How the Turtle Gets Its Shell

FIGURE  .   Development of the carapace. (A). Entry of cartilaginous rib precursor (arrow) into carapa-

cial ridge of Trachemys embryo around stage 16. The following show bone formation in Trachemys scripta, 

stained with Alcian blue (cartilage) and alizarin red (bone). (B) 1.2-cm embryo showing cartilaginous ribs 

forming the outline of the shell. (C). Ventral view of 3.1-cm carapace, showing intramembranous ossification 

of the nuchal bone and around and in the anterior ribs. (D) Lateral view of the same carapace, showing region 

of rib chondrogenic growth (blue, arrow) and transition zone (white) between cartilage and bone (red). (E) 

Dorsal view of 118-day (CL = 3.1 cm) hatchling carapace showing expanded nuchal bone region, the fusion 

of the anterior costal ossification centers, and the peripheral bone ossification centers that start anteriorly. 

The pigmentation of the epidermal scutes can be seen. (F) Dorsal view of 185-day (CL = 4.5 cm) hatchling 

carapace showing fusion of marginal ossification regions anteriorly, as well as the pygal ossification center 

posteriorly. The costal ossification centers have created bony armor dorsally (the blue staining is beneath the 

carapace). (G) Predominant pattern of the adult carapacial bones. (Modified from Gilbert et al., 2001; G modi-

fied from Zangerl, 1969.)

The unpaired midline dermal bones of the carapace, called neurals, are fused with the neural spines of the 10 thoracic vertebrae (Figure 1.1). The costal bones extend from the neurals toward the periphery. There are eight pairs and each is intimately associated with a rib (Figure 1.1E). Gennrally, there is a one-to-one correspondence between the vertebral spines and the neural bones, and How the Turtle Gets Its Shell

between the ribs and the costal bones of the carapace. This relationship does not hold in the anterior and posterior ends of the shell, where the vertebral centra are shortened and have little or no contact with the shell. The first costal bone overlies ribs one and two, and the eighth overlies ribs nine and ten (variants have nine pairs of costal bones). The pygal and suprapygal bones form the rear of the carapace. These bones have no contact with vertebra and ribs but project over the sacrum and pelvis. The peripheral bones form the edge of the carapace. There are generally 11 pairs of peripheral bones; before making contact with the costals, they form a socket around the distal tip of ribs two through nine. The nuchal bone forms the anterior margin of the carapace, which overhangs but is not attached to the posterior cervical vertebra. This bone extends laterally around the margins of the carapace to the level of the second rib. It is overlaid by the first three peripheral bones laterally and contacts the first costals and neural bone posteriorly. Each of the carapacial bones is connected by sutures to its neighbors. The distal edge of each costal is attached by suture to the peripheral bones. This contact often does not occur until later stages of post-hatching growth, leaving open a peripheral ring of fontanels that surround the distal tips of the ribs.

Sections across the carapaces of adult turtles show a three-layered arrangement of the bone. The central portion of the bone is a spongy layer containing spherical cavities. On either side of the spongy layer are layers of more compact lamellar bone. This compact bone is thought to form beneath the inner and outer periosteal membranes. The shapes and relative sizes of these bony regions determine the general form of the shell in different genera (Yntema, 1970; Ewert, 1985; Cherepanov, 1997).