alone could separate all reptiles from all other animals.
The characteristics of reptiles are numerous, therefore can not be
characteristics of reptiles are: cold-bloodedness; the presence of lungs;
scales but not feathers or hair; an amniote egg; internal fertilization; a
one; a metanephric kidney; twelve pairs of cranial nerves; and skeletal
features such as limbs with usually five clawed fingers or toes, at least
two spinal bones associated with the pelvis, a single ball-and-socket
connection at the head-neck joint instead of two, as in advanced amphibians
and mammals, and an incomplete or complete partition 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 or pterodactyls, are now thought to have been
warm-blooded and covered with hair. Also, 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 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.
of these subclasses are completely extinct. The subclasses contain about 24
orders, but only 4 of these are still represented by living animals.
Of the living orders of reptiles, two arose earlier than the age of
reptiles, when dinosaurs were dominant. Tuataras, of the order
ancient turtles, order Chelonia, occur nearly worldwide. The order
Crocodilia emerged along with the dinosaurs. Snakes and lizards, order
The Rhynchocephalia constitute the oldest order of living reptiles; the
only surviving representative of the group is the tuatara, or sphenodon
(Sphenodon punctatus). Structurally, the tuatara is not much different from
related forms, also assigned to the order Rhynchocephalia, that may have
appeared as early as the Lower Triassic Period (over 2 000 000 000 years
ago). The tuatara has two pairs of well-developed limbs, a strong tail, and
a scaly crest down the neck and back. The scales, which cover the entire
animal, vary in size. The tuatara also has a bony arch, low on the skull
behind the eye, that is not found in lizards. Finally, the teeth of the
tuatara are acrodont – i.e., attached to the rim of the jaw rather than
inserted in sockets.
Chelonia, another ancient order of reptiles, is chiefly characterised by a
the head and limbs. The protective shell, to which the evolutionary success
of turtles is largely attributed, is a casing of bone covered by horny
shields. Plates of bone are fused with ribs, vertebrae, and elements of
shoulder and hip girdles. There are many shell variations and modifications
from family to family, some of them extreme. At its highest development, the
shell is not only surprisingly strong but also completely protective. The
lower shell (plastron) can be closed so snuggly against the upper (carapace)
that a thin knife blade could not be inserted between them.
A third order of the class Reptilia is Crocodilia. Crocodiles are generally
large, ponderous, amphibious animals, somewhat lizardlike in appearance, and
clawed, webbed toes. The tail is long and massive and the skin thick and
plated. Their snout is relatively long and varies considerably in
proportions and shape. The thick, large horny plates that cover most of the
body are generally arranged in a regular pattern. The form of the is adapted
to its amphibious way of life. Finally, the elongated body with its long,
muscular paddletail is well suited to rapid swimming.
The final living order of the class Reptilia is Squamata. Both snakes and
lizards are classified in this order, but lizards are separated into their
own suborder, Sauria. Lizards can be distinguished from snakes by the
presence of two pairs of legs, external ear openings, and movable eyelids,
but these convenient external diagnostic features, while absent in snakes,
are also absent in some lizards. Lizards can be precisely separated from
snakes, however, on the basis of certain internal characteristics. All
lizards have at least a vestige of a pectoral girdle (skeletal supports for
the front limbs) and sternum (breastbone). The lizard’s brain is not
totally enclosed in a bony case but has a small region at the front covered
symmetrically and to the rear; in snakes the kidneys are far forward, with
the right kidney placed farther front than the left. Finally, the lizard’s
ribs are never forked, as are one or two pairs in the snake.
A natural classification of reptiles is more difficult than that of many
Earth); 13 of 17 recognized orders are extinct. There is still little
agreement on reptile taxonomy among herpetologists and paleontologists.
Even the major categories of reptile classification are still in dispute. On
the other hand, there is general agreement that the base reptilian stock is
the Cotylosauria, which evolved from an amphibian labyrinthodont stock. It
is also quite clear that the coty losaurs early divided into two lines, one
of which (the pelycosaurs) represented the stock that gave rise to the
mammals. Another branch led to all of the other reptiles, and later, to the
birds as well. Thus, most of the questions of reptilian evolution and
classification deal with the reptiles’ interrelationship, rather than with
their relationships with other animals.