Theropods are bipedal dinosaurs that appeared 230 million years ago and are still extant as birds. Their history is characterized by extreme variations in body mass, with gigantism evolving independently and on several occasions between many theropod groups. However, no study has shown whether all theropods evolved the same limb adaptations to high body mass or whether they had different morphologies. Here we studied the shape variation across 68 femora from 41 species of theropods using a 3D comparative approach, multivariate statistics, and phylogenetically informed analyses. We demonstrated that all the heaviest theropods evolved similar adaptations regardless of their phylogenetic affinities by enlarging muscular attachments and articular surfaces. We also highlighted that the lightest theropods evolved femoral adaptations to miniaturization, which occurred close to the bird lineage (Avialae). In addition, our results support a gradual evolution of known “avian” features, independent from body mass variations, which may relate to a more “avian” type of locomotion, where the knee drives hindlimb movement instead of the hip, like in earlier theropod relatives. The distinction between body mass variations and a more “avian” locomotion is represented by a decoupling in the mediodistal crest morphology, whose biomechanical nature should be studied to better understand the importance of its functional role in gigantism, miniaturization, and the evolution of a more “avian” type of locomotion.

Theropods are obligate bipedal dinosaurs that appeared 230 Ma and are still extant as birds. Their history is characterized by extreme variations in body mass, with gigantism evolving convergently between many lineages. However, no quantification of hindlimb functional morphology has shown whether these body mass increases led to similar specializations between distinct lineages. Here we studied femoral shape variation across 41 species of theropods (n = 68 specimens) using a high-density 3D geometric morphometric approach. We demonstrated that the heaviest theropods evolved wider epiphyses and a more distally located fourth trochanter, as previously demonstrated in early archosaurs, along with an upturned femoral head and a mediodistal crest that extended proximally along the shaft. Phylogenetically informed analyses highlighted that these traits evolved convergently within six major theropod lineages, regardless of their maximum body mass. Conversely, the most gracile femora were distinct from the rest of the dataset, which we interpret as a femoral specialization to “miniaturization” evolving close to Avialae (bird lineage). Our results support a gradual evolution of known “avian” features, such as the fusion between lesser and greater trochanters and a reduction of the epiphyseal offset, independent from body mass variations, which may relate to a more “avian” type of locomotion (more knee than hip driven). The distinction between body mass variations and a more “avian” locomotion is represented by a decoupling in the mediodistal crest morphology, whose biomechanical nature should be studied to better understand the importance of its functional role in gigantism, miniaturization, and higher parasagittal abilities.