DOES THE MAXIMUM BODY SIZE OF THEROPOD DINOSAURS INCREASE ACROSS THE TRIASSIC-JURASSIC BOUNDARY? USING ONTOGENY AND PHYLOGENY TO UNDERSTAND TRANSITIONS IN EARTH HISTORY

Dinosaurs originated in the Late Triassic as small generalists, but by the Early Jurassic they had evolved a wide range of body sizes. A sharp increase in the maximum body size of theropod dinosaurs has been reported across the T-J boundary in central Pangaea based on footprint data, but quantification of the maximum body size of Triassic theropod body fossils is necessary to enable comparison with large-bodied theropods of the Early Jurassic (e.g., Dilophosaurus wetherilli; Cryolophosaurus ellioti). Several larger Triassic theropods are known; e.g., Gojirasaurus, Liliensternus, and Zupaysaurus (~79%, ~70%, ~60% the size of D. wetherilli, respectively), and we incorporated analysis of skeletal maturity of available specimens to better understand body size evolution. Ignoring indicators of the ontogenetic stage of specimens when undertaking an analysis of body size can skew results, especially in such a poorly represented group as Triassic theropods.

We assessed the maturity of two large neotheropods from the Late Triassic Bull Canyon Formation, New Mexico. We histologically sampled a partial fibula, ~68% the size of D. wetherilli, and observed three annual lines of arrested growth (LAGs) and highly vascularized primary woven bone, suggesting that rapid growth had not ceased. The other partial individual was ~56% the size of D. wetherilli. The tarsal elements of this individual are only partially fused, and the pelvic elements are completely unfused. We found no LAGs in the rib and long bone cortex, and the highly vascular primary woven bone throughout suggests that this individual was still undergoing rapid growth. Gojirasaurus, Liliensternus and Zupaysaurus individuals also lack tarsal and pelvic fusion.

These unfused skeletal elements, an indicator of skeletal immaturity in the closely related Coelophysis bauri, suggest that these individuals had not yet reached skeletal maturity, although they are of sufficient size to form the largest Triassic tracks in central Pangaea. Osteohistology of the two neotheropods indicates that the examined specimens were still rapidly growing. These data suggest that the sharp increase in theropod track size in central Pangaea is a local, not worldwide, trend. The maximum body size of neotheropods does not expand much or at all across the T-J boundary.