Reptile Classification

Reptiles are a fascinating group of animals. They have horny scales made of a particular kind of protein, paired limbs with five toes, and skulls that lack the pineal opening that most amphibians have.


The class Reptilia (or Reptiles*) includes snakes, lizards, turtles, and crocodiles. It also includes modern mammals and extinct dinosaurs that were close to them.

Class Reptilia

Reptiles are the first vertebrates to evolve a life on land. They have a number of unique features that set them apart from amphibians and other mammals. These include a dry scaly skin that is watertight and offers protection from desiccation and physical injury, limbs adapted to walking on land, a three-chambered heart, and fully ossified skeletons.

In Class Reptilia, you can find the turtles and tortoises (Order Chelonia), lizards and snakes (Order Squamata), and alligators and crocodiles (Crocodilia). Birds are traditionally placed in Class Aves (“Aves”); however, fossil evidence, comparative anatomy, and molecular data now show that birds are more closely related to lepidosaurs than they are to any other group of modern reptiles.

As a result, most systematists now place birds in Class Reptilia. This is similar to the way in which the traditional class Amphibia* is replaced with Amniota to include both mammals and amphibians.

Most reptiles are carnivores and scavenge for food on land or in the water. In the event of danger, most reptiles will flee to a hiding place or into the water. For example, turtles have streamlined shells that make them fast swimmers and they can hide their heads and limbs inside the shell. Alligators and crocodiles are more likely to strike or bite at potential predators but will also retreat into the water.

Sub-class Chelonia

The sub-class Chelonia contains turtles and their extinct relatives from the extensive turtle fossil record. The green sea turtle (Chelonia mydas) is the only modern species in this genus and family. The genus name is from the Greek words chelone (“turtle”) and testudo (“moveable shelter”).

It has an egg-laying chamber in its tail that resembles a shell and it has lungs rather than gills for respiration. It also has a four-chambered heart, like mammals. Its front limbs are modified into flippers for swimming, but they can’t support the body weight of the animal while on land.

These animals are typically found in shallow shoals in tropical and subtropical seas, primarily the Atlantic coast of Mexico and the Pacific coastline of Central America. They are herbivorous, eating marine algae and plants such as Zostera, Thallassina, and Enhaus. They use their tongues to sample the environment and a pit in the roof of the mouth called Jacobson’s organ to analyze it.

The Green Sea Turtle is a large marine reptile in the order Testudines and the family Cheloniidae. The family derives from the ancestors of all modern turtles that appeared in the fossil record by the Triassic period. Its members are covered in a tough leathery skin and spend most of their lives underwater. Their ancestors evolved from marsh-inhabiting reptiles, and they were the first terrestrial vertebrates to swim. They are the only living members of the order Testudines; it is one of the 16 orders that evolved during the Triassic period. The other three remain today are Crocodilia (alligators, crocodiles, and caimans), Squamata (all snakes, lizards, and salamanders), and Eurypteridia (pythons and caimans).

Sub-class Parapsida

These reptiles are terrestrial, burrowing or aquatic animals with scales on the body. They are cold-blooded, air-breathing and oviparous. They have a 4-chambered heart and are much smarter than other reptiles. They are the first reptiles to have adapted to land life, evolving from amphibians. They have long limbs that can be used to hunt for their prey. They have a well-developed loop of Henle for water absorption. Their excretion is generally uriicotelic for more water absorption.

The sub-class Parapsida is divided into two orders namely, Cotylosauria and Chelonia. Both these groups have a skull roof but the chelonian group has a single supratemporal fossa while the cotylosaurian group does not have temporal vacuities. Their jaws and teeth are poorly developed. They have a pair of eversible copulatory organs, which are useful in the process of copulation.

This is a subclass of reptiles that are primitive in nature. They have a skull roof made of dermal bones, which is also called a anapsid skull. They do not have a temporal fossa in the skull. They have poor jaws and teeth, which are modified into paddle-like structure in aquatic forms. They have a body covered with dorsal carapace and ventral plastron. Their neck, limbs and tail are covered with scales. Their pentadactyls are also reshaped into paddles in aquatic reptiles.

Sub-class Diapsida

Traditional classifications of reptiles often rely on a single key character, the presence and style of temporal fenestration in the skull. The skulls of snakes, lizards and other diapsid reptiles typically have two openings (fenestrae) in the temporal region behind the eyes. Those with one or none are called anapsids, and include turtles.

But a new analysis of a fossil specimen from Madagascar, the lizardlike Acerosodontosaurus, suggests that the single supratemporal fossa in this group is an example of convergent evolution, not a true sign of diapsid affinities. The analysis also challenges the use of turtles in phylogenetic reconstruction, since their peculiar morphologies may mask their phylogenetic relationship to other groups in a broader analyses.

This reassessment of the position of Testudines highlights the dangers of using a single character to define a group; the hazards of basing phylogenetic conclusions on a limited dataset; and the difficulties in judging the appropriateness of outgroup choice for wide-ranging, inclusive analyses that attempt to encompass the full diversity of Recent and extinct groups.

The reassessment also supports the idea that diapsid reptiles are sister to anapsids, and that all living reptiles, including crocodiles, belong in class Reptilia. The clade of diapsids, anapsids and Testudines is therefore known as Reptilia (see the dendrogram page). The other major subclass in this group is Lepidosauromorpha, which contains all living lizards and crocodiles.