NON-AVIAN THEROPOD SKULL MORPHOLOGY TENTATIVELY SUPPORTS ONTOGENETIC NICHE SHIFT AMONG NORTH AMERICAN TYRANNOSAURIDS

Understanding the ecological role of juvenile carnivorous dinosaurs and how this may have changed through time is essential to documenting patterns of evolutionary paleoecology. Recent work describing dinosaur growth and development has permitted paleontologists to identify ontogenetic changes in the skeletons of many specimens that had previously been assumed to represent distinct adult taxa. Here, we use geometric morphometrics to assess the prevalence of ontogenetic niche shift among theropod dinosaurs. We analyze maxilla shape between the late Jurassic theropod Allosaurus fragilis and Campanian-Maastrichtian tyrannosaurids including Tyrannosaurus, two species of Daspletosaurus, Gorgosaurus, Albertosaurus, and Tarbosaurus. Multivariate regression showed significant correlation between maxilla size and shape among tyrannosaurids (p=0.00013), with smaller specimens exhibiting more gracile morphology (NPMANOVA: 0.0026). Within tyrannosaurids, maxillary shape was significantly different through ontogeny only in Tyrannosaurus (p=0.0072). Furthermore, Tyrannosaurus was the only lineage significantly and consistently more disparate than the other taxa, including A. fragilis (p=0.033), which demonstrated no significant ontogenetic correlation in maxilla shape (p=0.204, NPMANOVA: p=0.642). These results suggest that among the lineages tested, Tyrannosaurus likely had the most drastic shifts in its feeding niche through ontogeny. With extreme specialization towards robust adult skulls, and the largest disparity in neonate to adult mass of the species examined, it is unsurprising to find that Tyrannosaurus significantly modified even phylogenetically conserved traits through ontogeny. Conversely, lacking the skull-only hunting limitations of tyrannosaurids, Allosaurus demonstrates less of a reliance on changing maxillary morphology through ontogeny. Sharing its ecosystem with other megatheropods, Allosaurus may have had a more restricted niche through ontogeny than tyrannosaurs.