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Reptiles are a group of vertebrates that includes crocodilians, lizards, snakes and turtles. There are about 8,000 species of reptiles alive today. Early reptiles diverged from other amniotes between 320 and 310 million years ago during the late Carboniferous Period. The articles listed below provide information about the characteristics, classification and evolution of reptiles.
Crocodilians (4)
Squamates (21)
Tuataras (1)
Turtles (11)
Reptiles
Reptiles are vertebrates that colonized terrestrial habitats more extensively than their amphibian ancestors. This profile explores basic facts about reptiles including how they are classified, what they eat, where they live and the characteristics that make them different from other animal groups.

Reptile Characteristics
Learn about the key characteristics of reptiles including cold-bloodedness, amniotic eggs, scales and more.

Facts About Reptiles
Learn interesting facts about reptiles and find out about their diversity, evolutionary history and the characteristics make them different from other animal groups.

The Basic Reptile Groups
An introduction to the four basic groups of reptiles including crocodilians, lizards, snakes and turtles.

Reptile Pictures
Pictures of reptiles including anoles, giant ground geckos, chameleons, rattlesnakes, green turtles, Komodo dragons, eyelash vipers, and more.

What Do Reptiles Eat?
Most reptiles are carnivores that feed on small invertebrates, mammals and other reptiles. A few reptiles are herbivores that feed on plant material as varied as grasses, fruits, shrubs and marine plants such as algae and kelp.

Crocodilians
Crocodilians are a group of reptiles that includes crocodiles, alligators, caimans and the gharial. There are 23 species of crocodilians alive today. The articles listed below provide information about the characteristics, classification and evolution of crocodilians.
Crocodilians
Crocodilians are a group of large reptiles that includes crocodiles, alligators, caimans, and the gharial.

Crocodilian Pictures
Pictures of crocodilians, including crocodilian photos such as Nile crocodiles, saltwater crocodiles, gharials, American alligators, spectacled caimans and many more.

Gharials
Gharials have a long slender snout that makes the easily distinguishable from their wider-faced crocodile, alligator and caiman cousins. This profile explores basic facts about gharials including how they are classified, what they eat, where they live and the characteristics that make them different from other crocodilian groups.

Gharial Pictures
Pictures of gharials, a species of crocodilian recognizable by their long, slender snout. Gharials are among the largest of all crocodilians, smaller in size than only the saltwater crocodile.

Tuataras are a group of reptiles that includes only two living species. The articles listed below provide information about the characteristics, classification and evolution of tuataras.
Tuataras
Tuataras are rare reptiles that include only two living species that inhabit the small rocky islands off the coast of New Zealand. This profile explores basic facts about tuataras including how they are classified, what they eat, where they live and the characteristics that make them different from other reptiles.

What is a reptile? To answer this question, we must understand what characteristics are common to all reptiles—characteristics such as amniotic eggs, scales, scutes, and ectothermy (cold-bloodedness). Although it's easy to say, for instance, that a snapping turtle, a Galapagos land iguana or a leaf-tailed gecko is a reptile, it's more challenging to explain precisely why they are reptiles.

In this article, we'll explore the basic characteristics that are shared by all reptiles—from snakes and lizards to crocodiles and turtles. These characteristics help us to determine that, for example, a green sea turtle is indeed a reptile and not a crustacean or a mammal.

To begin our exploration of reptile characteristics, I've listed the basic characteristics shared by all reptiles. Then, in the sections that follow, you'll find additional information about each of these characteristics and how they help define what, exactly, a reptile is.

Key Characteristics

The following are the key characteristics of reptiles:

vertebrates
tetrapods
amniote
ectothermic
scales or scutes
All reptiles are tetrapod vertebrates. This means that, as tetrapods, all reptiles either have four limbs or, as is the case with legless reptiles such as snakes and amphisbaenians, are descended from four-limbed ancestors. Additionally, reptiles are vertebrates which means they possess a backbone. Reptiles are not the only tetrapod vertebrates alive today; other tetrapod vertebrates include amphibians, birds, and mammals.

All reptiles are amniotes. Amniotes are a group of tetrapods that produce an egg that has an amnios (an elastic sac within which the embryo develops). The amnios, along with other adaptations such as a hard, porous eggshell and (in the case of mammals) a placenta, enabled tetrapods to better adapt to life on land. Most reptiles are oviparous, meaning they lay hard-shelled eggs, although a few species of squamates are viviparous, meaning the bear live young.

All reptiles have skin that is covered with scales or scutes. The scales of reptiles are small, hard plates that are made of the protein keratin. The scales of reptiles develop from the epidermis (the outermost layer of cells in the skin). Reptilian scutes (such as the shell of turtles or the armour of crocodilians) are similar in appearance and function to their scales. But scutes differ from scales in that they are bony structures that form in a deeper layer of the skin (the dermis) than scales. Scales and scutes provide reptiles with physical protection and also help to prevent water loss. In many reptiles, scales and scutes have developed into different shapes and colors and play a role during territorial disputes and courtship display. Although all reptiles have scales, scales are not a characteristic that is unique to reptiles (other animals that have scales include butterflies and moths, birds, pangolins, and fish).

Reptiles are cold-blooded or ectothermic animals. The body temperature of cold-blooded animals (such as invertebrates, fishes, amphibians, and reptiles) is determined by the temperature of their environment. This contrasts with warm-blooded animals (such as birds and mammals), whose body temperature is maintained within a small, constant range that is mostly independent of their environment. Cold-blooded animals must work with their environment to gain or reduce temperature. For example, reptiles bask in the sun to increase their temperature. Basking in the sun enables reptiles to raise their metabolism and increase muscle activity (warm lizards run faster than cool lizards). When their body is gets too hot, reptiles seek shelter in the shade to cool themselves back down to a safer temperature.

Side-necked turtles (Pleurodira) are one of the two modern groups of turtles and include about 76 species. The other main group of turtles, the hidden-necked turtles, consists of about 200 species.
Side-necked turtles are distinguished from hidden-necked turtles by the way they fold their neck into their shell. Side-necked turtles fold their neck and head sideways and tuck it under the edge of the shell closer to the shoulder so that the head and neck bend at an angle relative to the axis of the spine. Hidden-necked turtles, in contrast, retract their neck inwards along the axis of the spine, curving it in an S shape along the spinal plane so that their head moves directly into the shell.

Side-necked turtles also differ from hidden-necked turtles in the structure of their carapace and plastron. The carapace and plastron of side-necked turtles are more oval in shape relative to those of the hidden-necked turtles, whose carapace and plastron are rounder in shape. Additionally, the bones and scutes take on different arrangements in the two clades of turtles.

Classification:

Animals > Chordates > Reptiles > Turtles > Hidden-Necked Turtles

There are 3 families of side-necked turtles, the Austro-American side-necked turtles, the African side-necked turtles and the American side-necked river turtles (a group that also includes the Madagascan big-headed turtle).

Evolution:

The first turtle-like reptiles appeared during the late Triassic, about 220 million years ago. The earliest known turtle that was equipped with a compete shell is Proganochelys, a creature that roamed the earth during the late Triassic. Another ancient turtle was Odontochelys which had a semi-soft carapace. Hidden-necked turtles arose and diversified during the Jurassic, displacing side-necked turtles from many of their aquatic habitats.