Reptile Classification

Reptiles are cold-blooded, ectothermic animals with a backbone. They are primarily lizardlike, crocodilian, snakelike or amphisbaeniform.


The traditional class Reptilia includes crocodiles and alligators, tuatara (a New Zealand species), lizards, snakes and amphisbaenians. This is still the classification most often found in zoos and college-level textbooks. It is based on morphological criteria.


Crocodilians, the members of the order Crocodylia, are primarily found in tropical and subtropical areas. They are the last surviving members of the Archosaurs, a group of large reptiles that existed during the Triassic period. Fossil records show that modern-day alligators, caimans, gharials, and crocodiles appear remarkably similar to those first appearing 160 million years ago.

The crocodilian brain is more advanced than that of other reptiles and allows the animals to hunt in the dark, a capability attributed to the presence of specialized photoreceptors on their skulls. The skulls of crocodilians feature a wide, U-shaped snout and an overbite, where the teeth in the lower jaw fit inside (closer to the tongue) those of the upper jaw. The snout also has special bony structures to help grip objects as the animal hunts.

Crocodilians display a level of social behavior not seen in other reptiles, particularly mammals and birds, that involves long-term relationships between individuals. They can recognize each other and communicate through vocalization, posture, and chemical signals. These animals exhibit parental care, a trait not observed in amphibians, turtles, and other reptiles. Unlike most other reptiles, crocodilians can walk with a semi-erect posture, rather than in a sprawled position; some can even gallop across land, as one Australian species has been known to do.


Squamata is the largest reptile order and contains over 7400 living species of lizards, snakes, and amphisbaenians (worm lizards). The name reflects their scaled skin, which has two characteristics that unite them: movable quadrate bones allow movement of the upper jaw relative to the neurocranium, making it possible for the group’s members to open their mouths wide to accommodate comparatively large prey; and they are all capable of regenerating lost or damaged scales by adding new ones from below.

Many squamates evolved from visual ambush predators with low activity levels and poorly developed chemosensory systems, but others adapted to more active foraging with advanced chemoreceptor systems and the ability to use their tongues to manipulate prey (lingual prehension). The latter feature helped free up the hands for locomotion and allowed squamates to develop a more complex feeding strategy. Vomerolfaction has varied within the order, remaining relatively undeveloped in basal scleroglossans but becoming acutely sensitive in more derived groups such as snakes and varanids.

Several crown group squamates were previously classified based on morphology, but their relationships to one another have been significantly revised by molecular studies. For example, it now appears that geckos are not closely related to lizards, but form a distinct clade with crocodiles and alligators. It is also now clear that the limbless squamate groups – snakes, amphisbaenians, and dibamids – are not closely related to each other.


The order Testudines includes turtles and tortoises. They can be found in many different habitats, from deserts and savannahs to oceans, lakes, rivers, streams, forests, and more. Testudines have a hard, bony shell that helps them protect themselves from predators and the elements. In addition, they are slow-moving creatures that can hide from prey. These traits have allowed them to survive the conditions that caused the extinction of dinosaurs and other ancient vertebrates, the shifting of continents, and the ebb and flow of glaciers.

There are two suborders in Testudines: Peurodira (side-necked) and Cryptodira (hidden neck). Peurodires have neck vertebrae that flex laterally, allowing them to draw their heads into sideways pockets in their shells. Cryptodires, on the other hand, have neck vertebrae that flex vertically, enabling them to completely hide their heads inside their shells. These features separate them from side-necked turtles and make them more adapted to terrestrial habitats.

Testudines are monophyletic, with recent molecular analyses placing them sister group to archosaurs. The allometry and morphology of their middle ear cavities are also similar to that of Archosaurs, which reinforces old hypotheses that they evolved from an aquatic ancestor.

The common names for Testudines vary from place to place and from language to language. In North America, for example, terrapin refers to freshwater chelonians and tortoise refers to land-dwelling ones; in other places, they may use the terms pond turtle or marine turtle instead.


A reptile in the Chelonian class (American English) or the turtle family of Testudines (British English). Members of this group are called chelonians, and they are known for their long-lasting lives. They are cold-blooded, but they are able to adjust their body temperature to match the surrounding temperatures. They are one of the few reptiles that can crawl over land and swim in water. Their shells protect them from predators.

The chelonids are divided into two families: the Cheloniidae and the Dermochelyidae. The cheloniids include the loggerhead turtle Caretta caretta, the green sea turtle Chelonia mydas, and the hawksbill turtle Eretmochelys imbricata. The dermochelyids are represented by the Kemp’s ridley turtle (Lepidochelys kempii) and the olive ridley turtle (Lepidochelys lepidoceles). They have a more advanced carapace, and their ribs fuse together to form a protective shield.

These are some of the oldest reptiles, dating back to the late Permian or Triassic periods. Today, they are some of the most resilient species on Earth. Their longevity is a result of a number of factors, including their ability to adapt to changing environments. However, they are still vulnerable to predation by sharks, crocodiles, and jaguars. Other causes of death for chelonids include habitat loss, direct harvesting for meat consumption (which can cause food poisoning), boat collisions, and interactions with fishing gear.