Evolutionary developmental perspective for the origin of turtles: the folding theory for the shell based on the developmental nature of the carapacial ridge

The body plan of the turtle represents an example of evolutionary novelty for acquisition of the shell. Unlike similar armors in other vertebrate groups, the turtle shell involves the developmental repatterning of the axial skeleton and exhibits an unusual topography of musculoskeletal elements. Thus, the turtle provides an ideal case study for understanding changes in the developmental program associated with the morphological evolution of vertebrates. In this article, the evolution of the turtle-specific body plan is reviewed and discussed. The key to understanding shell patterning lies in the modification of the ribs, for which the carapacial ridge (CR), a turtle-specific embryonic anlage, is assumed to be responsible. The growth of the ribs is arrested in the axial part of the body, allowing dorsal and lateral oriented growth to encapsulate the scapula. Although the CR does not appear to induce this axial arrest per se, it has been shown to support the fan-shaped patterning of the ribs, which occurs concomitant with marginal growth of the carapace along the line of the turtle-specific folding that takes place in the lateral body wall. During the process of the folding, some trunk muscles maintain their ancestral connectivities, whereas the limb muscles establish new attachments specific to the turtle. The turtle body plan can thus be explained with our knowledge of vertebrate anatomy and developmental biology, consistent with the evolutionary origin of the turtle suggested by the recently discovered fossil species, Odontochelys.