Reptiles

Abstract

Reptiles are a class of vertebrate animals characterized by a skin composed of dry scales. They appear in the fossil record during the Paleozoic era (about 300 million years ago), constituted a major fraction of the vertebrate fauna during the Mesozoic era, but have decreased in number since Mesozoic times. Recent reptiles fall into four orders: Chelonia (turtles and tortoises), Crocodilia (alligators, crocodiles, caimans, and gavials), Squamata (snakes, lizards, and amphisbaenians) and Rhynchocephalia (the tuatara or Sphenodon punctatum). Mammals derive from an evolutionary lineage that diverged from the stem reptiles prior to the Mesozoic era. Thus, none of the four orders of recent reptiles are ancestral to mammals. The central nervous system of reptiles consists of a spinal cord and brain whose total weight per unit body weight is generally intermediate between those of fishes and amphibians, on one hand, and birds and mammals, on the other. As in other vertebrates, the brain develops from five embryonic vesicles: the myelencephalon, metencephalon, mesencephalon, diencephalon, and telencephalon. The following sections survey the spinal cord and brain in reptiles.