the most numerous superorder of Mesozoic extinct reptiles of the subclass Archosauria. It is divided into two orders: Saurischia and Ornithischia, which include seven suborders comprising several hundred species.
The earliest dinosaurs are known from the middle Triassic, and the last ones lived to the end of the Cretaceous period. Morphologically and ecologically the dinosaurs are an extremely varied group. The ancestors of the dinosaurs were the more primitive Triassic archosaurs, the Thecodontia, from which the dinosaurs inherited the capacity for locomotion on their two hind limbs (bipedalism), which in time was reinforced in some (predatory dinosaurs and Ornithopoda) and lost in others (Sauropoda, Stegosauria, Ankylosauria, and Ceratopsia). Initially, all dinosaurs were land predators, but by the end of the Triassic the Prosauropoda branched off. Possibly omnivorous, the prosauropod dinosaurs gave rise to the gigantic herbivorous sauropods, which underwent a change and became inhabitants of large intracontinental water basins or the coastal regions of the sea. The Heterodontosauria, with front teeth as in the predatory dinosaurs, and back teeth as in the ornithopods, appeared during the Triassic. The heterodontosaurs probably gave rise to the ornithischian dinosaurs, all of which were herbivorous. The majority of ornithischians (except the ornithopods, which became, as did the sauropods, water dwellers) developed protective adaptations against predatory dinosaurs (spikes and bony armor in stegosaurs and ankylosaurs and horns in ceratopsians). Dinosaurs reproduced by means of eggs, whose shells are found quite often. Possibly they were warm-blooded animals and even possessed other indications of high physiological organization.
Dinosaur remains—numerous skeletons, skulls, and the like—are found on all continents. In the USSR they are found primarily in Kazakhstan as well as in Tadzhikstan and Kirghizia (in the vicinity of Fergana). The study of dinosaurs is important in gaining knowledge about the evolution of vertebrates. The wide geographic distribution of dinosaurs and the rapid replacement of species in time makes them valuable guiding forms for determining the geological age of continental deposits of the Mesozoic. Over the entire Mesozoic new groups were constantly appearing to replace certain species, genera, families, and suborders. The time of the disappearance of the last dinosaurs corresponds to the boundary between the Mesozoic and Cenozoic. The principal reasons for the extinction of dinosaurs are believed to be unfavorable changes in the habitation environment (redistribution of water and land on the earth’s surface, with the possible disappearance of natural zones of habitation, change of climate, disturbance in the balance of flora and fauna, that is, the food source, and so forth) primarily caused by geological transformations of the earth’s crust, which were very significant during the Mesozoic, especially in the Late Cretaceous.
A. K. ROZHDESTVENSKII