Evolution of the vomer and its implications for cranial kinesis in Paraves

Cranial kinesis is an important feature of neornithine birds. However, due to the rarity of palatal elements in fossils, its origin is poorly understood. Here we reconstruct the vomer of the troodontid Sinovenator and the stem bird Sapeornis, and provide the palatal reconstruction of this Early Cretaceous bird. Using these data we conducted a comprehensive morphological study of the palate and a 3D shape analysis of the vomer across Paraves. The results indicate that stem birds, like palaeognaths, had limited cranial kinesis, a conclusion also supported by the identification of an ectopterygoid in Sapeornis. This suggests that the remarkably flexible avian skull is a neognathous innovation, with greater cranial kinesis ultimately facilitating the radiation of this highly successful lineage.