Crocodiles And Their Rellatives

Crocodiles, alligators, caimans, and gavials have been on earth almost unchanged in form and habits since the great age of Reptiles, more than 160,000,000 years ago. They are reminders of that fardistant past, when there were great numbers of fierce-looking armored animals that have long since become extinct. In that remote, almost unimaginable era, the "ruling lizards" , or archosaurus. Among these were the dinosaurs, the crocodiles, and the reptilian ancestors of our birds. Although these animal had four four legs, the hind legs were long and powerful and the front the front legs were short. This caused them to develop what is called bipedal habits. When they really wanted to get somewhere in a hurry, they would rear up and lumber along on their hind legs. They were heavily protected by thick, horny armor, just as are their descendants in tropical swamps and rivers today.

Modern crocodiles use all four legs to heave their cumbrous bodies along the ground. But their front legs are short, like those of the crocodiles that lived in the Age of Reptiles. When they swim they fold their legs close to their bodies and move through the water by means of powerful strokes of their tails. The hind feet are webbed in most species. Sometimes the forefeet are also webbed.
Crocodiles and other living members of the subclass Archosauria form the order Crocodilia. These animals are found only in temperate and tropical countries. The crocodile itself in parts of five continents.: Africa,Asia, the tropical parts of North and South America, and northern Australia, as well as in the long chain of islands that extended southeastward from the Malay Peninsula. Alligators live in China and the southeastern United States. The caiman lives in Central and South America. The gavial, or gharial is found primarily in India.To teh anatomist, the vital organs of crocodiles are particular interest , for, like those of the birds, they are the reminder of the great dinosaurs. This especially true of the arrangement and structure of the hearts and lungs, which are highly specialized in these animals.
Laying Eggs On Land
Another characteristic that the crocodile order shares with the birds is that they hatch their young from eggs. A crocodile may lay from 20-90 eggs in one clutch. The eggs have a thick, tough shell and the young crocodile is equipped with a strong egg tooth at the end of its snout to enable it to chip its way out of the shell when  the time comes.This egg tooth drop off almost immediately after the baby is hatched, and the youngster itself  is at once completely able to cope with the world about it without any help from the parent crocodiles.
The laying of tough-shelled eggs on land was one of the very significant steps in the evolution of vertebrates. This habit freed the animals  who practiced it from a complete dependence on water, thus making possible the conquest of land. It is true that the early amphibians could leave the water and remain away for some time, but  they had to return in order to reproduce-that is, to provide a watery environment in which their eggs could develop. But when pioneering descendants of the amphibians began to lay their eggs on land, a new dimension was given to the life of vertebrates. The reptiles inherited  the earth.

The tough-shelled reptilian egg contained its own watery medium, the amniotic fluid. This provided for the needs of the embryo just as the aquatic environment had provided for the developing young of amphibians. A large store of food, in the form of yolk, sustained the reptilian embryo. The porous shell allowed for the exchange of oxygen and carbon dioxide yet kept the amniotic fluid from leaking out. When the development of the embryo was completed, the young reptile, a miniature of its parents, emerged from the egg.