Reptile Classification

Reptiles are a group of tetrapod vertebrates with an ectothermic metabolism and amniotic eggs. The study of modern reptiles (Class Reptilia) is called herpetology.

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The class includes turtles (Testudines), snakes and lizards (Lepidosauria), and crocodiles and their relatives (Crocodilia). All living reptiles are amniotes. Mammals (Class Mammalia) are the only other amniotes that are warm-blooded and have hair.

Order Crocodilia

The order Crocodilia consists of alligators, crocodiles and gharials. These are large, semiaquatic reptiles that have strong bites and specialized respiratory systems. They also have short, powerful tails that assist them in chasing and submerging prey. Their specialized ears are well adapted for both air and water, and they have glands that secrete salty lubricants to keep their eyes clear of mud and debris while they are submerged.

Like other reptiles, crocodilians have bilateral symmetry (their bodies are divided into mirror-image halves). They have a special system for cooling themselves by drawing blood toward the head. Their stomachs are very acidic, and they ingest stones to aid in digestion. They also use the nictitating membranes in their eyes to secrete a salty lubricant that keeps the eyes moist and enables them to see underwater.

Crocodilians are solitary animals, but they have been observed hunting together when conditions are favorable. They also show some cooperative behaviors, such as helping each other dismember and consume larger prey. Their specialized mouths allow them to breathe while eating, and they have the ability to hold their breath for extended periods of time.

While modern classification schemes have shifted slightly with the advent of molecular genetics and reinterpretation of phylogenetic relationships, the traditional groups of turtles (Order Testudines), snakes and lizards (Order Squamata) and crocodiles and alligators (Order Crocodylia) are still useful as practical groupings.

Order Squamata

The order Squamata, encompassing lizards and snakes (suborder Serpentes), contains more than 95% of all reptiles. Its members have a wide range of body shapes and ecological niches, from the tiny sandfish-like genus Geckodactylus to the 8 m (26.2 ft.) green anaconda. The order’s most prominent feature is the overlapping, shield-like horny scales that cover their bodies. Squamates also have movable quadrate bones that allow them to move their upper jaws relative to the skull. This makes it possible for some species to open their mouths extremely wide, allowing them to take in even the largest prey items.

Squamates are among the most successful vertebrates, with a remarkable diversity of ecomorphs and body forms. Phylogenetic studies suggest that the rise of modern squamate diversity was triggered by environmental change in the Late Cretaceous, leading to the rapid expansion of the dietary ecomorphospace they currently occupy.

Squamates are also distinguished by a transverse cloacal opening, external ear openings, and a flexible neck that can rotate and retract. They have the ability to eject mucus from their eyes and skin, and many have a tongue that can be notched or forked. Some squamates, such as chameleons and iguanas, are oviparous (egg-laying), while others are viviparous or ovoviviparous. They all undergo periodic shedding or molting of their skins, which is called ecdysis.

Order Testudines

Turtles and tortoises are members of the order Testudines (pronounced CHEL-onia) which has 360 living and extinct species. They are easily recognized by the hard protective shells that surround much of their bodies. These shells, called carapace and plastron, come in a wide variety of shapes and sizes. The upper shell, or carapace, can be tall and rounded or flat and broad, and it connects to the lower shell, or plastron, with a portion of shell known as a bridge. Unlike most other vertebrates, turtles have a single bony top and bottom shell, instead of two separate shells joined by an interconnecting area.

The hard protective shells of the order Testudines help to protect these slow moving reptiles from predators and other dangers. These animals are found all over the world in a variety of habitats. They occupy both terrestrial environments like deserts and savannahs, aquatic environments like rivers, lakes, ponds, swamps and brackish water estuaries, and the ocean.

The turtles and tortoises of the order Testudines are unique among all other vertebrates in that they can float in water for extended periods of time. This ability is due to the fact that they have an extra air chamber inside their body, which allows them to fill with water when in a sinking position. This feature makes turtles excellent swimmers and helps them navigate in shallow waters.

Order Archosauria

Living birds and crocodiles plus their extinct relatives like non-avian dinosaurs and pterosaurs comprise the crown clade Archosauria. But this group also includes a whole lot of other Permian and Triassic reptiles, including weird hippo-size herbivores like rhynchosaurs, long-necked predators called prolacertiforms, and evil-looking terrestrial predators known as erythrosuchians and proterosuchids. These early archosauromorphs are all limited to the Triassic and did not survive the end-Triassic mass extinction that ended their own lineage.

The defining features of archosaurs include a large opening in the skull (the antorbital fenestra) and an unusual opening on the lower jaw called the mandibular fenestra, a high narrow skull, and teeth set in sockets (known as thecodont tooth implantation). Archosauromorphs are unique among Reptilia for their tendency to fuse and reduce multiple bones in the head. This allowed them to reduce weight and allow for greater skull flexibility (kinesis) when eating.

Traditionally, archosaurs were based on morphological characteristics, meaning they were classified mainly by their physical appearance. For example, a rauisuchid lizard called Prestosuchus has a swollen rim delimiting the antorbital fenestra and a deep pit on the caudodorsal corner of the lateral surface of the squamosal that distinguishes it from other reptiles (see illustration). However, phylogenetic taxonomy now defines archosaurs using evolutionary relationships rather than morphological characters. This new approach still includes crocodiles, birds, non-avian dinosaurs and pterosaurs, but it no longer includes the old “thecodont” groups like phytosaurs and aetosaurs.