Reptile Classification

Reptiles are a group of tetrapod (four-legged) vertebrates that are characterized by scaled skin. They are ovoviviparous, meaning that they lay shelled eggs.

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In traditional classification, reptiles were grouped together as class Reptilia (“to creep”). This included all reptiles except birds and mammals. Modern phylogenetic studies show that this is no longer correct.

Orders

More than 8,200 living species are placed in four reptile orders: Crocodilia, which includes all modern crocodiles, alligators, caimans and gharials; Sphenodontia, which contains the single living species known as tuatara, found only in New Zealand; Squamata, which comprises lizards and snakes; and Testudines, which encompasses turtles, tortoises and terrapins.

Crocodilians are carnivorous, semi-aquatic predators. Their ancestors were close to dinosaurs. They have a flattened snout, tough skin, and rows of large teeth. Their skulls have a foramen in the frontal bone near the occipital suture. Their jaws can tear rather than chew their food. They are the largest ectothermic reptiles on the planet and they spend most of their lives in or near water.

Sphenodontia comprises a group of lizards that are small and terrestrial. Their skulls have one temporal opening, and they have a foramen in the frontal bones near the frontoparietal suture. Squamata comprises a group of reptiles that have scales on their bodies, and their limbs are usually adapted for crawling. They have a foramen in the frontal and prefrontal bones, and they have a skull with two temporal vacuities (or openings) called supratemporal fossae.

Testudines contain the turtles, tortoises and turtiles that are able to retract their heads into their shells for protection. They have a skull with two temporal fossae, and they have a bony external plate for their body called a plastron.

Classes

Reptiles are a group of tetrapod vertebrates that have scales or other protective coverings. Their eggs are aided and protected by an amnion, a structure that is not found in the first terrestrial vertebrates (amphibians). In addition to their scales, reptiles have several distinguishing features of the skull that help them to chew and hear better. In particular, they have a skull bone that is very close to the one that forms the malleus and incus in mammals, helping them to pick up higher-frequency sounds.

Traditionally, reptiles have been grouped together as the class Reptilia, from the Latin word for “to creep.” These animals were described by Linnaeus and included crocodiles, alligators, tuatara, snakes, lizards, turtles and frogs. They have bony skeletons, lay eggs and are cold-blooded (ectothermic).

The modern classification of reptiles includes birds as well. Birds are not considered to be reptiles in the strict sense of the word, but the fact that they evolved from a tetrapod ancestor means that they have a close relationship with the other living and extinct reptiles. Therefore, most zoologists agree that birds should be a subgroup of the class Reptilia, rather than an independent class.

The class Reptilia is divided into four orders: Crocodilia, which includes crocodiles and alligators; Sphenodontia, or tuataras; Squamata, or snakes and lizards; and Testudines, or turtles, tortoises and terrapins. Each of these groups can be further divided into sub-orders, families, genera and species.

Phylogeny

The class Reptilia includes lizards, turtles and snakes. It is also known as the Reptilian order, and its study is called herpetology. Reptiles are air-breathing vertebrates with internal fertilization and epidermal scales covering part of their bodies. Unlike mammals and birds, they are ectothermic and cannot control their body temperature by changing environments.

The phylogeny of these animals has changed rapidly as molecular research progresses. The results often conflict with morphological data and have resulted in lengthy discussions about classification. However, the new phylogenies remain relatively robust and provide insight into the evolution of these remarkable animals.

A phylogeny is an evolutionary tree showing how one group relates to other groups. It provides information about the relationship between different species and can reveal novel relationships that would not be apparent from separate analyses.

Molecular data shows that reptiles are classified into two major groups. One group, the Diapsida, includes modern crocodiles and alligators. The other, the Testudines, includes turtles and tortoises and the tuatara of New Zealand. The Testudines are characterized by a double-penis, which is an organ that allows the female to lay eggs without the male having to enter her shell.

The tuatara is the only living member of the lizard-like Sphenodontia order and has unique features, including biconcave vertebrae, a rudimentary diapsid skull, and a laterally compressed tail. This clade is sister to a clade that contains most modern pleurodont lizards, including many Colubroid and Serpentine snakes.

Reproduction

Reptiles are the class of air-breathing vertebrates with internal fertilization, amniotic eggs, and horny epidermal scales covering part or all of their bodies. They include crocodiles and their relatives (order Crocodylia), snakes and lizards (class Squamata) and turtles (order Testudines). Birds (class Aves) have been traditionally placed outside of Reptilia, but fossil and comparative data now strongly suggest that they share a close relationship with the lepidosaurs and crocodiles and therefore should be considered part of this group.

Most reptiles are oviparous, laying eggs that develop inside a shell on land. A few lizards and snakes are viviparous, delivering their young alive.

Some reptiles have seasonal breeding patterns, spawning at particular times of the year. This may occur because of environmental conditions, or it can be caused by a change in the behavior of males to optimize spermatogenesis.

Because reptiles are cold-blooded, their internal temperature varies with the environment. This sensitivity makes them very sensitive to changes in their captive environment, and it also influences their natural behaviors. The ability of many species to quickly suck in air or water at the surface is an important evolutionary adaptation that reduces their metabolic costs and helps them maintain their body heat. This is called a thermoregulatory system. Some reptiles can also store sperm for up to six years, which can be used for subsequent spawning events without further sexual contact with the female.