Reptilian skin is a tough armor that allows these animals to live on land. Its outer layer consists of b-keratin and, in the case of snakes, turtles, and crocodiles, dermal scutes.
Chameleons change color by modifying the space between guanine crystals in their specialized cells called chromatophores. The lower jaw hinges loosely with the rostral part of the skull and can be moved forward and backward, increasing the mouth opening.
Skull
Skulls of modern reptiles are distinctively different from those of early amphibians. They lack an otic notch and several small bones on the rear of the skull roof. They also have a moveable joint at the articular bone, which connects with the quadrate near the back of the lower jaw. This movement, called kinesis, allows lizards to change the geometry of their bite force to more precisely capture prey.
Researchers studied skulls of living and long-extinct lizards and snakes to find out how diverse shapes evolved. They found that the shape of a skull can be determined by its diet and habitat. This information can help us predict how a reptile will behave as it catches and swallows its food.
The study’s results showed that the morphospace of fully fossorial taxa is comparatively confined. This is indicative of a compact skull diameter, pronounced elongation, and low rfp. Furthermore, the trait pair compactness-diameter strongly converges according to a univariate analysis. This suggests that the absence of limbs may act as a deviating constraint on the cranial microanatomy and morphology of lizards.
Bones
Reptiles do not have the strong, solidly ossified bones of mammals. The skull of a snake is a good example, with separate frontal and parietal bones that are united to form a large basisphenoid bone at the rear of the skull. The jaws of most reptiles are loosely held together, with a hinge in the quadrate bone that allows it to open very wide to swallow large prey.
In the lower limbs, there is no patella and few sesamoid bones. The tibia (Fig. 135) on the preaxial or big-toe side of the leg is larger than in mammals and articulates with both condyles of the femur.
The hind legs of the extinct marine reptiles ichthyosaurs and plesiosaurs became shortened, transforming them into flippers. A second bone, the ulna, was reshaped into a long plate called the platecarpus. The bones of the wrist and hand of these aquatic reptiles were also shortened and shaped into paddles. In some marine reptiles, such as clidastes and the ichthyosaur Lariosaurus, these bones were reduced to a single, broad carpal bone.
Teeth
Reptiles generally have a very different type of tooth structure than mammals. Their teeth are usually not fused to the jaw bone like those of mammals, but they are only lightly infused with it and are thus more vulnerable to breakage. They also tend to replace their teeth continuously throughout life. Herbivorous reptiles can often “regrow” their teeth and they can take on different shapes depending on the foods they eat. Carnivorous snakes can also have teeth of varying sizes and shapes.
In venomous snakes the maxillary teeth are usually modified into fangs for venom delivery (Fig. 5.40). They have a series of anterior and posterior grooves for venom injection, as well as ridges interpreted as cutting edges.
Ultrasonography can be a useful tool in the evaluation of the heart and vascular system of many reptiles. A few studies describe normal ultrasonographic appearance of the cardiac structures in snakes; however, examination of a matched conspecific is always advisable to help guide the clinician.
Scales
Reptiles’ skin is covered with scales made of durable keratin, which protect them from damage and help keep them water-tight. They also serve as a sort of armor that helps to differentiate them from amphibians, which lack such protection.
Scales are a useful model system for studying how gene regulatory networks can lead to the formation of novel biological structures such as hairs, feathers, and scales (Chuong, 1998). Indeed, it has been speculated that the evolution of these appendages began with the modulation and reorganization of the same regulatory network that assembles skin appendages in vertebrates (Alibardi, 2003).
A reptile’s skin is typically a hard, densely arranged integument composed primarily of a thick layer of stratified epidermis. In crocodiles and many turtles, the epidermis is reinforced with a tough, abrasion-resistant material called b-keratin. The narrow hinge regions between scales contain a thinner epidermis that is strengthened with a-keratin. Some scales have ridges to increase the animal’s grip (e.g., a keel on the neck of anole). Others form pits that act as tactile sensory organs.
Eyes
A reptile’s eyes are an important part of their visual system. They use light to create an image and send signals to the brain. The morphology of the eye adapts to different lighting environments. For example, lizards with well-developed parietal eyes have photoreceptor cells that can occupy both scotopic (light limited) and photopic environments. The outer segments of these cells vary in length and diameter depending on behavior and habitat. Diurnal lizards have narrow, short outer segments, while nocturnal lizards increase light absorption by having long, thick, cylindrical outer segments.
Reptiles may have horizontal, vertical, or elliptical pupils. This is a result of their evolutionary history and adaptations to their habitats. Horizontal pupils are common in prey species, as they allow them to see predators approaching from all directions. Vertical pupils are more commonly seen in ambush predators, as they give the animal the ability to gauge distances.
Snakes do not have eyelids, and instead rely on a transparent scale called a brille to cover the eyes. This specialized scale is not a separate piece of skin, but is fused to the surrounding scaly skin. The brille can be shed when the snake molts. During a molt, the snake will secrete a proteinaceous fluid that causes the old layer of skin to separate from the new layer.