Reptiles are vertebrate, or backboned, animals constituting the class Reptilia. They are characterized by a combination of features, none of which alone could separate all reptiles from all other animals. Among these features are (1) cold-bloodedness; (2) the presence of lungs; (3) direct development, without larval forms as in amphibians; (4) a dry skin with scales but not feathers (a characteristic of birds) or hair (a characteristic of mammals); (5) an amniote egg; (6) internal fertilization; (7) a three- or four-chambered heart; (8) two aortic arches (blood vessels) carrying blood from the heart to the body; mammals have only one aorta, the left; birds also have but one, the right; (9) a metanephric kidney; (10) twelve pairs of head (cranial) nerves; amphibians have ten; and (11) (skeletal features such as (a) limbs with usually five clawed fingers or toes, (b) at least two spinal bones (sacral vertebrae) associated with the pelvis; amphibians have but one, (c) a single ball-and-socket connection (condyle) at the head-neck joint instead of two, as in advanced amphibians and mammals, and (d) an incomplete or complete partition (the secondary palate) along the roof of the mouth, separating the food and air passageways so that breathing can continue while food is being chewed.
These and other traditional defining characteristics of reptiles have been subjected to considerable modification in recent times. The extinct flying reptiles, called pterosaurs, are now thought to have been warm-blooded and covered with hair; and the dinosaurs are also now considered by many authorities to have been warm-blooded. The earliest known bird, Archaeopteryx, is now regarded by many to have been a small dinosaur, despite its covering of feathers; and the extinct ancestors of the mammals, the therapsids, or mammallike reptiles, are also believed to have been warm-blooded and haired. Proposals have been made to reclassify the pterosaurs, dinosaurs, and certain other groups out of the class Reptilia into one or more classes of their own, and these issues are now receiving a great deal of attention from paleontologists and zoologists.
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