Reptiles are cold-blooded, air-breathing vertebrates that have skin covered in scales as opposed to hair or feathers.
Reptiles are ‘tetrapods’ (vertebrate animals that have four feet, legs or leg like appendages) and amniotes, animals whose embryos are surrounded by an amniotic membrane and members of the class Sauropsida.
Amphibians, reptiles, dinosaurs, birds and mammals are all tetrapods and even the limbless snakes are tetrapods by descent. The earliest tetrapods developed from lobe-finned fish, into air-breathing amphibians in the Devonian period. There are around 8,225 species of reptile on our planet. Today they are represented by four surviving orders:
Crocodilia (crocodiles, gharials, caimans and alligators): 23 species
Sphenodontia (tuataras from New Zealand): 2 species
Squamata (lizards, snakes and amphisbaenids (‘worm-lizards’): 7,900 species
Testudines (turtles and tortoises): 300 species
Ancient reptiles were known as the dinosaurs. Todays reptiles are much smaller when compared to the gigantic sizes of the Diplodocus or the Brachiosaurus. However, they are all descendants of these magnificent beasts that survived millions of years ago.
Reptiles have existed since the beginning of time. Even snakes were around in the Cretaceous period. Modern reptiles inhabit every continent except for Antarctica, although their main distribution comprises of the tropics and subtropics. There are many species of reptile that survive in Rainforests throughout the globe.
Many reptiles are referred to as ‘cold-blooded. Reptiles rely on gathering and losing heat from the environment to regulate their internal temperature, for example, by moving between sun and shade, moving warm blood into the body core, while pushing cool blood to the periphery (the external surface of a body).
In their natural habitats, most species of reptile are adapted to this and can usually maintain core body temperatures within a fairly narrow range. Reptiles are thick-skinned and unlike Amphibians, they do not need to absorb their water because reptiles can retain water.
While this lack of adequate internal heating imposes costs relative to temperature regulation through behaviour, it also provides a large benefit by allowing reptiles to survive on much less food than comparably-sized mammals and birds, who burn much of their food for warmth. While warm-blooded animals move faster in general, an attacking lizard, snake or crocodile can move very quickly.
Except for a few members of the Testudines (Turtles, Tortoises and Terrapins whose bodies is shielded by a special bony or cartilaginous shell developed from their ribs), all reptiles are covered by scales.
Most reptile species are oviparous (egg-laying). Many species of squamates, however, are capable of giving live birth. This is achieved, either through ovoviviparous (egg retention), or viviparous (babies born without use of calcified eggs).
Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals. They often provide considerable initial care for their hatchlings. This is very unlike their ancestors – the dinosaurs – who would leave their hatchlings to defend for themselves after laying their eggs.
Hylonomus (an early reptile from around 315 million years ago during the Carboniferous period) is the oldest-known reptile. It was about 8 to 12 inches (20 to 30 centimetres) long. Westlothiana (a reptile-like creature that lived during the Carboniferous Period about 350 million years ago) has been suggested as the oldest reptile, but is for the moment considered to be more related to amphibians than amniotes (a taxon of tetrapod vertebrates).
Petrolacosaurus (a small diapsid (two holes in side of their skulls) reptile, one of the earliest known) and Mesosaurus (one of the first reptiles to return to the water where its amphibian ancestors originally came from) are other examples.
The first true ‘reptiles’ (Sauropsids) are categorized as Anapsids (skull does not have openings near the temples), having a solid skull with holes only for nose, eyes, spinal cord, etc. Turtles are believed by some to be surviving Anapsids, as they also share this skull structure. However, this point has become quarrelsome lately, with some arguing that turtles reverted to this primitive state in order to improve their armour. Both sides have strong evidence and the conflict has yet to be resolved.
Shortly after the first reptiles, two branches split off, one leading to the Anapsids, which did not develop holes in their skulls. The other group, Diapsida, possessed a pair of holes in their skulls behind the eyes, along with a second pair located higher on the skull. The Diapsida split yet again into two lineages, the lepidosaurs (which contain modern snakes, lizards and tuataras, as well as, debatably, the extinct sea reptiles of the Mesozoic period) and the archosaurs (today represented by only crocodilians and birds, but also containing pterosaurs and dinosaurs).
The earliest, solid skulled amniotes also gave rise to a separate line, the Synapsida. Synapsids developed a pair of holes in their skulls behind the eyes (similar to the diapsids), which were used to both lighten the skull and increase the space for jaw muscles. The synapsids eventually evolved into mammals and are often referred to as mammal-like reptiles, though they are not true members of the class Sauropsida.