A turtle without a shell would be unrecognizable. The shell forms a major part of their body and is what helps to contain their organs. A turtle without a shell would be kind of like a human without skin – a wobbly mess.
There are 2 main categories of turtle shells – hard and soft. Turtles with hard shells are more common, although there is a whole family (Trionychidae) that have soft shells.
Hard shell turtles have many segments on their shell, referred to as scutes. These are formed from keratin, the same material used to make human fingernails. These scutes provide a significant level of additional strength and protection to the turtle’s shell. These scutes are shed periodically and replaced with new growth.
This means that when a predator bites the turtle’s shell, it will damage one of the scutes. This prevents damage to the actual shell and means that the turtle will be okay.
Soft shell turtles do not have these scutes. Instead, their shell is covered in a layer of skin similar to leather. These are less tough than hard shells but still fairly resilient.
What does a turtle shell look like?
A diagram of the bones making up the plastron
What is the structure of a turtle’s shell?
A large part of a turtle’s body is made up of their shell. All that is not covered by it are their extremities – the 2 arms, 2 legs, head, and tail. There are 3 main elements to a turtle’s shell, the carapace, the plastron, and the bridge.
There are in excess of 300 different turtle species known to exist. There is a variation in the shell shape – they can be flat, round, or domed – but they all have identical bone structures on the interior.
Carapace
This is the upper area of the turtle’s shell, the part that you can see with ease. This is the area that people without much herpetological knowledge would likely call the shell. It covers the turtle’s back and the backbone is mostly fused along the center.
The carapace also has flattened and wide ribs fused into it. This shows how integral the shell is to the survival of a turtle. Turtles are the only animal species in existence with all of the ribs fused into one area of bone. All other species have multiple ribs that are separate from one another.
It is estimated that there are about 50 different bones that make up the carapace of a turtle.
Plastron
This is the underside of the turtle shell, kind of like a protective chest plate. It consists of 9 bones and 2 epiplasts, as shown in the diagram above. It is unsure how the turtles evolved to grow a plastron.
The general consensus is that the sternum overdeveloped and eventually became the plastron. This theory was suggested in the 19th century by a French zoologist. Despite being widely accepted, this theory remains unproven.
Bridge
This is the area of the shell that links the carapace and the plastron. The bridge regions are on the sides of your turtle and are believed to have evolved as an extension of the fused ribs. They are designed to protect the inner organs and to keep the rest of the shell in one piece.
The bridge is a vital part of the turtle’s shell, without which it would not be able to survive.
Can they survive without a shell?
No, turtles cannot survive without a shell. As we have detailed above, the shell contains integral parts of the turtle’s skeleton, without which their bodies would have no structure.
A turtle without a shell would appear similar to a puddle of warm jello on the sidewalk. They would ooze everywhere with nothing to keep them in one piece.
Turtles rely on their shells for protection too. Even if they figured out a way to not disintegrate without their shell coat, they would have no defense against predators. Their inner organs would be completely exposed and they would be easy pickings for all of their natural enemies.
Turtles are born with a well-developed shell as soon as they hatch. They are capable of healing minor shell injuries of their own accord, but serious cracks will never heal.
How did turtles get their shells?
Millions of years ago, it is believed that turtles did not have shells. So how did they come to be so utterly useless without them? No one really knows the answer to this for sure. There are 2 main theories when it comes to this question.
The first is that the older turtles had rudimentary scutes mounted on their backs. The theory goes that over time, the turtles grew more and more scutes, which eventually ended up fusing together and creating the carapace.
The other theory is that the ribs of the turtles grew and expanded over time, eventually being forced to fuse together. This has been proven to be more correct than the first theory, although the reasoning behind this happening is still uncertain.
Using fossil records, we can see that some of the early turtles had the plastron area, just no carapace. These older turtles are from the species odontocetes and are believed to have lived around 220 million years ago. They are the most visually similar species to modern-day turtles.
We know that the older species of turtles lived primarily on land and enjoyed burrowing into the earth. This is different from modern turtles that are primarily aquatic. Scientists in 2016 theorized that turtle ribs are so wide as they gave a lot of stability for burrowing and digging.
While handy for digging, these wide ribs can be very cumbersome and greatly reduce the animal’s mobility. This then leaves them much more vulnerable to predation. The theory goes that the ribs began to fuse and form the carapace to protect the turtles while still allowing them to dig vigorously.
