The cranial and mandibular bone fusion that characterizes the skull of adult crown birds mainly occurs during postnatal development, when existing sutures between neighbouring bones are fully closed 12, 13. Although less drastic, a similar relationship was found for the connectivity modularity in the skull of the non-avian theropod Tyrannosaurus rex and crown bird Gallus gallus, which based on anatomical network analysis (AnNA) 11 (Fig. However, when compared to their non-avian theropod ancestors, the number of modules is significantly reduced 10. Another recent application of geometric morphometrics showed that the avian cranium has extensive variational modularity, consisting of seven to eight semi-independent regions evolving in a mosaic pattern 9. A recent comparison of ontogenetic series of non-avian theropods and extant crown birds using geometric morphometrics demonstrated that avian skull shape is the result of a sequence of at least four paedomorphic events in the evolution of Eumaniraptora, meaning that the shape of adult bird skulls retain juvenile features like the enlarged orbit and associated brain regions 7, 8. In contrast, Mesozoic Avialae outside the crown, such as the Late Jurassic Archaeopteryx lithographica or the Late Cretaceous Ichthyornis dispars, still retain numerous ancestral theropod characters 4, 5, 6. In contrast to their non-avian theropod ancestors, which possess a typical diapsid skull morphology 1, adult crown birds have highly apomorphic skulls, characterized by a toothless beak, enlarged round orbits, an enlarged and highly pneumatized chrondrocranium, the loss and fusion of bones and skull openings, and a complex kinetic system that allows the simultaneous motion of both jaws, which magnitude is, however, restricted by the morphology and articulation of the quadrate and palate 2, 3. Succeeding the general paedomorphic shape trend, the occurrence of an additional peramorphosis reflects the mosaic complexity of the avian skull evolution.īirds represent highly modified reptiles and are the only surviving branch of theropod dinosaurs. Phylogenetic comparisons indicate that skull bone fusion and the resulting modular integration represent a peramorphosis (developmental exaggeration of the ancestral adult trait) that evolved late during avialan evolution, at the origin of crown-birds. Due to the greater number of bones and bone contacts, early juvenile crown birds have less integrated skulls, resembling their non-avian theropod ancestors, including Archaeopteryx lithographica and Ichthyornis dispars. To understand this process in an evolutionary context, we investigate postnatal ontogenetic changes in the skulls of crown bird and non-avian theropods using anatomical network analysis ( AnNA). In contrast to the vast majority of reptiles, the skulls of adult crown birds are characterized by a high degree of integration due to bone fusion, e.g., an ontogenetic event generating a net reduction in the number of bones.
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