Another problem that arises from life on land is the influence of gravity. Whereas fishes are supported by the dense water in which they live and have developed a system of locomotion through body movements and fin stabilization, the first land-dwelling vertebrates had to evolve a system of moving over the land and supporting the body in the relatively thin atmosphere. A strong backbone and sturdy limbs were developed in the earliest stages of their evolution. The vertebral column was supported by a pectoral girdle at the front (which in turn was supported  by the arms and the hands) and a pelvic girdle at the rear part of the body (which was supported by the legs and the feet). These limbs served not only to hold the body in counter-reaction to gravitational pull, but were the means by which the animal could propel itself over the land surface. The early amphibians retained a tail, which probably functioned as an organ of balance. In addition, the tail could still be used as an organ of propulsion when the animal returned to a watery environment. Although amphibians made a major contribution to life on land, they never really found a satisfactory solution to terrestrial reproduction. Throughout their history these animals have had to return to water or at least damp areas in order to lay their eggs.

From the original ichthyostegids a number of lines evolved that not only eventually led to our modern amphibians but also radiated into the ancestral lines of reptiles, birds, and mammals. Most of the earlier amphibians were large salamander-like creatures.

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The body of the vertebrae is called a “centrum ;  the space between centra is the  intercentrum.  The vertebral centra may be one of two types , either amphicouelous ( biconcave) or procoelus ( concave anterior , convex posterior).  The backbone runs the entire length of the tail, but should this member be lost. the replacement will forever be devoid of true vertebrae.  Along the rail section , when true vertebrae are present. one can locate the plane of autonomy, which is usually a cartilaginous plate before or behind the transverse process of the vertebra.

The forelimbs are supported from the pectoral girdle.  The humerus bone articulates at a point between the sternum and interscapula.  Similarly the femur fits into an opening of the ischium.  At the lowest point of the pectoral girdle is the sternum ,  a bone which protects a good part of the internal organs.  Between the sternum and the pelvis may be a bony or cartilaginous union of the ribs, known as the parasternum.  Extending from beneath the scapula  and uniting with the sternum is the clavicle , a bone always present in pairs.

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