Essay on Reptiles Term Paper

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Essay on 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 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.

Leonardo da Vinci (I wish)

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