Massetognathus pascuali and Santacruzodon hopsoni (Cynodontia, Traversodontidae) from the late Middle-early Late Triassic of South America: new insights on their premaxillary dentition

AGUSTÍN MARTINELLI; MARTÍN EZCURRA; TOMÁS MELO; LEANDRO GAETANO; MARINA B. SOARES; LUCAS E. FIORELLI; JULIA DESOJO

Vitoria

Simposio; IX Simpósio Brasileiro de Paleontologia de Vertebrados; 2014

Sociedade Brasileira de Paleontologia

Massetognathus pascuali and Santacruzodon hopsoni are the commonest recovered vertebrates from the late Ladinian-early Carnian Chañares Formation (Villa Unión-Ischigualasto Basin, Argentina) and early Carnian Santacruzodon Assemblage Zone of the Santa Maria Sequence 1 (Paraná Basin, Brazil), respectively. Both species are based on well-preserved specimens; nonetheless, some details of their dentition are still poorly documented, such as features regarding incisors morphology. The discovery of new specimens (CRILAR-PV-192, 193, and 194;UFRGS-PV-576) of these species allows us the observation of the incisor morphology and the replacement process. M. pascuali and S. hopsoni have four upper and three lower incisors. They are strongly labio-lingually compressed and leaf-shaped, with denticulate mesial and distal margins. Adjacent to the main cusp, there are up to five cuspules mesially and up to six distally. Upper incisors are close, but not in contact with each other, forming a semi-circular cutting edge. In M. pascuali, upper and lower incisors are similar in size, whereas in S. hopsoni lower incisors are larger and more procumbent than the upper ones. In both taxa, leaf-shaped incisors are replaced by the same kind of teeth but particularly in S. hopsoni, the new incisors have larger, more discrete accessory cuspules. This may suggest some changes in the food reduction strategy during ontogeny. Incisor morphology is rather variable in traversodontids. Basal forms, such as Andescynodon and Pascualgnathus, have simple chisel-like incisors, usually subequal in size. Generally, large-sized forms, such as Exaeretodon and Scalenodontoides, have procumbent and large incisors, with only an enamel layer on the labial surface. The incisor morphology of M. pascuali and S. hopsoni is one of the most bizarre among traversodontids, also shared by Arctotraversodon pelmmyridon from the Carnian of Nova Scotia. Dadadon isaloi (Carnian, Madagascar) has an accessory distal cusp on the I4 and the lower incisors have small accessory distal cusps, but they are rather different from the leaf-shaped condition of the aforementioned traversodontids. The detailed analysis of the incisor morphology among traversodontids will permit the character-states refinement, given that the latest phylogenetic analyses only consider three characters dealing with incisor anatomy (procumbency, cutting margin, and size). Finally, it is worth mentioning that denticulated leaf-shaped incisors were acquired convergently in some archosauromorph lineages (e.g., Azendohsaurus, Revueltosaurus, basal ornithischians, and sauropodomorphs) during the Triassic, suggesting a partially similar ecological strategy among them. Nonetheless, the basic functional difference is that in traversodontids this morphology is limited to anterior teeth (incisors) whereas posteriorly the food was processed by gomphodont postcanines.