GEOCHRONOLOGICAL PERSPECTIVES ON THE TRIASSIC RISE OF DINOSAURS

The early dinosaur evolutionary history is obscured due to a fragmentary fossil record, taxonomic uncertainties and speculative correlations among widely dispersed fossil-bearing successions. The Triassic early dinosaurs and their dinosauromorph precursors were comparatively low in abundance (and diversity) and were only minor components of tetrapod paleocommunities. Complete, articulated fossils are rare, complicating phylogenetic relationships among Triassic dinosauromorphs. In addition, the paucity of reliable radioisotopic geochronology has historically plagued unambiguous correlation of terrestrial rock formations and their faunas independent of controversial lithostratigraphy or vertebrate biostratigraphy.

New high-fidelity U-Pb geochronologic data (CA-ID-TIMS method) from Triassic vertebrate-bearing continental successions of western Pangea have begun to shed new light on the mode and tempo of early dinosaur radiation. In South America, despite the near-complete stratigraphic separation of non-dinosaurian dinosauromorph record and that of the earliest dinosaurs, their first appearances are now shown to be much closer in time than expected. The earliest dinosaur record of the Colorado Plateau (Southwest US) now appears to be truncated at its base by a ca. 16 Myr depositional and/or fossil preservation gap, whereas the overlying strata of the Chile Formation mark the sustained coexistence of dinosaur precursors, early saurischian dinosaurs and neotheropods. Accumulating geochronological data now demonstrate that the supposed age disparities between geographically disparate Triassic dinosaur-bearing units are largely artifacts of incomplete preservation coupled with erroneous correlations.

New Triassic chronostratigraphic frameworks based on high-precision geochronology do not support models that depict ‘diachronous’ evolutionary radiation of early dinosaurs, despite persistence in the literature. The same applies to paleoenvironmental phenomena such as geomorphic barriers or ecological instability invented as explanations for the presumed delayed latitudinal radiation of dinosaurs. There is no evidence yet that the first appearance of evolutionary advanced (neotheropod) dinosaurs in South America predated those elsewhere in the Triassic.