LOEWEN, M. A.; SAMPSON, S. D.: Hindlimb ontogeny in the Late Jurassic theropod dinosaur Allosaurus fragilis

The Cleveland-Lloyd Dinosaur Quarry (CLDQ) of central Utah contains an extensive sample of Allosaurus fragilis, the largest known for any large theropod taxon. This collection, along with isolated articulated specimens, forms the material basis of a study of ontogenetic change in the hindlimb of this Late Jurassic theropod dinosaur. Previous research assumed isometric growth in the hindlimb of Allosaurus. However, recent work has called this hypothesis into question, suggesting instead that juveniles possessed relatively longer hindlimbs than adults. Hindlimb elements of Allosaurus have been subjected to gross morphological study and analysis of cross-sectional geometry. The femur exhibits strong negative allometry in length as compared with ilium length. This indicates that juveniles had relatively longer hindlimbs than adults. In contrast, ontogentic series of femoral cross-sections exhibit strong positive allometry in circumference, cortical bone thickness, and second moment of area. Moreover, femoral cross-sections exhibit a dramatic decrease in circularity. Specifically, greatest thickness in adult femora occurs in the caudomedial-craniolateral plane, directly associated with insertion of the major femoral retractor musculature (m. caudofemoralis). In total, these findings indicate substantial shifts in loading regimes during ontogeny. Ultimately, comparisons with extant taxa will be used to estimate changes in locomotor function.