasilodontids, and Mammaliaformes).