ANATOMY OF THE TURTLE SHELL

ANATOMY OF THE TURTLE SHELL

The character and homology of the bony elements of the turtle shell have a long history of controversy. The shell is comprised of the endochondral axial elements of the trunk overlaid by a mosaic of dermal bones and an outer epidermal layer made of keratinous scales (also called scutes or shields). All turtles possess 10 trunk vertebrae associated with the carapace. Each vertebra possesses a single-headed rib that often shares an articulation with the next anterior vertebra. The first and tenth ribs are diminutive and normally extend a short distance before making contact with the second and ninth ribs, respectively. The tenth rib is often indistinguishable in both embryos and adults, but the presence of a large tenth rib in embryos is a normal variation. The thoracic ribs enter the dermis of the shell a short distance from their articulation with the vertebrae, and they extend laterally within the carapacial dermis, terminating at the periphery (reviewed by Zangerl, 1969). In the dermal layer of the shell, there are generally 59 bones: the carapace has 38 paired and 12 or 13 unpaired bones (sometimes the suprepygeal bone is divided and sometimes it is not). The plastron contains one unpaired and eight paired bones. With the exception of a few key taxa, the only real variations in this general scheme occur as individual variations around the neck and tail where the axial skeleton is not closely joined to the carapace. The shapes and relative sizes of the bones determine the general form of the shell in different genera.

The shell’s epidermal layer generally consists of 38 scutes in the carapace and 16 in the plastron. However, this can vary depending on the shape of the shell (domed, hinged, flapped, and so on; see Chapter 3). The shield and bone patterns are not in register; each shield covers a particular area of the bony mosaic. The pattern of the sulci that form between neighboring scutes and the sutures that form between neighboring bones form two minimally overlapping patterns. The epidermal shield pattern develops long before the shell bones begin to ossify, and the underlying dermis may play a major role in the formation of the epidermal scutes, similar to the influence of somitic dermis of feather patterns in the chick (Yntema, 1970; Cherepanov, 1989; Alibardi & Thompson, 1999a,b).