The spine and expanded ribs of a turtle are fused through ossification to plates beneath the skin to form a bony shell. Both upper and lower sections of the shell have an outer layer of plates called “scutes” made primarily of keratin (as are hair, feathers, hooves, claws, horns and nails). Scutes protect the shell from scrapes and bruises.
In most land turtles and tortoises, scutes remain on the shell for life, which causes the shell to thicken and protects it. Growth of the scutes occurs through the addition of keratin layers to the base of each scute.
For most water species, as the turtle grows, the epithelium, or thin layer of tissue between the scutes and the bony plates, produces a new scute beneath the old one that is a larger diameter than the one layered on top of it, allowing the shell to expand.as the turtle and its shell grow. The old scutes shed or peel away to make way for the newer, larger scutes (see top of shell, or plastron, of Northern Map Turtle on right in photo). Basking in the sun helps turtles shed scutes by drying them and leaving them ready to fall off. Usually this happens without any assistance, though there are some species of turtles which do pull loose scutes off each other’s shells.
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