Abstract

The head of birds is immediately and profoundly distinguishable from those of other major vertebrate clades, owing to a bewildering number of derived features including an expanded brain and eyes, a shortening of the maxillary face, and an expansion of the premaxillary region at the tip of the snout into the characteristic avian beak. Considered separately, multitudes of individual transformations are required to bridge the gap between an ancestral reptilian cranium and a bird skull. However, in our work, we have found that large‐scale cranial evolution operates in such a way that multiple transformations have a single underlying developmental cause, resulting in character nonindependence and a simpler and more elegant evolutionary story than has previously been assumed. Foremost, we find that the avialan skull is fundamentally a juvenilized or paedomorphic version of the ancestral dinosaurian skull, and that many if not most of the iconic features of birds can be explained using a simple growth or scaling mechanism. In addition, the beak and skull roof — the entire modern avian feeding system, which features a great deal of mobility or kinesis — is patterned by one or a few early developmental influences. Finally, the highly modified avian skull roof is primarily influenced by the transformed brain, and indeed evidence suggests that a tight linkage between these structures is ancestral for Osteichthyes. We also found indications that the expanding brain restricted the size of the jaw‐closing musculature. Remarkably, the fossil record supports many of these hypotheses, showing clear signals of juvenilization and correlated shifts in brain and skull, and also showing simultaneous transformation of the rostral beak and the palate as suggested by our developmental work. We find particular support for our hypotheses using new material of the most important fossil taxa demonstrating the “modernization” of the avian skull, the Cretaceous toothed stem‐birds Ichthyornis and Hesperornis. However, this material simultaneously demonstrates that the avian adductor chamber was, in at least some taxa, remarkably plesiomorphic even after the expansion of the brain, falsifying our hypotheses of a strict causal relationship between brain expansion and adductor diminution. Thus, despite the importance of developmental data in demonstrating the ways in which character transformations are interrelated during major evolutionary transitions, the fossil record is always the final arbiter of the merit of any large‐scale evolutionary hypothesis.Support or Funding InformationFunding provided by Yale University, the Yale Peabody Museum of Natural History, the National Science Foundation, and the Human Frontier Science Foundation.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

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