Turtle shells are ribcages worn outside the body
Order Testudines wraps reptiles in shells built mainly from ribs: a domed bony carapace above and flatter plastron below, sheathed in keratin scutes like hair and claws. Living turtles split into side-necked Pleurodira and hidden-necked Cryptodira by how they tuck the head.
About 360 living and recently extinct species make up the order Testudines, a name coined from Latin testudo by the German naturalist August Batsch in 1788; the rival labels Chelonii and Chelonia came later and lost on priority. Living members fall into two camps defined by how the head is stowed: Pleurodira bend the neck sideways, Cryptodira draw it straight back. They range from sea turtles crossing oceans to land tortoises and freshwater terrapins on most continents.
The shell, made of 50 to 60 bones, is unlike anything else among vertebrates. In the embryo, the ribs splay outward along a structure called the carapacial ridge, which no other animal has, burrowing into the skin of the back and widening into plates that fuse with the spine, a process signalled by growth factors such as FGF10. The belly plastron comes from shoulder-girdle bones, sternum and abdominal ribs. Because both girdles sit inside the shell, the shoulders and hips effectively live within the rib cage. A typical shell wears 54 keratin scutes, 38 above and 16 below.
Shapes track lifestyle. Land species carry high domes that resist crushing, while aquatic ones are flat and smooth; box turtles have hinges, softshells rubbery rims, and the leatherback almost no shell bone at all. Every species has eight neck vertebrae, a constancy unusual among reptiles. Instead of teeth they have keratin beaks, and a hard-shell eater, Mesoclemmys nasuta, bites with about 1,920 newtons. The red-eared slider has seven types of cone cell in its eyes.
Speed varies wildly: a desert tortoise ambles at under half a kilometre per hour, whereas sea turtles reach 30 km/h by beating their front flippers like wings, producing twice the thrust of paddling freshwater species. The leatherback, the biggest living turtle, can exceed 2.7 m and 500 kg, and the extinct Archelon reached 4.5 m. At the other extreme, South Africa's Chersobius signatus stays under 10 cm. Hunting, bycatch and habitat loss now threaten many species.
Source: Turtle
