The evolution of femoral morphology in giant non-avian theropod dinosaurs

Theropods are obligate 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 convergently
between many lineages. However, no quantification of hindlimb functional
morphology has shown if 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 the bird
lineage (Avialae). Our results support a gradual evolution of known
“avian” features, such as the fusion between lesser and greater
trochanters and a reduction of the epiphyses’ offset, independently 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.