The Development and Evolution of the Turtle Body Plan: Inferring Intrinsic Aspects of the Evolutionary Process from Experimental Embryology

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Abstract
The body plan of turtles is unique among tetrapods in the presence of the shell. The structure of the carapace involves a unique relationship between the axial and the appendicular skeletons. A common developmental mechanism, an epithelial-mesenchymal interaction, has been identified in the early stages of carapace development by means of basic histological and immunofluorescence techniques. By analogy to other structures initiated by epithelial-mesenchymal interactions, it is hypothesized that carapace development is dependent on this interaction in the body wall. Surgical perturbations were designed to test the causal connection between the epithelial-mesenchymal interaction in the body wall and the unusual placement of the ribs in turtles. By comparison to data available on body wall formation in avian embryos, these experiments also shed light on the segregation of somitic and lateral plate cell populations and the embryonic origin of the scapula in turtles. This study specifically addresses the ontogeny of a unique tetrapod body plan. The ontogenetic information can be used to make inferences about the phytogeny of this body plan and how it could have evolved from the more typical primitive tetrapod. On a more general level this studyexplores the potential role of common developmental mechanisms in the generation of evolutionary novelties, and the developmental incongruities between homologous skeletal elements in different groups of tetrapods.

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