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Zoological Science
Abstract
Our knowledge of vertebrate cranium evolution has relied largely on the study of gnathostomes. Recent evolutionary and developmental studies of cyclostomes have shed new light on the history of the vertebrate skull. The recent ability to obtain embryos of the hagfish, Eptatretus burgeri, has enabled new studies which have suggested an embryonic morphological pattern (the “cyclostome pattern”) of craniofacial development. This pattern is shared by cyclostomes, but not by modern jawed vertebrates. Because this pattern of embryonic head development is thought to be present in some stem gnathostomes (ostracoderms), it is possible that the cyclostome pattern represents the vertebrate ancestral pattern. The study of cyclostomes may thus lead to an understanding of the most ancestral basis of craniofacial development. In this review, we summarize the development of the hagfish chondrocranium in light of the cyclostome pattern, present an updated comparison of the cyclostome chondrocranium, and discuss several aspects of the evolution and development of the vertebrate skull.
We thank Osamu Kakitani of Shimane Fishery Association in Gotsu City and Kiyomi Kayano of Sekikatsu Inc. for helping with hagfish sample collection. We also thank Tadafumi Kawamoto for his technical advice on paraffin sectioning, and Itsuro Kamimura for advice on the Avizo technique. Our gratitude is extended to Dominique Adriaens and Per Ah I berg for valuable discussion. This research was supported by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS), the Ministry of Education, Culture, Sports, Science, and Technology of Japan, and MOST grant 102-2311-B-001-012-MY3 from the Ministry of Science and Technology of Taiwan.
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