Abstract

Allosaurus was a large (10-12 m) bipedal theropod dinosaur from the Late Jurassic of North America. Although previous studies have explored how Allosaurus preyed upon herbivorous dinosaurs, understanding the function of its relatively small yet robust forelimbs remains challenging. We chose to study how the dimensions of the Allosaurus humerus compare to those of American alligators and of domestic turkeys, two extant archosaurs that bracket Dinosauria. As birds are more closely related to theropods, we hypothesized the dimensions of the humerus would be more similar between Allosaurus and the turkey as compared to the alligator. For all archosaurs in our sample, our data show that humerus robustness (length v. circumference) and deltopectoral crest length scale with positive allometry. This suggests that as the humerus lengthens, the deltopectoral crest extends relatively further away from the shoulder, indicating a relatively higher mechanical advantage for muscles associated with protraction and adduction coupled with a lower range of motion. In contrast, the medial condyle compared to the lateral condyle showed negative allometry; because the medial condyle is associated with flexor muscles, this would indicate the mechanical advantage of manus flexion decreases in larger animals. In all of these instances, Allosaurus humeri scaled like those of alligators compared with the turkeys. Therefore, we reject our hypothesis and suggest Allosaurus likely had a range of forelimb movement similar to alligators. This would preclude the ability of Allosaurus to have a predatory forelimb stroke hypothesized for bird-like theropods.