reptilium

A class of vertebrates composed of four living orders, the turtles or Chelonia, the tuatara or Sphenodonta, the lizards and snakes or Squamata, and the crocodylians or Crocodylia. Numerous extinct orders are also known. The group first appeared in the Carboniferous and underwent a culminating evolutionary radiation in the Mesozoic, often called the age of reptiles. Although the major portion of the class is now extinct, several Recent groups, particularly the Squamata, are very successful, and there are approximately 5000 living species of reptiles as compared to about 4000 living mammals. A classification is given below; see separate articles on each of the groups listed.

Class Reptilia

     Subclass Anapsida

          Order: Captorhinida

                    Mesosauria

                    Chelonia

     Subclass Diapsida

          Infraclass Sauropterygia

               Order: Nothosauria

                    Plesiosauria

                    Placodontia

          Infraclass Lepidosauria

               Order: Araeoscelida

                    Eosuchia

                    Sphenodonta

                    Squamata

          Infraclass Archosauria

               Order: Protorosauria

                    Rhynchosauria

                    Thecodontia

                    Crocodylia

                    Pterosauria

                    Saurisachia

                    Ornithischia

     Subclass Ichthyopterygia

     Subclass Synapsida

               Order: Pelycosauria

                    Therapsida

The reptiles are the most primitive of the completely terrestrial vertebrates and are consequently the first to exhibit amniote features. Reptile eggs are covered by a complex series of protective layers, including a leathery or calcareous shell. A rich supply of food material in the form of yolk is deposited inside the ovum to furnish food for the developing embryo. A series of protective extraembryonic membranes, the serosa and amnion, appears later in embryogenesis to protect the embryo from water loss and shock. A third such membrane, the allantois, functions as a storage sac for nitrogenous wastes. The serosa and allantois usually fuse to form a respiratory structure. Gaseous exchanges take place across the shell and seroallantoic membrane between the outside air and the blood vessels of the allantois. These adaptations allowed the reptile egg to be deposited on land, undergo its development there, and hatch into a fully developed form without a gilled larval stage. Most reptilian eggs are buried in the soil or in rotting vegetation out of direct sunlight. See also Amniota.

Paleoherpetology, the study of fossil reptiles, is especially important for two reasons. First, the class Reptilia lies at the center of vertebrate history; the reptiles evolved from the amphibians (which themselves had originated from the fishes), and both birds and mammals evolved from the reptiles. Thus reptiles are concerned in three of the four major “jumps” (the class-to-class transitions) in vertebrate evolution. The distinction between the living representatives of two successive classes is always very clear, being based on a number of features of their anatomy, physiology, and embryology; the distinction between their fossil members, however, is inevitably less clear, not only because the distinction must be based almost entirely upon characters of the skeleton but also because there must have been animals with a mixture of the characters of both classes during the transitional period. These help elucidate the reasons for the jumps and the precise mechanism by which each occurred. See also Amphibia; Animal evolution.

Second, the Reptilia were the dominant class of land vertebrates (and were also important in the sea and in the air) during a very long period of the Earth's history. Knowledge of the extinct reptiles, their morphology and their habits, is vital to an understanding of the life of those times, of how the animals and plants and the physical environment reacted upon each other (paleoecology). See also Paleoecology; Paleontology.