Dinosaurs and their kin

The dinosaurs and their kin — the crocodiles and flying pterosaurs — dominated the air, land and waters of the earth during the Mesozoic Era. This "Age of the Ruling Reptiles" began more than 200 million years ago, and ended some 65 million years ago. During this span of almost 140 million years, some of nature's most awesome beasts evolved — carnivorous dinosaurs standing 20 ft/6 m tall, plant-eating dinosaurs some 85 ft/26 m long, and pterosaurs with a wingspan of some 40ft/12.2m. The sole surviving members of this great assemblage of "ruling reptiles" (the archosaurs) are today's crocodiles, with an evolutionary history stretching back some 230 million years.

The first step in the evolution of the ruling reptiles was taken in the Late Permian period, some 250 million years ago. A new line of small, diapsid reptiles evolved, called the protorosuchians (see p. 89). From them radiated a variety of reptiles called the thecodontians (see pp. 94-97). They thrived during the subsequent Triassic period, and some of their members became progressively more skilled at walking upright on 2 legs. The dinosaurs (see pp. 106-169) arose from such a stock of 2-legged thecodontians, called the ornitho-suchians. The crocodiles (see pp. 98-101) descended from the same line, and probably also the flying pterosaurs (see pp. 102-105).

Dinosaurs: Past masters of the world

The first-recorded dinosaur bones were found in the rocks of southern England, and belonged to the giant carnivore Megalosaurus (see p. 116) and the giant herbivore Iguanodon (see p. 144). They were described to the scientific community in 1824 and 1825. More than a decade was to pass before it was realized that they were representatives of a totally different, and totally extinct, group of reptiles. It was Sir Richard Owen, the

The "Age of the Ruling Reptiles" spanned the Jurassic and Cretaceous periods, when the great dinosaurs dominated the land, the pterosaurs ruled the skies, and the crocodiles flourished in the seas and rivers. The ancestor of all these reptiles was a small, 2-legged thecodontian, one of the ornithosuchians.

With the exception of the crocodiles, all these ruling reptiles, great and small, had perished by the end of the Cretaceous period — the close of the Mesozoic Era. The birds probably evolved from among the lizard-hipped (saurischian) dinosaurs. (For key to silhouettes, see p. 312.)

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