How Do Turtles Grow Shells And What Are They Made Of?
Just like our bones, calcium phosphate is the main mineral in a turtle shell. Its embryos develop shells through the fusion of backbones and rib bones. The carapace is the shell’s upper portion, and the plastron is the lower portion. The plastron and carapace are joined together with a bridge, which is a structural material.
In almost all the turtles, a protective layer of scutes composed of keratin covers the shell, though some turtles such as leatherback and soft-shelled turtles lack scutes. Turtles represent a unique type of structure in biology because turtle shells form through the fusion of bones.
Tyler Lyson, a scientist from the Smithsonian Institution, analyzed 45 specimens of Eunotosaurus africanus, a 260-million-year old fossil reptile from South Africa. He found that his species has several features in common with modern-day turtles, including paired rib bones and the absence of rib muscles that are used for breathing in many vertebrates. He stated the findings in a news release.
After they get the intermediate shell, a transitional form that connects the gap between the turtles and other reptiles and helps explain how the turtle shell evolved. Eunotosaurus was an early offshoot of the lineage that gave rise to modern turtles.
Those findings suggested that the evolutionary origin of turtle shells may have begun about 40 million years earlier than thought. The evidence on fossils was consistent with genetic studies that also suggested that the turtle shells’ evolution started during the Permian period 260 million years ago.
One of the most massive extinctions on earth happened at the end of the Permian period about 252 million years ago. It was amazing to think that these prehistoric turtles managed to survive.
The shell of a turtle is composed of many layers, each having a different composition and function. The outermost part of the turtle’s shell is made up of large scales known as scutes. Scutes and human hair are both made of keratin. Keratin tissue has no nerves or blood supply, and is therefore dead. Hair may not be the best comparison. Human fingernails are. The keratin in fingernails is similar to that of a scute, with the same shape and hardness.
Under the scutes is reptilian-like skin. The same skin is also found on the rest of the turtle’s body including its head, feet and tail.
The innermost part of the turtle’s shell, called his carapace, is made up of fifty fused bones. The carapace, or top of the shell, is made up of modified vertebrae. These are the bones that form the spine. The spinal cord runs along the carapace. The lower part of the shell, called the plastron, is made up by the sternum and ribs.
The shell of a turtle will grow with it, just as our skeleton does as we age. He will go through molting periods where each scute is shed. The shell looks as if it is peeling away. As the old scute sheds, a new, larger scute appears underneath.
Turtles may also lose their scutes when they become infected, damaged or covered with algae. Turtles have cold blood. The sun is used to raise the internal temperature of turtles. The scutes can be damaged or covered with algae which will block the heat absorption from the sun. The turtle will then shed the damaged scute in order to form a healthier, newer one.
Look at the top surface of the tortoise shell, also known as its carapace. You’ll notice what’s called scutes. These are bony plates made from keratin. You should remember those numbers. Remember how the moon has 13 phases in a calendar year? Guess how many larger scutes are on each tortoise shell: 13. You’ll find 28 smaller scutes around the shell if you count them. This is the same number of days between full-moons.
Oldest Known Turtle Fossil
In China, a partially formed fossilized shell from the turtle species, Odontochelys semistestacea, was discovered in 2008. The shell was estimated to be about 220 million years old. It had a fully developed plastron, however only a partially developed carapace was made of broadened ribs. Scientists believed that this fossil represented an early view of how turtle shells evolved. The broadened ribs on the carapace led the scientists to take a closer look at a similar-looking species known from fossils in South Africa.