title: The endoskeletal origin of the turtle carapace.
abstract: The turtle body plan, with its solid shell, deviates radically from those of other tetrapods. The dorsal part of the turtle shell, or the carapace, consists mainly of costal and neural bony plates, which are continuous with the underlying thoracic ribs and vertebrae, respectively. Because of their superficial position, the evolutionary origins of these costo-neural elements have long remained elusive. Here we show, through comparative morphological and embryological analyses, that the major part of the carapace is derived purely from endoskeletal ribs. We examine turtle embryos and find that the costal and neural plates develop not within the dermis, but within deeper connective tissue where the rib and intercostal muscle anlagen develop. We also examine the fossils of an outgroup of turtles to confirm that the structure equivalent to the turtle carapace developed independently of the true osteoderm. Our results highlight the hitherto unravelled evolutionary course of the turtle shell.
authors.affiliation: Laboratory for Evolutionary Morphology, RIKEN Center for Developmental Biology, Kobe 650-0047, Japan. hirasawa@cdb.riken.jp
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Hirasawa
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Nagashima
authors.forename: Shigeru
authors.initials: S
authors.lastname: Kuratani
mesh_terms: D000489:Alligators and Crocodiles / Q000033:anatomy & histology / Q000196:embryology; D060105:Animal Shells / Q000033:anatomy & histology* / Q000196:embryology; D000818:Animals; D005075:Biological Evolution; D019521:Body Patterning; D002642:Chick Embryo; D002645:Chickens / Q000033:anatomy & histology / Q000254:growth & development; D004625:Embryo, Nonmammalian; D005580:Fossils; D010802:Phylogeny; D013131:Spine / Q000033:anatomy & histology* / Q000196:embryology; D014426:Turtles / Q000033:anatomy & histology* / Q000196:embryology