Historic Background of Reptiles (Last Part)

It pays a creature like man to keep in mind the vast time spans of the "unsuccessful" lives of these extinct groups. In evolution, of course, there is nothing novel about lines becoming extinct. Extinction is the rule, and survival the breathtakingly improbable exception. When you look about at the animals and plants on earth today, what you see are the favored few. For every single stock running down into the present, thousands have been cut off. This applies not just to species, but to genera and families, and even to orders. During the Mesozoic, there were times when 16 orders of reptiles lived on earth at one time. Today there are four. One of these, the tuatara, is a single species with a drastically diminished range on a few New Zealand islands. Two of the orders are decadent, and only one, the Squamata, has shown recent evolutionary vigour. In brief, out of the whole teeming array of Mesozoic reptiles, only these stocks, listed here in the order of their branching out from the main reptile stem, remain on earth today:
1) the mammals, which split off as the mammal like reptiles in the Permian, some 250 million years ago;
2) the turtles, which were probably derived during Permian times and were well developed by the Triassic;
3) the crocodilians, which were derived from Triassic archosaurs;
4) the tuatara, the only species of its order still alive, which has remained almost unchanged since its origin in Triassic cosuchian stock;
5) the lizards and the snakes-- the lizards having split off in the Upper Triassic, the snakes in the Cretaceous;
6) the birds, which first appeared in the Jurassic in the intermediate form of Archaeopteryx and almost certainly were derived from Triassic archosaurs.

If we had been alive in the Age of Reptiles the great sight would have been the sweep of the deployment -- the adaptive radiation of unprecedented types of life into unprecedented ways of living. Today, looking at what remains, we see the lesson of extinction and survival. The whole spirit of the grand, doomed days seems held in the small, cold body of the tuatara on the few chill islets of its dwindled range. Living on as unaccountably as the giants it once lived with died, it creeps out of its burrow in the evening, plods about in the mist to gather crickets to keep the old, small flame of its life alive; then, in the early morning, goes back again into the earth to which it has clung so stubbornly for some 200 million years.

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