Re: how is a turtle shell formed?

Greetings!

The most unique and recognizable feature of a turtle is its shell. The design of this successful adaptation has changed little in almost 200 million years. The turtle's shell surrounds and protects its body. There are two parts of the shell, an upper half called the carapace, and a lower part known as the plastron. The carapace and plastron of most turtles are joined at the sides by a bridge. The shell is actually made of many seperate bones. These bones are joined together by sutures. Scutes of tough keratin cover the surface of the bones. The area where scutes meet is called a seam.

The carapace of most turtles has about 50 bones. In most turtles, the bones are covered by 26 scutes. Some turtles, most notably softshell turtles and the leatherback sea turtle, have reduced bones in the carapace and a covering of leathery skin instead of scutes. The plastron of most turtles has 11 bones and 12 scutes. The plastron of many species (including box, mud, and musk turtles) has a hinge. The hinge allows the turtle to tightly clamp the plastron and carapace together, protecting the turtle from predators.

The turtle's shell has remarkable ability to regenerate. If part of the shell is injured by predators, fire, or other harm, the undamaged areas will continue to grow and the damage may heal.

As a turtle grows, a new layer of keratin is added under the existing keratin. In some species (including box turtles, tortoises, and wood turtles), the new layers form prominent growth rings. Despite popular notion, the number of growth rings does not always tell the exact age of the turtle. If the conditions are right, several rings may be laid down in one season. Also, growth rings may become worn as the turtle ages. In very old turtles, the shell may become completely smooth.

I hope this answers your question.