cavity cavity

Land adaptations for adult reptiles

Although protection of the developing embryo from drying out was a great step forward in the conquest of the land, 2 other innovations were necessary before that conquest was complete. First, reptiles had to be protected from desiccation after they emerged from the egg. This was achieved by evolving a horny layer that covered their scales or armor, making them impermeable to water loss.

Second, in order to remain active, reptiles had to develop a more efficient breathing method than that of their amphibian ancestors. Amphibians ven-. tilate their lungs by means of a throat-pump, which forces air into the lungs. Reptiles developed a new system, in which the rib cage was expanded and contracted, resulting in air being sucked into the lungs, and then expelled. The capacity of this system was limited only by the volume of the lungs, not merely by the volume of the mouth.

However, even with all these adaptations, living reptiles are still, like amphibians, limited in one respect. They are "cold-blooded" — that is, they obtain nearly all of their energy from the heat of the sun. When the weather or climate becomes cooler, their body temperature is lowered, and they become inactive. The physiology of reptiles is, therefore, geared to a low and varying body temperature, and they cannot sustain prolonged periods of activity.

In contrast, "warm-blooded" birds and mammals obtain their energy from

There are 4 membranes inside the shelled egg of a reptile — the amnion, chorion, allantois and yolk sac. Each plays a particular role to enable the embryo — in this case a turtle — to develop to maturity within its protective egg, independent of its surroundings.

The developing embryo is suspended in a fluid-filled cavity surrounded by the amnion. It receives nourishment from the surrounding yolk sac through blood vessels connected to its gut. Waste products are excreted into the allantoic cavity. Oxygen enters the egg via the chorion, which lies just beneath the egg's porous shell.

their food, and have a high, constant body temperature, independent of their surroundings, which allows them to sustain a high rate of activity for much longer periods. Paleontologists are currently debating whether dinosaurs were cold- or warm-blooded (see p. 93).

Like their amphibian ancestors, and the fish before them, reptiles move by lateral flexure of their bodies (below). The upper parts of a reptile's limbs project laterally from the body, since the length of each stride depends on the distance across the body from one knee or elbow joint to the other. The feet are also angled somewhat to the side, to resist the lateral forces produced by this type of movement. The toes have to be of different lengths if they are to leave the ground at the same time, so that the weight of the body is shared evenly between them.

As a result of all these adaptations, both structural and physiological, reptiles were able to colonize the land to the full, living even in the hottest deserts. The peak of their evolutionary development was reached in the form of the great dinosaurs, which grew to sizes that even their successors, the mammals, were unable to rival.

Radiation of the reptiles

The reptiles evolved from the amphibians some time in the Late Carboniferous (Pennsylvanian) period. The earliest-known reptile is Hylonomus (see p. 64), preserved in the Late Carboniferous rocks of Nova Scotia. These rocks are about 300 million years old — some 60 million years after the first amphibian Ichthyostega crawled out of the water to form the spearhead for the invasion of the land (see p. 52).

From Hylonomus, many different types of reptile evolved. Like the early amphibians, all of the early reptiles seem to have been confined to the ancient continent of Euramerica (see p. 49). Fortunately for paleontologists, most of these different lineages of reptile can be distinguished by the pattern of openings in the skull (see p. 61).

In addition to skull structure, evidence for the interrelationships of reptiles can be seen in the structure of

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