The evolution of hindlimb anatomy and function in theropod dinosaurs

Substantial differences in hindlimb osteology and soft tissues separate extant crocodylians (Crocodylia) and birds (Neornithes). A phylogenetic perspective including data from dissections and fossil vertebrates reveals that these disparities arose in a stepwise pattern along the line to Neornithes: Important transitional character states that preceded the origin of Neornithes can only be observed or inferred in extinct taxa, blurring the distinction between “avian” and “non-avian” anatomy. The bird sub-clade Ornithurae evolved the last few apomorphies that characterize extant birds, in conjunction with more flexed hip and knee joints.; Previous reconstructions of dinosaurian thigh musculature lack a phylogenetic approach to homology or soft tissue evolution, and many do not incorporate sufficient evidence from dissections and fossils. I compare and contrast these studies with my new reconstructions of the thigh musculature of Tyrannosaurus and other theropod dinosaurs (Syntarsus and Velociraptor). I use my reconstructions to infer the sequence of thigh muscle evolution on the line to crown group birds. A major caveat from this analysis is that functional or other higher-level analyses that use muscle reconstructions should carefully address the ambiguities in the anatomical data by using sensitivity analysis.; When integrated with a soft tissue reconstruction, these data from osteological and soft tissue anatomy form a powerful database for analyzing musculoskeletal biomechanics. I create realistic biomechanical models of the hindlimbs of bipedally running animals to estimate how much hindlimb muscle mass (as a percentage of body mass) would be needed to maintain static equilibrium at mid-stance of bipedal running. These simulations, in conjunction with a preliminary sensitivity analysis, support the hypothesis that Tyrannosaurus rex (and probably other giant theropods) was incapable of running, especially using low duty factors, because the required muscle mass would be an unreasonably large proportion of the whole body mass. Smaller theropod dinosaurs likely enjoyed a wider range of locomotor performance than their larger relatives, much as in many extant vertebrates. By remaining relatively small, birds and other small dinosaurs maintained a wide range of locomotor performance on land as well as in the air once flight evolved.