The Macmillan Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals

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, planteating dinosaurs some 85 ft/26 m long, and pterosaurs with a wingspan of some 40 ft/12.2 m. The sole surviving mem¬ bers 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 ornithosuchians. The crocodiles (see pp. 98101) descended from the same line, and probably also the flying pterosaurs (see pp. 102-105).