Search

The ascent of dinosaurs in the Triassic is an exemplary evolutionary radiation, but the earliest phase of dinosaur history remains poorly understood. Body fossils of close dinosaur relatives are rare, but indicate that the dinosaur stem lineage (Dinosauromorpha) originated by the latest Anisian (ca 242–244 Ma). Here, we report footprints from the Early–Middle Triassic of Poland, stratigraphically well constrained and identified using a conservative synapomorphy-based approach, which shifts the origin of the dinosaur stem lineage back to the Early Olenekian (ca 249–251 Ma), approximately 5–9 Myr earlier than indicated by body fossils, earlier than demonstrated by previous footprint records, and just a few million years after the Permian/Triassic mass extinction (252.3 Ma). Dinosauromorph tracks are rare in all Polish assemblages, suggesting that these animals were minor faunal components. The oldest tracks are quadrupedal, a morphology uncommon among the earliest dinosauromorph body fossils, but bipedality and moderately large body size had arisen by the Early Anisian (ca 246 Ma). Integrating trace fossils and body fossils demonstrates that the rise of dinosaurs was a drawn-out affair, perhaps initiated during recovery from the Permo-Triassic extinction. [ABSTRACT FROM AUTHOR]

SYNOPSISAllosauroidea, a clade of large-bodied theropod dinosaurs that ranged from the Middle Jurassic until the Late Cretaceous, has been the subject of extensive phylogenetic study. However, despite the publication of 12 cladistic analyses little phylogenetic consensus has emerged, frustrating attempts to use these dinosaurs to study character evolution, biogeography and the quality of the fossil record. Here we analyse a core subset of allosauroids using cladistic methodology and several comparative methods. After reviewing previous studies and their points of disagreement, we present a new cladistic analysis that integrates data from these studies with new characters. This analysis finds strong support for placing Sinraptoras a basal allosauroid, Neovenatoras a basal member of Carcharodontosauridae and Acrocanthosaurusas a more derived member of Carcharodontosauridae, rather than the sister taxon of Allosaurusas sometimes advocated. The current dataset is compared in detail to those of previous studies to determine the degree of overlap and the basis for differing topologies. These comparisons show that scoring differences, character choice and taxonomic sampling all play a major role in generating incongruence. Finally, the recovered most parsimonious topology is used to assess character evolution, stratigraphical congruence and biogeography. This topology shows a strong overall match with the stratigraphical record and is much more congruent with stratigraphy than any alternative topology. The biogeographical history of core allosauroids is found to be congruent with the breakup sequence of Pangaea, but more complete assessment is hampered by sampling biases. [ABSTRACT FROM AUTHOR]