Middle Triassic Continental Faunas From Gondwana: The Chañares Formation Tetrapod Assemblage, A Case Study From Western Argentina.

MANCUSO, ADRIANA CECILIA; GAETANO, LEANDRO; LEARDI, JUAN MARTÍN; ABDALA, FERNANDO; ARCUCCI, ANDREA

Los Angeles, California

Congreso; Annual Meeting of Society of Vertebrate Paleontolgy; 2013

Ladinian tetrapod faunas are poorly represented worldwide and only a few assemblages allow the analysis of these early Mesozoic communities. The Chañares Formation in western Argentina is unique in its diverse and well-preserved non-marine tetrapod assemblage. The taphonomical analysis of the Chañares fossiliferous levels, under precise stratigraphical and paleoenvironmental control, reveals that a good representation of the original tetrapod composition is present in the recovered assemblage. Hence, it is possible to hypothesize on the paleoecological significance of the assemblage and the interactions among its different components. Mass estimations and morphology-based paleobiological inferences allowed us to propose the putative trophic structure of this community. The tetrapod record includes a high diversity of archosauriforms, with at least twelve taxa, but only known by few specimens (17.3% of the specimens collected). Synapsids are very abundant (82.7% of the specimens collected) but only represented by four genera. The Chañares tetrapod fauna was numerically dominated by middle-sized herbivorous (i.e., Massetognatus pascuali) and small faunivorous cynodonts (i.e., Probainognathus jenseni). Large herbivorous dicynodonts and middle-sized faunivorous cynodonts were also present but only constitute ~15% of the specimens found. Non-archosaurian archosauriforms and ornithodirans were mainly represented by small to middle-sized slender forms (~16% of the specimens collected). The paracrocodylomorphs (e.g., Luperosuchus) are identified as the top-predators of the Chañares community with estimated body masses between 350 and 500 kg. The traversodontid cynodont M. pascuali and the dicynodont Dinodontosaurus, with body masses that reached approximately 43 and 360 kg, respectively, are the only taxa recognized as exclusively herbivorous. Compared with the Chañares tetrapod assemblage, the Dinodontosaurus Assemblage Zone from southern Brazil shows remarkably low archosauriform diversity and the large dicynodont Dinodontosaurus dominates the assemblage numerically. The putative Ladinian Upper Omingonde Formation assemblage of Namibia shows clear dominance of traversodontid cynodonts, as occurs in the Chañares assemblage, and low representation of the other forms. In contrast, non-marine tetrapod assemblages from Laurasia of equivalent age are dominated by temnospondyl amphibians not known from the Chañares levels in spite of the fluvial/lacustrine nature of its deposits.