The Evolution Of Reptiles
The origin of the reptiles lies about 320–310 million years ago, in the steaming swamps of the late Carboniferous period, when the first reptiles evolved from advanced reptiliomorph 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).
Reptiles first arose from amphibians in the swamps of the late Carboniferous. Increasing evolutionary pressure and the vast untouched niches of the land powered the evolutionary changes in amphibians to gradually become more and more land based.
Environmental selection propelled the development of certain traits, such as a stronger skeletal structure, muscles, and more protective coating (scales) became more favorable; the basic foundation of reptiles were founded. The evolution of lungs and legs are the main transitional steps towards reptiles, but the development of hard-shelled external eggs replacing the amphibious water bound eggs is the defining feature of the class Reptilia and is what allowed these amphibians to fully leave water. Another major difference from amphibians is the increased brain size, more specifically, the enlarged cerebrum and cerebellum. Although their brain size is small when compared to birds and mammals, these enhancements prove vital in hunting strategies of reptiles. The increased size of these two regions of the brain allowed for improved motor skills and an increase in sensory development.
Anapsids, Synapsids, Diapsids and Sauropsids
A = Anapsid, B = Synapsid, C = Diapsid. 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's 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 both "mammal-like reptiles" and modern, extant 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.
The main points to the transition from reptile to bird are the evolution from scales to feathers, the evolution of the beak (although independently evolved in other organisms), the hallofication of bones, development of flight, and warm-bloodedness.
The evolution of birds is thought to have begun in the Jurassic Period, with the earliest birds derived from theropod dinosaurs. Birds are categorized as a biological class, Aves. The earliest known species in Aves is Archaeopteryx lithographica, from the Late Jurassic period. Modern phylogenetics place birds in the dinosaur clade Theropoda. According to the current consensus, Aves and Crocodilia are the sole living members of an unranked clade, the Archosauria.