Reptiles 2


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.

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