| Paleobiological studies must often cope with small sample sizes, so for most taxa, it is unclear what effect ontogeny and intraspecific variation have on functional morphology during an organism's lifetime. The large theropod dinosaur Allosaurus---known from numerous associated and articulated specimens and thousands of individual elements---currently provides the best opportunity to address this critical, unresolved issue. This study focused on the skull and hindlimb, and included a morphometric analysis of over 570 bivariate comparisons for 1,300 specimens.;Analysis of hindlimb allometry reveals significant growth-related changes in both species, including: relative shortening and thickening of the entire limb, reduction of the femoral medullary cavity, and distal movement of the insertion point for the major limb retractor muscles. These patterns are interpreted as indicative of an ontogenetic shift in locomotor strategy from more agile, cursorial juveniles to a more graviportal adult condition. Taken together, cranial and hindlimb changes during the growth of Allosaurus suggest a shift in lifestyle from agile, generalist juveniles to graviportal, specialist adults.;Analysis of the skull, combined with stratigraphic assessments of individual specimens, indicates the presence of two temporally-separated species of Allosaurus in the Late Jurassic Morrison Formation: A. fragilis (Brushy Basin Member) and A. n. sp. (Salt Wash Member). Cranial elements reveal an increase in skull height vs. skull length during growth for both species, suggesting that these ontogenetic trends are conservative across the two taxa. In A. fragilis, but not in A. n. sp., the caudal portion of the skull exhibits extreme positive allometry, substantially increasing in transverse breadth and ventrally displacing the jaw joint relative to the tooth row, an unusual condition among theropods that mirrors the ontogenetic trajectory of T. rex. Significant increases in skull height and breadth, as well as an overall increase in robustness and reorganization of skull architecture, are postulated to be functionally linked with heightened stress and loading associated with adult predation and feeding. |
