The turtle shell

The turtle shell is a remarkable evolutionary novelty that defines the order Chelonia. The turtle shell is found in three general forms based on the nature and degree of ossification: hardshells, softshells, and leatherbacks. This section will concentrate almost totally on the bony component of those shells of the hardback turtles of the Emys and Chelydae families. This shell is composed of two main parts, the dorsal carapace and the ventral plastron, connected along the midflanks by lateral bridges.
Altogether, the shell contains over 50 dermal bones that are homologous to no other bone in anyother vertebrate order. Moreover, the presence of this bony casing has necessitated extensive modifications of the tetrapod body plan (Zangerl, 1969). Whereas dermal ossification itself is a primitivecharacter for vertebrates (Smith & Hall, 1993), the turtle shell represents an extreme developmentof the dermal skeleton among tetrapods.

The shell clearly has adaptive value for turtles as physical protection, but it also serves physiological functions in different species as a site of hematopoiesis, a reservoir for water, fat, or wastes,and a buffer for pH. The embryonic development of the shell involves a dramatic hypertrophy of thedermis in the dorsal body wall and a resultant rearrangement of the typical relationship between thepectoral girdle and the axial skeleton. Thus, turtles are the only vertebrates whose limbs are founddeep to the ribs.

The paraxial and limb-girdle musculature—the neck and skull—are also greatlymodified. As we detail here, the key innovation for the chelonians appears to be the carapacialridge, a bulge of ectoderm and mesoderm that influences the growth of the ribs (Burke, 1989a). Theribs are enveloped within the dorsal dermis, resulting in their lateral displacement as the dermis rapidly expands. Thus instead of extending ventrally and enclosing the thoracic cavity, the turtle ribs become integrated into the carapacial dermis.

The neural arches of the vertebrae also fuse with the midline of the carapace. As the anonymous author (1676) of the letter to the Royal Society of London wrote in 1676:“The Anatomie of a Tortoise, showing that what were the Ribs in other Animals one upper Shell is in the Tortoise, and that to that upper Shell are firmly fastened the spinal Vertebrae, so that the Animal cannot go out of its Home, as Snails do.”