This week, we were lucky enough to examine two turtles which had been donated to the RVC. Turtles, tortoises and terrapins belong to a group of reptiles called Testudines or chelonians, which is thought to be one of the very oldest groups of reptiles. They have (slowly) walked the Earth for more than 220 million years, survived the mass extinction that killed off the (non-avian) dinosaurs and are found on land, in freshwater and in oceans across the world. These highly unusual animals clearly have a few tricks up their scaly sleeves, so we took a closer look at two loggerhead turtles (Caretta caretta)…

The instantly recognisable feature of chelonians is, of course, their protective shell or carapace. The shell is formed from the fusion of the spine, pelvis and ribcage, and covered in keratinous scutes (see below). With the ribs being fused to the inside of the shell, breathing may have posed a challenge to turtles’ ancestors; most amniotes (reptiles, birds and mammals) use the intercostal muscles (spanning the mobile ribs) to expand and contract the rib cage, altering pressure in the lungs and so drawing in and expelling air. In chelonians the ribs are immobile and the intercostal muscles, being useless, are lost during development. Instead, chelonians have a specialised pair of muscles which press the ventral internal organs against or away from the lungs to produce the necessary changes in pressure.

As has been reinforced by countless fables and folk tales, chelonians usually are infamously slow and cumbersome animals. Although it provides excellent protection, the shell is obviously heavy and restricts the motions of the limbs as well as preventing essentially all motion of the backbone. In other species these motions can help increase speed, so turtles have traded off some of these speed advantages for armouring advantages. Despite this, tortoises and turtles have some remarkable adaptations to keep them moving. We CT-scanned and dissected the forelimbs and hindlimbs from the loggerheads to look at their skeletal and muscular structure.

A feature of the chelonian forelimb which has interested biologists for many decades is the position of the scapula or shoulder blade. In many other tetrapod animals, the scapula sits outside of the ribcage, but the chelonian scapula lies vertically-oriented inside the ribs and shell. The strange angle of the forelimbs is demonstrated in the photograph of the tortoise above. It was thought that this condition was due to the growth of the shell, but some scientists now hypothesise that this is a much more ancient characteristic which ancestral vertebrates may have had, called a plesiomorphy.

One obvious difference between the turtles and tortoises is the use of the forelimbs, which form powerful paddles in our loggerhead. Although the ‘fingers’ appear to have been lost, the five digits are still present within the paddle (below, right). This is a secondary adaptation to life in water, and with a huge muscle mass these fins allow for fast and long-distance swimming with little input from the hindlimbs, which are tucked away beneath the shell to reduce drag and provide stabilisation.

To move such heavy bodies forward, the hind leg is specially adapted for pushing forwards at the unusual angle required to carry the shell. The digits, although elongate in sea turtles compared with those of normal land chelonians, are barely visible from the outside, with only short nails or claws protruding from the thick skin. The fifth metatarsal (base of the outer “pinky” toe, which in turtles is robust, not small like our fifth toe) is almost J-shaped, sticking out to the side of the foot (below). This simple adaptation gives extra leverage, pushing the outer side of the foot backward and outward into the ground, creating the sprawling gait that chelonians typically adopt. Indeed, this is a trait inherited from the distant reptilian ancestors of chelonians, and can be seen in many other sprawling reptiles today as well as in the fossil record. The “J-shaped fifth metatarsal”, as scientists often call it, is an important trait for identifying and classifying reptiles. Indeed, recent anatomical studies of turtles have revealed that this bone has a complex structure in turtles and is best called the “ansulate bone“, possibly having evolved separately from the similar bone in other reptiles. But plenty of mystery remains regarding this peculiar feature!

Hindlimb, indicating the fifth digit (arrow). Also note the microchip used to track the animal (red circle).

Hindlimb (skinned), indicating the fifth digit.

Despite their incredible survival story so far, many species of turtles and tortoises are heavily threatened today. Poaching, habitat destruction and litter could spell the end for this spectacular group who once roamed the planet alongside their dinosaur relatives. Studying turtle anatomy also has implications for our understanding of our own evolution, such as the shoulder blades. The better we understand them, the better we understand ourselves and the greater chance we have to help them.

Want to know more?

Read: How the turtle got its shell: through skeletal shifts and muscular origami. 

Introduction to Testudines, UCMP.

Watch: How the turtle got its shell, an animation.

Turtles and taxonomy, part of a series by the American Museum of Natural History.

Visit: The Natural History Museum in London, who have a dedicated turtle research project.

Glossary

Amniotes: vertebrate animals which lay amniotic (watertight) eggs; reptiles, birds and mammals.

Chelonian: a species belonging to the group Chelonia, also known as Testudines (the differences between these names can be considered negligible).

CT scanning: taking a series of X-rays of an object and using these to look at structures in 3D. Stands for computed tomography.

Intercostal muscles: the muscles between ribs which contract and relax to move the ribcage and facilitate breathing.

Keratinous: made from keratin, the protein which makes up hair and nails in humans.

Plesiomorphy: an ancestral characteristic; an evolutionary holdover.

Scapula: shoulder blade.

Scutes: a bony plate, such as those on the backs of tortoises and crocodilians that may be grouped together to form a carapace or shell.

Secondary adaptation: The evolution of a pre-existing feature (such as the forelimb, used for walking) for a new use (such as evolving a paddle-like shape, for swimming). Also called an exaptation.

Testudines: a taxonomic order, containing all turtles, terrapins and tortoises; essentially equivalent to the group Chelonia (above).

Tetrapod: four-limbed vertebrates, including amphibians, reptiles, birds and mammals.

Ventral: Front (belly) side of the body.