Members of the former group possess a reptile-like pelvis and are mostly bipedal and carnivorous, although some are giant amphibious quadrupedal herbivores. The Ornithischia were almost all plant-eaters, and eventually outnumbered the Saurischia. All five orders are thought to have evolved from a 'stem-order', the Thecondontia . Compsognathus, a meat-eater, was only the size of a chicken, and ran on its hind legs. Not all dinosaurs had small brains.
Many dinosaurs appear to have been equipped for a high level of activity.
The largest carnivorous dinosaur was Giganotosaurus carolinii . Its skeleton was discovered 1995. Theories of extinction A popular theory of dinosaur extinction suggests that the Earth was struck by a giant meteorite or a swarm of comets 65 million years ago and this sent up such a cloud of debris and dust that climates were changed and the dinosaurs could not adapt quickly enough. An alternative theory suggests that changes in geography brought about by the movements of continents and variations in sea level led to climate changes and the mixing of populations between previously isolated regions. This resulted in increased competition and the spread of disease.