Historic Background of Reptiles (Part 3)

For the embryo to thrive, such an egg must be kept warm and not too dry, and must never be submerged for long in the water. Thus the shelled egg, laid on land, is a kind of private secluded pool in which a tender, developing reptile can go through its perishable stages in much the same environment its ancestors lived in. The egg itself is a terrestrial adaptation, but the animal it harbours is still acquatic. The shelled egg is thus the essence of the reptiles. No amount of protection of any animal's adult stages will allow real pioneering on shore if the embryos remain unprotected. The adult's scales reduce water loss from the body, or it can move about to damp places to avoid drying up. But an embryo starts out as a helpless cell, almost without structure and without any capacity for movement at all, and it must be protected if it is to survive.

One of the frustrating features of the fossil record of vertebrate history is that it shows so little about the evolution of reptiles during their earliest days, when the shelled egg was developing. There are three fundamental body features that the earliest land vertebrates had, and it would be very nice if we could draw a neat, clear, logical picture of their development. These features are legs, lungs, and a connection between nostril and pharynx that allowed animals to breathe with their mouths shut. Unfortunately for logic, these things were not started by reptiles, nor even by the earlier amphibians. We must go even farther back, to the fishes, to find their beginnings. In trying to account for this, alternative explanations come to mind. Either the fishes that had the structures used them for some hidden purpose lost in time; or their bearers actually did use them for what seems the obvious purpose of such an adaptation--going out on land. The latter is what palaeontologists believe. The old lung-bearing, nose-breathing, stalk-finned fishes, known as Crossopterygii, remained fishes, but they apparently acquired the new adaptations in order to survive in the uncertainty of a shore-water environment that was subject to periods of drying up.

To be continued...

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