The Turtle Skeleton
A well preserved turtle Skeleton showing how the plastron and scapula connect to the rest of the skeletal structure to form a solid shell surrounding the entire body. Scales (left) and flippers (right) of the carapace of a turtle. The turtle shell is an extremely complex defensive shield for the turtle’s ventral and dorsal portions of the body, fully enclosing all of the internal organs of the turtle… but it’s not that tough! (click the picture for a larger and better view.)
In nature, turtles evolved over million years ago, long before the dinosaurians roamed the earth. The most well-preserved fossil turtle fossils come from the Cretaceous geological period, which lasted from the middle Jurassic to the end of the Triassic period. The first turtles lived in the rivers and seas of what is now Russia, India, China and Taiwan. During the Cretaceous geological period, millions of years ago, a giant armored turtle called the Mesozoic was dominate the oceans. This Mesozoic turtle has jaws that open wide and pierce their opponents with needlelike teeth.
Modern turtles are born with a shell, called a keratin egg, that grows continuously over a few months. The body is covered with a hard spongy or waxy outer layer, called a plastome (or “leathery” to modern turtles), which is shed on the ground after death. The external layer of plastome can harden into a solid mass called a carapace when the turtle molts, or gains muscle mass over time. Then, like a cornered turtle, the turtle starts to flatten towards the base of its tail.
After a month of continuous growth, the juvenile reaches the point at which the last plastrons or carapaces (the exoskeleton’s last defense layer) hardens into a solid structure called a plastome. At this time, the juvenile also grows a set of strong, spiked feet that assist him in walking on land. At this time, the adult’s last defense layer –the carapace – also thickens and hardens, forming the solid carapace that protects the animal through adulthood. The complete turtle skeleton grows continuously as long as it lives. However, at some point –usually near the tail – the turtle stops growing and begins to degenerate.
The turtle gets its name from the broad band of bony protrusions along its spine (which also protect the spinal cord in terrestrial species). These protrusions also help the turtle hold itself upright by creating an indented ribcage that helps support the weight of the turtle. The ribs begin to thin as the turtle grows older, and turn into slender tube-like carapaces instead of hard, solid, protective ribcages.
The most visible part of a turtle’s skeletal structure is its soft, fleshy underbelly. This is the region from where the turtle derives most of its nutrients, and its general condition is indicative of the health of that part of the turtle. The number and size of the claws, or plastrons, are also indicators of its health: the larger the plastrons, the healthier the turtle.
Most turtles grow slowly, reaching maturity not more than a few years in age. In the first year of life, they grow very slowly, and reach full size in the late years of their lives. This means that a specimen of one hundred years old would be as tiny as a mere two pounds in weight.
An adult turtle usually weighs between fifteen and twenty pounds in total, including the plastrons. At maturity, the turtle can grow to around forty pounds in weight. Most live for around eight to ten years.
Like all other reptiles and amphibians, turtles have soft shells, which are composed primarily of a hard outer shell, along with a series of grooves or channels inside it. The outer shell of a turtle can be completely smooth or patterned, or have an ridged, textured surface. They are also usually colored black. The channels and grooves inside the shell are covered with small hairs, and a turtle’s legs can protrude through these channels into the scutes. The inner shell is usually made up of a soft material, such as mucus, clay or similar material.
After becoming fully mature, a turtle gains protection within its shell by growing a set of bony plates around the head and neck, called carapace. These bony plates often end up behind the tail, where they are called ventral plastrons. The number of bony plates and the shape and size of them varies from species to species. Some have only two, while others can have as many as ten. Their legs, which are the ones that assist in movement, have long thin claws, but some have claws that are longer and more muscular than other turtles. While some trogons have unusually long legs, all possess relatively short legs.
The skeleton of a turtle looks somewhat different in relation to that of other land-dwelling vertebrates. All turtles have a relatively complex system of internal bones and organs, including the eye, the lungs, the heart, the intestinal tract and the limbs. In addition, the skeleton is made up of soft tissues, including fur, skin, hair and the bony plates. A well-developed skeleton is most vulnerable to attack from predators during the nesting period.