NEW DINOSAURS FROM THE EARLY JURASSIC HANSON FORMATION OF ANTARCTICA, AND PATTERNS OF DIVERSITY AND BIOGEOGRAPHY IN EARLY JURASSIC SAUROPODOMORPHS

The Early Jurassic Hanson Formation of Antarctica has yielded unprecedented insight into the evolution of high-latitude vertebrate faunas during the early Mesozoic. To date, the Hanson fauna includes the theropod dinosaur Cryolophosaurus ellioti, the sauropodomorph dinosaur Glacialisaurus hammeri, a pterosaur, and a 'Bienotheroides clade' tritylodont. New U-Pb zircon dates of ~194 Ma help constrain faunal age. Our recent expedition collected new material of Cryolophosaurus and Glacialisaurus, as well as several new sauropodomorph specimens. One individual is a nearly complete juvenile skeleton, and the other includes several vertebrae and pelvic material. The two specimens may represent distinct species, and differ in the curvature of the iliac preacetabular process, the concavity of the proximal pubes, and the shape of the femoral head.

Phylogenetic analysis of 57 taxa and 353 characters recovers the three Antarctic sauropodomorphs as distantly related to each other, and the new specimens as more closely related to Sauropoda than to Massospondylidae. Previous studies posited that some Early Jurassic sauropodomorph regional faunas were phylogenetically overdispersed (taxa from a given fauna are more distantly related to each other than expected by chance), but such patterns have not been tested quantitatively. We analyzed aspects of Early Jurassic sauropodomorph faunal structure using phylogenetic comparative methods and a time-sliced dataset with several branch length estimations. The phylogenetic diversity represented in the Antarctic sauropodomorph fauna is as high or higher than that of the other five Early Jurassic regional faunas analyzed. We found no significant support for phylogenetic overdispersion of sauropodomorphs in Early Jurassic regional faunas, though analyses generally recovered Antarctica as having the most evenly dispersed fauna. These results provide additional support for prevalent dispersal across Pangaea during the Early Jurassic. The presence of three distantly related sauropodomorphs in Antarctica also implies that although endemism in Antarctic vertebrates increased from the Early Triassic to Early Jurassic, climatic and/or physiographic barriers did not prevent dispersal into Antarctica during the Early Jurassic.