Comments on the Taxonomic Diversity and Paleogeography of the Earliest Known Dinosaur Assemblages (Late Carnian-Earliest Norian)
1. Martín D. Ezcurra(a)
a. GeoBio-Center, Ludwig-Maximilians-Universität München, Richard-Wagner-Str. 10 (D-80333),
The beginning of dinosaur evolution is currently known based on a handful of upper Carnian–lowermost Norian (232–225 Mya) localities situated in a paleolatitudinal belt of approximately 40–50° S in Argentina, Brazil, Zimbabwe and India. The taxonomic diversity of the oldest known dinosaurbearing assemblages, included within the Hyperodapedon Assemblage Zone, is reviewed here. The Brazilian “Teyuwasu barberenarai” is reinterpreted as a nomen dubium representing an indeterminate dinosauriform, the record of cf. Saturnalia from Zimbabwe is considered a basal saurischian and only one of the Indian specimens described by Huene can be unambiguously assigned to Dinosauria. The highest early dinosaur species richness sampled is concentrated in southwestern Pangean assemblages (Argentina and Brazil), with 10 to 11 different described species. By contrast, only one or two species can be currently recognized from the approximately coeval beds of south–central Pangea (Zimbabwe and India), which are much less well sampled. The oldest known dinosaur assemblages appear to have been mostly restricted to subtropical to cool temperate arid areas based on recent paleoclimatological reconstructions. This observation agrees with the hypothesis that the absence of dinosaurs in the upper levels of the Ischigualasto Formation of Argentina is related to an increase in humidity in the basin. Accordingly, climatic factors, with humidity as probably the most important, may have controlled the paleobiogeographic distribution of the oldest known dinosaur assemblages. The achievement of a worldwide dinosaur distribution during the latest Triassic may have occurred after a global climate change, such as the end of the “Carnian Pluvial Event”, and/or the invasion of more tropical humid climates by dinosaurs.