Reptiles 2 Essay, Research Paper
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 numerous, therefore can not be
explained in great detail in this report. In no special order, 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 characterised 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 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 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.