Evolution of reptiles

Reptiles arose about 320 million years ago during the Carboniferous period. Reptiles, in the traditional sense of the term, are defined as animals that have scales or scutes, lay land-based hard-shelled eggs, and possess ectothermic metabolisms. So defined, the group is paraphyletic, excluding endothermic animals like birds that are descended from early traditionally defined reptiles. A definition in accordance with phylogenetic nomenclature, which rejects paraphyletic groups, includes birds while excluding mammals and their synapsid ancestors. So defined, Reptilia is identical to Sauropsida.

Though few reptiles today are apex predators, many examples of apex reptiles have existed in the past. Reptiles have an extremely diverse evolutionary history that has led to biological successes, such as dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and ichthyosaurs.

The origin of the reptiles lies about 320–310 million years ago, in the swamps of the late Carboniferous period, when the first reptiles evolved from advanced labyrinthodonts. The oldest known animal that may have been an amniote, a reptile rather than an amphibian, is Casineria (though it has also been argued to be a temnospondyl amphibian).

A series of footprints from the fossil strata of Nova Scotia, dated to 315 million years, show typical reptilian toes and imprints of scales. The tracks are attributed to Hylonomus, the oldest unquestionable reptile known. It was a small, lizard-like animal, about 20 to 30 cm (8–12 in) long, with numerous sharp teeth indicating an insectivorous diet.

Other examples include Westlothiana (sometimes considered a stem-amniote rather than a true amniote) and Paleothyris, both of similar build and presumably similar habit. One of the best known early reptiles is Mesosaurus, a genus from the Early Permian that had returned to water, feeding on fish.

The earliest reptiles were largely overshadowed by bigger labyrinthodont amphibians, such as Cochleosaurus, and remained a small, inconspicuous part of the fauna until after the small ice age at the end of the Carboniferous.

It was traditionally assumed that first reptiles were anapsids, having a solid skull with holes only for the nose, eyes, spinal cord, etc.; the discoveries of synapsid-like openings in the skull roof of the skulls of several members of Parareptilia, including lanthanosuchoids, millerettids, bolosaurids, some nycteroleterids, some procolophonoids and at least some mesosaurs made it more ambiguous and it is currently uncertain whether the ancestral reptile had an anapsid-like or synapsid-like skull. Very soon after the first reptiles appeared, they split into two branches. One branch, Synapsida (including modern mammals), had one opening in the skull roof behind each eye. The other branch, Sauropsida, is itself divided into two main groups. One of them, the aforementioned Parareptilia, contained taxa with anapsid-like skull, as well as taxa with one opening behind each eye (see above). Members of the other group, Diapsida, possessed a hole in their skulls behind each eye, along with a second hole located higher on the skull. The function of the holes in both synapsids and diapsids was to lighten the skull and give room for the jaw muscles to move, allowing for a more powerful bite.

Turtles have been traditionally believed to be surviving anapsids, on the basis of their skull structure. The rationale for this classification was disputed, with some arguing that turtles are diapsids that reverted to this primitive state in order to improve their armor (see Parareptilia). Later morphological phylogenetic studies with this in mind placed turtles firmly within Diapsida. All molecular studies have strongly upheld the placement of turtles within diapsids, most commonly as a sister group to extant archosaurs.