The collective term “dinosaurs” (meaning terrible lizards) has been given to that distinctive group of reptiles prominent in Mesozoic life for some 140 million years. In size, the dinosaurs ranged from as little as 1 foot to as much as 85 feet in length and from a few pounds to perhaps 45 tons in weight. Some were _carnivorous_ (meat-eaters) but the majority were _herbivorous_ (plant-eaters). Some were _bipedal_ (walked on their hind-legs) while others were _quadrupedal_ (walked on all fours), and although most of the dinosaurs were terrestrial in habitat, aquatic and semi-aquatic forms were also present.
According to the structure of their hip bones, the dinosaurs have been divided into two great orders. These are the Saurischia (forms with lizard-like pelvic girdle) and the Ornithischia (dinosaurs with a bird-like pelvic girdle).
[Illustration: Plate 43 Dr. Brown, R. T. Bird, and Dr. Schaikjer with the skull of _Phobosuchus_, an extinct crocodile from the Cretaceous of Trans-Pecos Texas. Photograph courtesy of the American Museum of Natural History.]
Dinosaurs belonging to this order were particularly abundant during the Jurassic and are characterized by hip bones that are similar to those of modern lizards. These dinosaurs were first discovered in rocks of Triassic age and did not become extinct until the end of the Cretaceous. The lizard-hipped reptiles are divided into two rather specialized groups of dinosaurs: the _theropods_ (carnivorous bipedal dinosaurs that varied greatly in size) and the _sauropods_ (herbivorous, quadrupedal, semi-aquatic, usually gigantic dinosaurs).
This type of saurischian dinosaur walked on bird-like hind limbs, and they were exclusively meat-eating forms, such as _Allosaurus_ (Pl. 44) of Jurassic age. Some theropods were exceptionally large and were undoubtedly vicious beasts of prey. This assumption is borne out by such anatomical features as the small front limbs with long sharp claws for holding and tearing flesh, and the large strong jaws which were armed with numerous, sharp, dagger-like teeth. The largest of all known theropods was _Tyrannosaurus rex_ which, when standing on his hind limbs, was almost 20 feet tall. Some individuals were as much as 50 feet long, and _Tyrannosaurus_ is believed to have been among the most vicious animals to ever inhabit our earth. A cast of the skull of one of these great beasts is on display in the Texas Memorial Museum at Austin, and a _Tyrannosaurus_ tooth has been found in the Big Bend National Park in Trans-Pecos Texas.