What Is a Turtle Skeleton?
One of the most fascinating aspects of the marine reptile is its ability to change its shell. The majority of reptiles (including turtles) can change their shell according to their desire, food and habitat. The carapace is usually the first to break off, but sometimes other bones are left on the shell as well. Over half of all marine reptiles do not have a full set of back legs, they have one or two legs sticking out the bottom of the shell. Their head is held in a cavity that is partly or completely covered by the plastron.
As you look inside the shell the arrangement of the skeletal system becomes apparent. Each bone has a small ‘nose’ where it protrudes and the upper jaw has several suckers at the front end. There are tracheal bones that are present in the right and left sides of the snout. In addition there are numerous bones that fit in the crevice of the eye, and these are lined up in a specific way to aid the eyestalks with the vision process.
It is important to recognize each one individually and to be able to recognize the position of each bone in the right and left side of the snout. Also notice that the turtle skeleton has a series of bumps along its shell that correspond to the eye, teeth, claws and other protruding appendages. At the end of the carapace the ribs meet and this is the second most important part of the turtle skeleton. The ribs on the inside of the shell are called plastrons, while those on the outside are called tuning. The plastrons and tunica hold the tail and the ribs on either side of the shell to hold the head, or nevi.
On either side of the head you will see two sets of paired undulating scales. These are known as the scutellarii and the eloboard. The scutellarii show scales on the upper half of the shell, while the eloboard has scales on the lower half. The shape of the tail is very important to understand when studying the turtle skeleton. Its shape resembles a serpent with its end curled back, while the plastron forms a V-shaped ridge along the back of the shell.
The turtle skeleton has been excavated from fossils in a number of locations over the past century. The best locality for finding plastron and tunica are found in the limestone of Mexico, USA, Western Canada and Eastern Oregon. Over half of the turtle’s complete body has been discovered in these deposits. Excavations in other areas have yielded more fossils from the Cretaceous geological period of between ninety and three hundred million years ago.
One of the greatest finds from the Cretaceous is a complete female (adult) from the Stegonidae or broadened ribs group of turtles. This specimen is approximately two to four inches long and the head was nearly as wide as the entire body. It had broadened ribs that led to a short neck and rounded claws. Its legs were shorter than its arms and its body was marked by numerous “puffs” or ridges along its back and along its fringes.
In addition to the well-preserved ribs and the complete breastbone, this specimen also has plastron and tuning, the plastron (or broadened ribs) formed of soft bone. These bones are very important for determining an average age since they indicate a time of life before the extinction. Plastron is the outer covering of the shell. The tunica is the inner lining of the shell that provides protection for the interior of the shell. This shows how the soft bones of the turtle are separated into two shells that cover the inside of the shell.
Since turtles can grow to a full adult length of more than twelve feet, this means that they can reach the largest size of a turtle shell ever documented. The maximum size of a turtle shell is usually only half a foot in diameter and a quarter of an inch in depth. While this measurement may seem drastic, it is a common occurrence in the world of turtles. Some specimens have been found to exceed one and a half yards in diameter.