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Endoskeletal origin of turtle carapace (free pdf)

From: Ben Creisler

A new paper in Nature Communications (in open access):

Tatsuya Hirasawa, Hiroshi Nagashima & Shigeru Kuratani (2013)
The endoskeletal origin of the turtle carapace.
Nature Communications 4, Article number: 2107

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.