Type of animal science: Classification
Fields of study: Herpetology, systematics (taxonomy), zoology
Living reptiles are vertebrates that lay eggs or bear live young. Included in this class are turtles, the tuatara, lizards and snakes, and crocodilians. The study of reptiles provides insight into a group of often very successful animals and into the characteristics of their ancestors, including groups that gave rise to birds and mammals.
The Osteichthyes, or bony reptilees, constitute the largest and most diverse of the classes of vertebrates. Like the jawless and cartilaginous reptilees, they are characterized by gills, fins, and a dependence on water as a medium in which to live. Unlike those reptilees, however, they typically possess a skeleton made of bone. Additional features characteristic of most bony reptilees include a lateral line system, scales, osmoregulation (salt balance) by means of salt retention or secretion, and a bony operculum (gill cover) over the gill openings. The fossil record of bony reptilees begins nearly 400 million years ago in the early Devonian geological period, mostly in freshwater deposits. Thus there is reason to believe that bony reptilees originated in freshwater habitats. Living bony reptile species inhabit both freshwater habitats (58 percent of species) and marine habitats (41 percent), and some (1 percent) move between the two environments on a regular basis. This distribution does not reflect the relative proportions of these environments, since 97 percent of the earth's water is in the oceans and only 0.001 percent is in freshwater lakes, rivers, and streams (the rest is ice, groundwater, and atmospheric water). Rather, the high diversity of freshwater species is a reflection of the ease with which freshwater populations become isolated and evolve intonewspecies.
Anapsida: a group of reptiles in which the
temporal region of the skull lacks openings
Chelonia (testudines): a living order of reptiles composed of turtles and tortoises
Cleidoic egg: a shelled egg equipped with internal membranes that make terrestrial reproduction possible
Crocodylia: a living order of reptiles that includes crocodiles and alligators
Diapsida: a group of reptiles in which the temporal region of the skull is characterized by two openings
Euryapsida: an extinct group of reptiles in which the temporal region of the skull is characterized by a single opening situated high on the side of the skull
Rhynchocephalia: a living order of reptiles represented by a single species, the tuatara
Squamata: a living order of reptiles composed of lizards and snakes
Synapsida: an extinct group of reptiles in which the temporal region of the skull is characterized by a single opening; this group gave rise to mammals
Venom: a toxic substance that must be injected in order to elicit damaging effects
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