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Reptile is a member of the class of animals which includes
the snakes, lizards, turtles, crocodilians, and the
tuatara. The crocodilians include alligators, crocodiles,
the gavial of Asia, and the caymans of tropical America.
The tuatara, a large reptile, looks like a lizard and lives
on islands off the coast of New Zealand. It is the last
survivor of a whole group of ancient reptiles. 

About 6,000 different kinds of reptiles live throughout the
world. They are of many different sizes shapes, and habits,
because the reptiles have gone through many changes since
their ancestors lived millions of years ago. The reptiles
seen today resemble the twigs of a great tree of reptile
life. They are not new branches, but descent directly from
ancient animals. The four large separate limbs are the
turtles, crocodilians, tuatara, and the snakes and lizards
together. the huge dead limbs include such reptiles as the
It is not fully known why one reptile branch died out while
another survived. The lizards and snakes together form the
only large branch today. This branch includes about 95
percent of all the different kinds of living reptiles. 

Reptiles are vertebrate, or backboned animals constituting
the class Reptilia and are characterized by a combination
of features, none of which alone could separate all
reptiles from all other animals.
The characteristics of reptiles are: cold-bloodedness; the
presence of lungs; direct development, without larval forms
as in amphibians; a dry skin with scales but not feathers
or hair; an amniote egg; internal fertilization; a three or
four-chambered heart; two aortic arches (blood vessels)
carrying blood from the heart to the body, unlike mammals
and birds that only have 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. 

The class Reptilia is divided into 6 to 12 subclasses by
different authorities. This includes living and extinct
species. In addition, a number 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 Rhynchocephalia, are found only on
New Zealand islands, whereas the equally ancient turtles,
order Chelonia, occur nearly worldwide. The order
Crocodilia emerged along with the dinosaurs. Snakes and
lizards, order Squamata, are today the most numerous
reptile species.
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
characterized by a shell that encloses the vital organs of
the body and more or less protects 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 snugly 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
carnivorous. They have powerful jaws with conical teeth and
short legs 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 only by a
membranous septum. The lizard's kidneys are positioned
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 animals because the main evolution of the
group was during Mesozoic time (a time of transition in the
history of life and in the evolution of the 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.



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