The oldest known co-occurrence of dinosaurs and their closest relatives: A new lagerpetid from a Carnian (Upper Triassic) bed of Brazil with implications for dinosauromorph biostratigraphy, early diversification and biogeography

摘要

Late Triassic deposits from south Pangea (Argentina and Brazil) bear the oldest (so far) and most informative unequivocal records of dinosaurs worldwide. Herein we describe a new lagerpetid dinosauromorph from a classic dinosaur-bearing locality (Cerro da Alemoa site) from the Candelaria Sequence (Santa Maria Formation), southern Brazil, and report the oldest co-occurrence of dinosaurs, lagerpetids and silesaurids, in the same layer of a Triassic outcrop. Although tentatively regarded as a skeletally immature specimen, the new lagerpetid represents a new morphotype due to the occurrence of the following features: (1) distal condyles transversely wider than deep; (2) absence of flange in the craniomedial margin of the femur; (3) round shaped and medially deflected medial condyle; (4) round shaped crista tibiofibularis. This suit of traits differentiates the new specimen from Ixalerpeton polesinensis, (which was so far the only lagerpetid found in Brazilian strata) and from other known lagerpetids. As non-dinosaurian dinosauromorphs are still relatively scarce in Brazilian Triassic rocks, recent records from this region are gradually providing new data regarding faunal turnovers and biostratigraphy of south Pangean terrestrial deposits. Additionally, new insights on the oldest dinosauromorph-bearing sites throughout the Triassic, as well as regarding taxonomic diversity and geographic distribution of early dinosauromorphs reinforces previous hypotheses where dinosaurs and dinosaur-relatives consistently coexisted (for at least 21 Ma) before their extinction.