New prospects on the cranial evolution of non-avialan paravian theropods based on geometric morphometrics

The cranial morphology of theropod dinosaurs has been used to examine the phylogeny, ontogeny, ecology and biomechanics of the clade. Previous studies have recognized that paedomorphosis and peramorphosis occurred multiple times throughout theropod evolution, with skull paedomorphism being one of the major changes during the transition from non-avialan theropods to birds. This study supplemented previous works with more detailed sampling of the morphological data of non-avialan paravians. Principal component analyses based on the cranial geometry confirm that the small-bodied non-avialan paravians have paedomorphic skulls compared with the early-diverging theropods, but independent peramorphosis is also observed in various groups. The secondary elongation of the preorbital portion of the cranium was present in both the late-diverging troodontids and the late-diverging dromaeosaurids, but it was achieved through different morphological configurations in these two lineages.