The evolutionary radiation of Triassic dinosauriforms

NOVAS, F. E.; EZCURRA, M. D.

Jornada; VI Simpósio Brasileiro de Paleontologia de Vertebrados; 2008

Until recently, sister taxa of Dinosauria were restricted to Ladinian forms (e.g.,Lagerpeton, Marasuchus, Pseudolagosuchus), thus evolution of Dinosauriformesdepicted a step-wise pattern in which the dinosaurian diversification occurred after theappearance of non-dinosaurian dinosauriforms, without recording a temporaloverlapping between them. However, recent discoveries and re-study of previouslyknown dinosauriforms allow to change the understanding of the early evolution ofDinosauriformes. Discovery of the gracile archosaur Silesaurus opolensis (LateCarnian, Poland) demonstrates that non-dinosaurian Dinosauriformes survived intothe Late Triassic, being co-eval with the oldest known dinosaurs (e.g., herrerasaurids,Eoraptor, Saturnalia). Moreover, Silesaurus exhibits notable cranial and dentalapomorphies (e.g., leaf-shaped teeth, beaked-jaw), suggesting that an important,hidden morphological disparity among basal dinosauriforms remains to bediscovered. In the same line of evidence is the systematic reassessment ofEucoelophysis baldwini (Early Norian, United States), originally thought as acoelophysoid theropod. Contrarily to this interpretation, Eucoelophysis lacksneotheropod and dinosaurian features (e.g., femoral head strongly inturned, ligamentsulcus on femoral head, fibular crest on proximal tibia) indicating that Eucoelophysisis not a theropod dinosaur but a basal dinosauriform. This means that nondinosauriandinosauriforms survived into the Norian. In this regard, the currently available fossil record shows that non-dinosaurian dinosauriforms and herrerasaurids(e.g., Chindesaurus) survived into the early Norian, thus tends to blur the impact ofthe Carnian-Norian extinction event, as far as for dinosauriforms is regarded. In sum,a better understanding of early dinosauriform diversification emerges from recentdiscoveries and re-studies, which show that the early evolutionary radiation ofdinosauriforms was more complex than it was thought. In fact, the numericaldominance of dinosaurs at the end of the Triassic (Norian) was preceded by an stage(Carnian-early Norian, at least) in which non-dinosaurian dinosauriforms co-habitedwith the flourishing basal saurischian forms.