CEBRA-THOMAS, J*A*; TAN, F. E.; GILBERT, S. F.; Swarthmore College; Swarthmore College; Swarthmore College: How the Turtle Gets its Shell: A Provisional Outline
The turtle shell is a morphological innovation that is the basal synapomorphy for the Chelonian clade. In forming the shell, normal vertebrate development is altered and fifty novel bones are produced. Recent research has suggested four major steps in the formation of the turtle shell: (1) the formation of a pair of carapacial ridges in the dorsal trunk of the turtle; (2) the entry of the ribs into the carapacial ridges and the expansion of the ridges; (3) the formation and fusion of the costal bones of the carapace above the ribs; (4) the formation of the plastron bones. We have made a set of provisional hypotheses based on the development of the red eared slider Trachemys scripta to outline how these steps are accomplished. We propose that fibroblast growth factors are involved in the induction of the carapacial ridge and that they are important in directing the rib precursor cells into the dermis. We then hypothesize that the ribs, as they undergo endochondral ossification, produce other paracrine factors--bone morphogenetic proteins (BMPs) and hedgehog proteins--that induce the surrounding and overlying dermis to become bone. As the cells become bone, they also produce BMPs, thereby propagating the BMP signal for ossification. The plastron is thought to form from neural crest cells, and a mechanism is proposed through which trunk neural crest cells may be able to form bone. In this set of hypotheses, the turtle shell is seen as using many of the same processes used to form the vertebrate head.