Reptiles include more than 6500 species of turtles and tortoises, lizards and snakes, and crocodiles and alligators. The class of Reptiles also includes tuatara, the New Zealand lizard that dates back to the dinosaurs.
Unlike mammals and birds, most reptiles lack the ability to generate enough internal heat to keep themselves warm; they are thus cold-blooded. They rely on their surroundings to regulate their temperature by moving between sun and shade.
Crocodilia
Crocodilia comprises the alligators, caimans, and gharials (see the image above). These animals are a group of sluggish water-dwelling predators with long snouts and powerful tails. They have a wide range of body sizes, from the cuvier’s dwarf caiman at 4 to 5.6 feet (1.2 to 1.7 meters) to the saltwater crocodile at 7 m (23 ft). They are also some of the longest-living reptiles.
They are excellent swimmers. During aquatic locomotion, their muscular tail undulates from side to side to drive them through the water while their limbs are held close to their bodies to reduce drag and allow for precise steering and maneuvering. They can also walk on land.
While the body morphology of crocodilians differs among species, most have a broad, flattened snout and a powerful, laterally compressed tail. Their mouths have powerful closing muscles, and the jaw hinge is attached to the bottom of the skull. This allows the jaws to open a fairly large distance and provides a stable structure for holding prey.
Unlike other reptiles, crocodiles are ectotherms, meaning that they cannot generate heat internally and must rely on external sources to raise their body temperatures. They can warm up by basking in the sun, and they can cool themselves by immersing in cold water. They are often preyed upon by large mammals and birds, as well as other reptiles such as snakes.
Squamata
Squamates are the most diverse group of reptiles, comprising over 9416 extant species in 1818 genera. They are found worldwide in a wide range of habitats, from the dry conditions of deserts to the moist and warm rainforests. Some species, such as the wormlizards, are fossorial (foss-OR-ee-ul), living in underground burrows; others, such as the snakes of Asia, spend most of their lives among trees; and some, like the sea kraits, live in the water of coral reefs.
Most squamates are diurnal, but some are crepuscular (active at twilight), and some are nocturnal. Several have adapted to extreme conditions, such as deserts, savannas, chaparral, and thornscrub; others have specialized adaptations for aquatic habitats or for avoiding predators, including camouflage, a heightened sense of smell, or a heightened ability to jump. Squamates are also the only reptiles with paired penes, and they have the most diverse range of locomotion of all vertebrates.
In lizards, snakes, and most amphisbaenians, females bear young internally. This is called viviparity. It probably evolved as a response to the invasion of cold environments, since eggs can be protected inside the mother until they hatch and neonates are then extruded (ovoviviparity). It is not known why this development did not occur in teiids or varanids, but it is likely that carrying embryos for long periods increases predation risk on gravid females.
Rhynchocephalia
The order Rhynchocephalia contains only one living reptile, the tuatara (Sphenodon punctatus). This genus is unique among reptiles and is currently classified as an independent clade (Rhynchocephalida). It has a number of characteristics which set it apart from other reptiles. These include the fact that its teeth are fused to the jaw bone, and the absence of external copulatory organs. In addition, it has a palatal dentition where the lateral tooth row present on the palatine bones is enlarged and orientated parallel to the maxillary tooth rows. This allows three point bending of food items and, in combination with propalinal movement (back and forward motion of the lower jaw), shearing bites.
Tuatara are nocturnal, insectivorous and can be active in low temperatures, unlike other reptiles. They have a wide range of habitats including wetlands and forests. They can also swim and breathe through gills. They are also very fast and can move their heads side to side to change their direction of travel.
Rhynchocephalia was most diverse in the Early Cretaceous but became less diverse after that, probably due to competition with lizards and mammals. Nevertheless, it survived in high-latitude South America until the Early Paleogene when it disappeared.
The fossil record of Rhynchocephalia is very sparse outside of New Zealand, where only a fragmentary skull has been discovered. However, it helps bridge a gap in the rhynchocephalian fossil record of nearly 70 million years.
Testudines
Despite their distinctly different appearances, tortoises and turtles share some important characteristics. They both belong to the order Testudines and the class Reptilia. All members of this group have shells, and they are cold-blooded, egg-laying animals that breathe air through lungs. They also have scaly skins.
The turtles (Testudinidae) that we know and love today – think of our beloved loggerhead, hawksbill, and leatherback turtles – all have bony upper and lower shells that protect them from predators. The upper shell, called a carapace, can be tall and rounded or flat, and the lower shell, called a plastron, can cover most of the body or just a part of it. The carapace and plastron are connected by a bridge, and the turtles’ heads can retract into their shells to protect them from a threat.
The bridge in the middle of a turtle’s neck also provides the ability to vocalize. Researchers have found that Chelodina oblonga, the snake-necked turtle, can produce more than 17 types of sounds, including percussive and complex vocalizations. These sounds, produced in both air and water, are thought to help the turtle locate prey and warn of danger. They have also been shown to help turtles navigate underwater. These calls also show that turtles can differentiate between different environmental cues. For example, they can hear inaudibly low echoes of their own calls, but not the cries of other turtles in the area.