Decelerated dinosaur skull evolution with the origin of birds

The evolutionary radiation of birds has produced incredible morphological variation, including a huge range of skull form and function. Investigating how this variation arose withrespect to non-avian dinosaurs is key to understanding how birds achieved their remarkablesuccess after the Cretaceous–Paleogene extinction event. Using a high-dimensional geometric morphometric approach, we quantified the shape of the skull in unprecedented detailacross 354 extant and 37 extinct avian and non-avian dinosaurs. Comparative analysesreveal fundamental differences in how skull shape evolved in birds and non-avian dinosaurs.We find that the overall skull shape evolved faster in non-avian dinosaurs than in birdsacross all regions of the cranium. In birds, the anterior rostrum is the most rapidly evolvingskull region, whereas more posterior regions—such as the parietal, squamosal, and quadrate—exhibited high rates in non-avian dinosaurs. These fast-evolving elements in dinosaurs are strongly associated with feeding biomechanics, forming the jaw joint andsupporting the jaw adductor muscles. Rapid pulses of skull evolution coincide with changesto food acquisition strategies and diets, as well as the proliferation of bony skull ornaments.In contrast to the appendicular skeleton, which has been shown to evolve more rapidly inbirds, avian cranial morphology is characterised by a striking deceleration in morphologicalevolution relative to non-avian dinosaurs. These results may be due to the reorganisation ofskull structure in birds—including loss of a separate postorbital bone in adults and the emergence of new trade-offs with development and neurosensory demands. Taken together, theremarkable cranial shape diversity in birds was not a product of accelerated evolution fromtheir non-avian relatives, despite their frequent portrayal as an icon of adaptive radiations