Data from: Phylogenetic variation in hindlimb bone scaling of flightless theropods

The robusticity of the weight-bearing limbs of large terrestrial animals is expected to increase at a more rapid rate than in their smaller relatives. This scaling has been hypothesized to allow large species to maintain stresses in the limb bones that are similar to those seen in smaller ones. Curvilinear scaling has previously been found in mammals and non-avian theropods but has not been demonstrated in birds. In this study, polynomial regressions of leg bone length and circumference in terrestrial flightless birds were carried out to test for a similar relationship to that seen in non-avian theropods. Flightless birds exhibit curvilinear scaling, with the femora of large taxa becoming thicker relative to their lengths at a greater rate than in smaller taxa. Evidence was found for non-linear scaling in the leg bones of non-avian theropods. However, unlike avians there is also phylogenetic variation between taxonomic groups, with tyrannosaur leg bones in particular scaling differently to other groups. Phylogenetically corrected quadratic regressions and separate analyses of taxonomic groupings found little phylogenetic variation in flightless birds. It is suggested here that the non-linear scaling seen in avian femora is due to the need to maintain the position of the knee under a more anterior center of mass, thereby restricting femoral length. The femur of non-avian theropods is not so constrained, with greater variability of the linear scaling relationships between clades. Phylogenetic variation in limb bone scaling may broaden the errors for mass-predictive scaling equations based on limb bone measurements of non-avian theropods.