Reptile Classification

Reptiles are a diverse group of cold-blooded vertebrates belonging to the class Sauropsida. They include modern crocodiles, alligators and lizards as well as dinosaurs (except birds) and the tuatara of New Zealand.

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They are divided into the groups Archosauria and Lepidosauria. Birds have long been a special case, but comparative analyses now place them within the other two major groups.

Crocodilian

Crocodilians are semiaquatic reptiles that live throughout the tropics in Africa, Asia, and Australia. They are solitary, carnivorous predators and are at the top of the food chain. Their teeth grow into sockets in their jaws, and the skull has a characteristic upper and lower opening that resembles dinosaurs.

The crocodilian order, Crocodylia, contains three families: Alligatoridae (alligators and caimans), Crocodylidae (crocodiles, gharials, and gila monsters), and Gavialidae (piranhas and catfish). All crocodilian species are carnivorous. The crocodilian skin is covered with tough, armored scales that protect it from its enemies. Crocodiles and gharials have glands on their tongue that excrete salt ions into the water. Crocodilian jaws are very strong, and some species can eat fish, birds, and even elephants.

In addition to being carnivorous, crocodilian reptiles are cold-blooded and lay eggs. They also have a four-chambered heart, and they are able to breathe air through their lungs and skin. Their skeletons have a notochord, which is a structural element found in vertebrates like mammals.

The crocodilian class is sometimes referred to as Diapsida, along with reptiles such as snakes and lizards. However, it is more common to refer to the crocodilian class as Reptilia. The reason for this is that all crocodilian reptiles evolved from a group of animals known as Tetrapoda, which includes mammals and other groups.

Sphenodontia

The Rhynchocephalia, also known as sphenodontians or wedge-toothed reptiles, are an unusual group of lizard-like reptiles that arose in the Mesozoic and had a moderate radiation before becoming extinct. They are represented today by two closely related living species of tuatara (Sphenodon punctatus and Sphenodon guntheri), found on isolated islands off the coast of New Zealand.

Tuataras have a primitive diapsid skull with biconcave vertebrae, and are distinguished from other sphenodontians by their lack of external ears and a third eye on the forehead, which was positioned at the midline of the skull. Their jaws are characterized by a canine-like successional tooth series, hatchling teeth, and an alternating sequence of dentary teeth that are gradually worn away as the animal matures.

The fossil record for sphenodontians has been severely limited, and most specimens robustly assigned to them have consisted of isolated jaw elements or postcrania. This has made it difficult to reconstruct sphenodontian phylogeny and evolution. A new morphologically-based relaxed phylogenetic clock BI reconstruction with tip-dating has resulted in a significantly improved treetop, resolving many previously unresolved relationships and yielding much more accurate divergence times20.

Squamata

The suborder Squamata contains 9416 extant lizard and snake species in 84% of 1018 genera (as of December 2012). These are the most diverse vertebrates. Squamata are found in most natural habitats, and their body plans and lifestyles have made them particularly abundant in open, warm, semiarid regions. They are terrestrial, arboreal, fossorial, saxicolous, aquatic, and nocturnal. In addition, squamata are the only reptiles with the capacity for vivipary (living birth).

The squamate radiation has been intimately linked to continental drift and the origin of islands. For example, Australian pygopodid geckos arose from diplodactylid and ophidiid ancestors dispersed throughout the island continent. In contrast, varanids and skinks originated in the Northern Hemisphere and dispersed to South America and Africa.

Limb reduction is one of the most significant evolutionary innovations in Squamata. Ancestral squamates were essentially ambush predators, detecting prey by movement. They also had relatively low activity levels and poorly developed chemosensory systems. These early adaptations enabled squamates to survive periods of mass mortality, such as in the Permian extinction.

The phylogenetic relationships of the major groups within Squamata are poorly understood, in part because few molecular studies have employed large-scale taxon sampling. However, mitochondrial genomes suggest that squamates are sister to the tuatara of New Zealand. The tuatara is the last member of the ancient Rhynchocephalia.

Testudines

The order Testudines includes all turtles, terrapins, and tortoises. This clade is distinguished by its characteristic bony shell. In turtles, this shell is more than just an epidermal covering; it forms a vital part of the skeleton and is called a carapace. A separate ventral shell, the plastron, is likewise incorporated into the skeleton. Both shells are covered with keratinous plates, which are called scutes. The scutes are usually hinged, and allow the head, legs, and tail to be withdrawn into the shell.

In addition to their unique body structure, testudines also have special respiratory systems. They breathe by a process called buccal pumping, in which they use their mouths to push air up through their throats into their lungs. Some sea turtles have pairs of sacks in their cloacas that function like gills, taking in oxygen and expelling carbon dioxide.

Some testudines are oviparous, and lay eggs. Others, like the radiated tortoise (Geochelone radiata), are viviparous and give live birth to their babies. The radiated tortoise was presented to the Tongan royal family by British explorer Captain James Cook in 1777, and lived until 1965, when it died at the age of 188. This is one of the longest-lived reptiles ever recorded, and shows how important tortoises can be to their habitats. Other testudines, such as the loggerhead turtle (Caretta caretta), are also long-lived.