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Embryonic derivation and segmentation of the bony skull are not conserved among vertebrates: data from amphibians

Fundamental aspects of cranial anatomy in vertebrates were resolved initially well before the delineation and broad acceptance of Darwinian evolution, let alone modern studies of molecular genetics and cell‐lineage analysis and broad recognition of the prominent role of the embryonic neural crest. Contemporary work has validated many classical observations, yet others mandate reassessment and reevaluation of several underlying assumptions and conventions. Head segmentation is perhaps the most classical topic of all. Compelling accounts regarding the nature and extent of head segmentation abound, but the empirical base on which such accounts are grounded is surprisingly thin. We have utilized a novel transgenic line of the clawed frog, Xenopus laevis, to map neural‐crest derivation of the bony adult skull. This method circumvents technical limitations that have hindered previous studies of the embryonic origin of adult‐specific features in frogs, which form first at metamorphosis. The bony skull of Xenopus is segmented, but in important ways that differ from both those predicted by conventional theories of cranial organization and those reported for other vertebrates. Patterns of cranial segmentation are evolutionarily labile and susceptible to modification associated with the origin of specialized life‐history and developmental modes. Supported by NSF (EF‐0334846; AmphibiaTree).