What makes a turtle or tortoise different from all other animals is its shell. Turtle shells have fascinated children and scientists for centuries. This article will provide basic information on the anatomy, growth, and diseases of turtle shells.

Anatomy

Parts of a shell: There are two parts to the shell of a turtle: the upper portion is called the "carapace" and the bottom half is called the "plastron." Both shells are actually made of many fused bones. The carapace is the fusion of about 50 bones - the ribs and vertebrae. The plastron is the fusion of bones including the clavicles (collar bones), bones between the clavicles, and portions of the ribs. A bony bridge joins the carapace and the plastron along the side of the turtle. Some turtles have a moveable joint, usually in the plastron, which acts as a "hinge" and allows the turtle to pull the carapace and plastron together tightly, while the turtle retracts its body into the shell. Shells have a blood and nerve supply, so bleeding and pain can result if the shell is injured.

Scutes on the Carapace

Scutes: The shells are covered with a layer of keratin (the same type of material that makes up our fingernails or horses' hooves). The keratin is arranged in patches called scutes, or shields. The carapace usually has 38 scutes, and the plastron, twelve to fourteen. The names and numbers of the scutes roughly correspond to the adjacent bones and body portions. The scutes, however, do not precisely overlap the bones. Instead, they are staggered, which helps give the shell more rigidity.

Scutes on the Plastron

Some aquatic turtles, such as soft-shelled and sea turtles, may have fewer bones in their carapaces, and the scutes are replaced by leathery skin.

Scute patterns: Different species of turtles have scutes of different patterns and designs, and there is often individual differences among members of the same species.

Shell shapes: The shell shapes of turtles differ with each species, and are often related to habitat. Most aquatic turtles are generally flatter, allowing them to move faster through the water. Tortoises, on the other hand, have carapaces that are dome-shaped.

Shell Growth

As a shell grows, the number of scutes generally does not change, but their size does. In some turtles, old scutes are shed and replaced by larger, new ones. In other species, including box turtles, tortoises, and wood turtles, scutes enlarge in diameter as new keratin is laid down. The "growth rings" in scutes have been used be some experts to help determine the age of a turtle. Age estimation based on growth layers, however, can be erroneous for several reasons:

• Some turtles produce multiple growth zones per year.
• Growth is determined by changes in the environment (seasons), so age determination by examination of growth rings would be more accurate in wild turtles, than those kept in environments, which do not change significantly.
• Growth layers may wear with age, so older turtles may be estimated to be younger than they really are.