Bodyplan

The bodies of chelonians are evolutionary wonders. In order to deal with living in a shell, turtles had to go through some major changes to accomodate this new and improved lifestyle.

What's in a Shell.

The shell of turtles is the backbone of their success (pun intended; Sorry.). These amazing little marvels are what have kept chelonians in business for over 230 million years. But what is in a shell? What is it made up of?

Let's find out :)

The turtle shell is broken up into three parts. We have:

The carapace. This is the upper part of the shell.
The Plastron. This is the lower part of the shell.
The Bridge. A line of bone inbetween the fore and hind limbs that connects the plastron to the carapace.

These pieces of shell are all made up of the same component parts. First a layer of cartilaginous bones which form the shape of broadened ribs. This is then covered by a layer of membrane bones or thecals. Then this is followed by the tuff stuff. The osteoderms or epithecals. Also known as dermal armor, for that's exactly what it is. The osteoderms are fused plates of bone that make up the general shape of the shell and give it it's rigidity. The last covering in chelonians is the scutes. These are the tuff outer scales covering the shell and giving it it's color and texture.

The bones of the shell (the osteoderms) can be broken down into even further parts.

The Carapace

The top of the carapace in the middle is made up of many small pieces of bones called neurals. These end in the front at a large piece of bone known as the proneural and at the back in two pieces called the suprapygals.

The midsections of bones on the carapace are called pleurals and on the front and rear of the pleurals are two pieces of bones. In the front there is the axillary buttress, while in the back we have the inguinal buttress. Both pieces are used to strengthen the shell more.

Finally we come to the end of the shell. All around the side are many little pieces of bones called peripherals. In the front the peripherals meet up with the proneural and in the back the peripherals meet with the pygal.

The plastron also has a different array of bony plates.

Plastron

At the very front of the plastron there is two pieces of bone known as the epiplastron. These two pieces then meet up with a third piece near the bottom of them known as the entoplastron.

After the entoplastron there are two large plates of bone known as the hyoplastron.

Behind the hyoplastron is two more large plates called the hypoplastron.

Finally at the end of the hypoplastron are the last two pieces of bones. They are called the xiphiplastron.

In the middle on the sides is the bridge which composes two pieces only with a notch on both the front and hind ends called the axillary notch.

The osteoderms of the shell are then covered by the horny keratinous scutes. These scutes can also be broken into different parts.

Carapace

The piece of scute directly behind the head is known as the nuchal. Then all the scutes directly behind the nuchal are known as vertebrals.
Radiating along the sides of the shell are scutes called marginals. Named of course because they are at the margins or fringes of the shell.

Finally all the scutes inbetween the vertebrals and the marginals are called costals, thus making up the outer surface of the carapace.

The Plastron

The first pair of scutes directly behind the head are known as the gulars.

These are then followed by the humerals, then the pectorals, followed by the abdominals and femorals. Note how these scutes are basically named after the part of the body they are located at or near.

The final and most rearward scutes would be, of course, the anals.

On the front of the bridge there is the axillary and on the back of the bridge there is the inguinal.

Together these pieces make up the shell, one of the most amazing pieces of defensive machinery ever evolved.

Special adaptations of the shell

The above was a basic view of the many different parts to chelonian shells. But, not all chelonian shells are alike. Some major modifications have gone into these things.

For instance in some types of chelonians the shell has evolved a cartilaginous hinge inbetween certain bones. This hinge allows the chelonian to actually close up that part of the shell making it that much safer from predators.

The hinges themselves are different as well. Some types such as the genus Kinosternon (mud turtles) there is a hinge inbetween the pectoral and abdominal scutes which allows the turtle to close up the front half of it's shell. In the genus Kinixys (Hingebacks) the hinge has evolved on the carapace instead of the plastron and lies inbetween the second and third costals. It allows the tortoises to close the hind quarters of their shells. Finally we have the genus Terrapene (box turtles) which decided on having the best of both worlds. They developed two hinges. One inbetween the pectorals and the abdominals and another inbetween the abdominals and the femorals. Thus allowing the turtles to close up both halves of their shells making an impenetrable fortress or Box as their common names imply.

There are also species of chelonian who have decided to mess with their protective covering and make it more maneuvarable and less protective. In the tortoise Malacochersus tornieri (Pancake tortoise) the bony casing has been reduced so as to allow the animal to move more freely. The shell has also been widened and flattened so as to accomodate it's new lifestyle. These tortoises live in rocky areas where when danger threatens, they run to the nearest rocky crevice and cram themselves in. The new shape of the shell allows the tortoise to both move quickly and fit in very narrow spaces.

Then there are the Trionychids. They are more commonly refered to as the soft shelled turtles. There shells have undergone a major tranformation. Not only has the bony layer been radically reduced, but the horny keratinous covering of scutes has been dumped in favor of a tough leathery skin. The plastron has been modified into a set of strut like supports instead of a solid casing. This relieves a lot of weight of the turtle's backs making them very fast and maneuverable. This change in shell is not doubt due to the turtle's need for speed when catching their prey and avoiding predators.

Another species that has undergone some radical change in shell structure is that of Dermochelys coriacea (The Leatherback sea turtle). It is the only species of turtle to have basically dumped it shell completely. Instead of bony plates there are many small interwoven bones that provide it's support. It's entire outer covering is cartilaginous making it very streamlined in the water but very susceptible to cuts and bruises when hauling itself out on land to lay eggs.

Among other changes in chelonian shell structure there is plastron reductions. These reductions can be nearly to the point of nonexistence and are probably made to allow the turtle to move with greater ease as it's chases down frogs and insects.

So in the beginning there was the shell and it was good. But then some chelonians found that they could do better with a softer, lighter shell and one species even decided to basically remove the shell entirely. As I said before, a remarkable evolutionary achievement.

Shell Damage and Regeneration

Although the shell is made from horny scutes and plates of bone, it can still be damaged and even bleeds when damaged severely.

If the shell is damaged, it will regenerate, but depending on the severity of the damage and the health of the animal, regeneration could take a long or relatively short time.

Bone Structure: Making things work.

Along with an amazing protective shell, chelonians had to also evolve new ways for them to move.

Vertebrae

In most vertebrates, the vertebrae are a flexible series of bones that allow for fairly unrestricted movement on land and in the water. In chelonians though this would be a bad thing. The carapace has to attach from somewhere and the best place to be would be the vertebrae. So a flexible vertebrae would quickly put an end to their marvelous shells. The dorsal and sacral (the vertebrae that make up the middle of the backbone) vertebrae have been fused together and join with the neural bones of the shell. The verts here been shortened to ten to add strength. The cervical (neck) and caudal (tail) vertebrae are the most flexible parts. In chelonians there are eight cervical vertebrae that are extremely flexible. These verts allow the chelonians to perform the maneuvers needed to get in their shells also. Chelonian necks are also usually more flexible that that of mammalian necks and can be compared with birds in terms of maneuverability. The caudals are made up of up to 33 bones and are also highly flexible. While most chelonians have characteristicallly short tails, there are plenty of others with very long tails.

Ribs

In order for the carapace to form it needs to drape itself over some kind of supporting structure. It's already connected to the backbone so what else can it use? The answer is ribs. The ribs are peculiar in chelonians in that they actually envelope the limb girdles. The reason for this strange occurence can be found in the embryonic stage. Here the anlage (early carapace) is actually growing faster than the ribs forcing them to grow with it.

Limbs

As you can imagine the limbs have undergone some major changes themselves. The clavicle (collar bone) has become a part of the plastron known as the epiplastron. The scapula (shoulder blade) has joined to the carapace at it's outer surface where it can anchor the legs more effectively. The sternum which is normally used to protect the body has been totally lost since a nice bony plastron has taken it's place. The humerus and femur have been shortened considerably except near the end which has become considerably enlarged. The reason for this is that it has to support the weight bearing joints of the forelimb and shank. Further down the carpal and tarsal elements have fused thus strengthening the distal parts of the legs.

Turtles and tortoises are mostly digitigrade animals. That is they walk on their toes. All chelonians (except fully aquatic of course) are digitigrade in their forelimbs, while most tortoises are digitgrade in the hind limbs also. For the most part their forelimb terminates in five digits while the hind limb terminates in four. There are exceptions with some species have only four digits on the front leg or only three in the hind.

Aquatic chelonians have either fully webbed feet or as is the case with marine turtles, flippers.

Beaks

All chelonians have beaks instead of toothed mouths. This is a feature they share with birds and certain deinosaurs as well.

The beak is a horny piece of keratin that is self sharpening and continually grows. In some species this beak is covered over by a layer of skin which, combined with their strange noses, makes them look like their lips are in a perpetual pucker.

Breathing

One of the biggest problems with growing an immovable and impenetrable shell is that breathing can be a big problem. Since their is no movable flesh to allow for expansion of the lungs nor a diaphragm to expand them. So chelonians had to find a new way of handling this problem.

When this was first being considered, researchers thought that, like amphibians, chelonians achieved respiration by gular pumping. That is they thought that the constant throat pumping movements seen in these animals was used to force air into the lungs. This has turned out to be false and gular pumping in chelonians is now known to be of olfactory (smelling) signifigance.

So how do they breathe?

Breathing is accomplished by the creation of a negative pressure differential (i.e. the air outside has a higher pressure than the air inside, so through the process of diffusion, the air will enter this "negative air space" and fill the lungs). Mammals and crocodylians accomplish this through the use of their diaphragms and intercostal (between the ribs) muscles, while squamates use their intercostal muscles and, in some, gular pumping (throat breathing). It has already been established that they don't use gular pumping for the purpose of inhalation, they don't have diaphragms and the ribs now form a part of the shell, so intercostal breathing is out as well. Chelonians had to find another way.

This negative pressure differential (NPD) is partly achieved via the shell. In tortoises breathing is accomplished by the use of the rigid shell and the toroise's musculature. The muscles used for breathing expand into the limb pockets at the borders of the shell and serve to modify the internal pressure within a chelonian's body. So when the tortoise moves it is expelling air in one movement and taking in air with another. This would also explain why resting turtle's and tortoise's forelimbs move in and out.

Now in species like Chelydra serpentina (Common Snapper) the plastron has been severely reduced, which actually makes breathing on land easy since the weight of the animal's internal organs force air into the lungs in a kinda cantelevering process, which makes the expulsion of air the only muscular activity (imagine that, having to work to keep air out of your system.)

When in the water though, things change and inspiration is the newly required muscular activity, while expiration happens virtually spontaneously due to the water pressure. Of course marine species which dive to great depths have a better handle on this.

So in order to deal with a life enclosed within a shell, chelonians had to not only find a way to make such a thing, but also had to modify the ways in which they moved and breathed.

Absolutely amazing.

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