If you’re a “pet turtle” – a freshwater turtle species like the Eastern Mountain Blue, the Leatherback, Cappuccino, or Green turtles – you probably already know that a turtle Skeleton is an important part of keeping your pet healthy. Turtles are voracious predators and need to have their shells. Without the right sized, shaped, and placed turtle shell they can suffocate, drown, or get hit by a car. This is why it’s so important to have an expert give you a well-rounded health assessment prior to purchasing your pet turtle. You don’t want to purchase your turtle just because he has nice colored skin.
Let’s take a closer look at the turtle shell itself. The carapace (upper shell) is held together by a series of pleats that are stitched or glued to the upper shell surface. These pleats allow water to pass through while allowing air to stay trapped inside the shell. Also, because the pleats make it possible for the turtle to breathe while inside the shell, they are very useful for the turtle in terms of thermoregulation. A properly sized and fitted shell will allow a turtle to endure temperatures of up to ninety degrees Fahrenheit, while the average temperature inside a carapace in the wild is around twenty degrees Fahrenheit.
The Plastron is the underside of the turtle skeleton. It is the foundation for the turtle’s ribs, which are its heart and lung. The Plastron is made up of two to four stiff ribcages held together by their cartilage. A turtle may have as few as five ribs or as many as fifty. The plastrons are also connected to the side of the turtle’s head.
Another important structural element of the turtle skeleton is called the Ischial (neck). This is the thickest part of the shell and is lined with a soft padding called a keratin coating. The Ischial serves a very valuable purpose: it acts as a “rebounder” for the plastrons. The Ischial pushes outward, so the ribs can expand outward. The plastrons are held in place by the Ischial, and when it becomes stretched out, water fills the opening. This keeps the ribs in place and keeps the turtle skeleton itself from moving.
To help keep the plastrons in place, the Ischial also has a sticky backside to adhere to. This backside is what allows the plastrons to grow, replacing decaying bones with new ones as it is continuously pushed forward. On the outside of the plastrons there are two kinds of openings: the vesicle or skin cavity, and the osteoid. The vesicle is lined with an acid protecting it, while the osteoid is lined with connective tissue, which serves as padding for the vesicle.
The first portion of the turtle shell is the osteoid, which is lined with osteoblasts (specialized bacteria). Osteoids are also where the first teeth come in, and are where the eye (the iris) comes into the animal. The turtle has an additional bony covering on its underside called the plastron, which is actually a hard outer sheath that protects the internal organs. This plastrum is made up of various types of keratin and has a very important job: making sure that the internal organs stay healthy.
Lastly, the three layers of the turtle shell are made up of the nautiloid, the dentiloid, and the plastroid. The nautiloid is made up of long thin bony plates that line the inside of the turtle shell; these plates keep the bones inside tightly together. The dentiloid is a set of bony plates along the bottom of the shell, while the plastron is a thick hard outer sheath.
The fossil record shows us that all three of these important parts of the turtle shell have been around since the beginning of the species. While we cannot accurately pinpoint when the shell first became solid, the oldest known fossils come from the genus Eutheria, and they date from the Triassic period. This means that the earliest known turtles have had a solid shell since the Triassic period. The most recent evidence of turtle shell evolution comes from the anterior carapace of some species of modern turtles.