320 Million B.C.T. - Reptiles Emerged

Around 320 Million B.C.T., the first reptiles evolved.

Reptiles arose about 310-320 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 (outside heat reliant) metabolisms. So defined, the group is paraphyletic, excluding endothermic (internal heat reliant) animals like birds and mammals that are descended from early reptiles. A definition in accordance with phylogenetic nomenclature, which rejects paraphyletic groups, includes birds while excluding mammals and their mammal-like reptile ancestors.

A group is said to be paraphyletic if it consists of all the descendants of the last common ancestor of the group's members minus a small number of monophyletic groups of descendants, typically just one or two such groups. For example, the group of reptiles, as traditionally defined, is paraphyletic: it contains the last common ancestor of the reptiles — including the extant reptiles as well as the extinct mammal-like reptiles — along with all descendants of that ancestor except for mammals and birds.

Though few reptiles today are apex predators, many examples of apex reptiles have existed in the past. Apex predators (also known as alpha, super, top- or top-level predators) are predators with no predators of their own, residing at the top of their food chain.  Reptiles have an extremely diverse evolutionary history that has led to biological successes such as dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and ichthyosaurs.

Reptiles first arose from amphibians in the swamps of the late Carboniferous Period. 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; and, thus, 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.

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 an 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 (for the moment considered a reptiliomorph amphibian 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.