Fig. 328. Skull of a Permian Theromorph
At the opening of the Mesozoic era reptiles were the most highly organized and powerful of any animals on the earth. New ranges of continental extent were opened to them, food was abundant, the climate was congenial, and they now branched into very many diverse types which occupied and ruled all fields,—the land, the air, and the sea. The Mesozoic was the Age of Reptiles.
The ancestry of surviving reptilian types. We will consider first the evolution of the few reptilian types which have survived to the present.
Crocodiles, the highest of existing reptiles, are a very ancient order, dating back to the lower Jurassic, and traceable to earlier ancestral, generalized forms, from which sprang several other orders also.
Turtles and tortoises are not found until the early Jurassic, when they already possessed the peculiar characteristics which set them off so sharply from other reptiles. They seem to have lived at first in shallow water and in swamps, and it is not until after the end of the Mesozoic that some of the order became adapted to life on the land.
The largest of all known turtles, Archelon, whose home was the great interior Cretaceous sea, was fully a dozen feet in length and must have weighed at least two tons. The skull alone is a yard long.
Lizards and snakes do not appear until after the close of the Mesozoic, although their ancestral lines may be followed back into the Cretaceous.
We will now describe some of the highly specialized orders peculiar to the Mesozoic.