Segment Theory and the Homologizing of Cranial Bones

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Abstract
The possibility of homologizing the bones of the cranial roof of vertebrates has been discussed since the beginning of comparative anatomical studies. The head of the vertebrate has been assumed to be segmented and even the bones of the cranial roof have been associated with segments, but no system has been convincing. The key to the problem lay in the extrascapular series, or the lack of this series. The papers of Graham-Smith have refocused attention on sone aspects of this problem. Using an approach similar to Graham-Smith, but combining information on segmentation, a basic pattern can be demonstrated for all osteichthyans and their derivatives, the tetrapods. This pattern can even be extrapolated in part to the arthrodires. This common pattern of roofing bones, reflecting basic developmental mechanics, not only simplifies comparative study but becomes another bit of evidence (reciprocal illumination of Hennig) supporting the segmental hypothesis. It helps determine the number of head segments and expands the concept of homology.

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