The first reptiles evolved in the Upper Carboniferous period, at least 300 million years ago. The Class Reptilia consists of three orders:
# Order Crocodilia, consisting of roughly 30 species and subspecies of crocodile, alligator and caiman
# Order Chelonia (tortoises and turtles), at least 244 species
# Order Squamata, includes lizards (over 3,750 species), snakes (about 3,000 species) and the lesser known Amphisbaenians or "worm lizard."
# There are about 160 species of Amphisbaenians, and they are found in Africa, Europe, Asia, North and South America. They are burrowing animals, up to 2 feet long, whose ring like scales gives them an earthworm like appearance.
# The fourth Order, Rhynchocephalia, flourished in pre-historic times but is now almost extinct. The number of species making up the single living genus, represented by the Tuatara, is still being argued.
# The Tuatara is extremely rare, found on just a few islands near New Zealand. Superficially lizard like, the Tuatara has unique eye and jaw anatomy, which among other factors separate it taxonomically (that is, the factors which determine its place in our classification of order, family, genus and species). Unusually for reptiles, Tuataras are adapted to life at temperatures as low as 6 degrees Celsius (43 degrees Fahrenheit).
Other interesting facts:
# Reptiles, like birds, have voluntary control over the muscles in their eyes, which determine their pupil size. This means that they are able to constrict or dilate their pupils at will, not just in response to light.
# The brain of a reptile is not more than 1 percent of his body mass. This means that the brain of a 70-pound python is no larger than a lima bean. Unlike amphibians, however, the reptilian brain has two hemispheres. Since man appears to use very little of his brain mass, the reptilian brain appears to be highly and efficiently adapted. The nervous systems of reptiles are sufficiently complex and similar to those of mammals that we can conclude that their senses and pain perception are highly refined. We are only beginning to understand just how highly specialized these animals are.
# Reptiles were the first vertebrates to evolve with 12 cranial nerves. "Lower" vertebrates have 10 pairs of these important nerves, which govern activities of the senses, such as sight, hearing and taste.
# The jaw structure of a reptile does not permit chewing; they can only tear their food.
# Some reptile species are known to store sperm and produce young 3 and perhaps 6 or more years after a single, successful mating. In some cases, it is possible to have an infertile clutch followed by a fertile clutch without further matings.
# The sex of a turtle is determined by the temperature at which the egg is incubated, with warmer temperatures producing females, cooler temperatures producing males and temperatures in the middle resulting in a mixed clutch. The situation is reversed for crocodiles, with males predominating at higher temperatures. The gender of a snake is determined by chromosomes, as it is in the case of mammals and birds.
# Reptiles do not have sweat or sebaceous glands; therefore, they are not slimy. They are, however, waterproof, and this, in addition to lacking a metamorphic stage is one of the distinctions between amphibians and reptiles (tadpoles metamorphose, or change, into frogs). The reptilian egg, with calcium in its shell is not dependant on development in water, as is the amphibian egg.
# Reptiles are not "cold blooded"; rather they are ectothermic animals, which rely on heat to be provided by the environment, as they produce little of their own. Behavior, such as seeking shade, gaping and shunting blood toward or away from the body surface allows impressively fine control of body temperature several degrees above or below the ambient temperature. When housed in suitable environments, most reptiles when active, maintain a body temperature similar to that of mammals.