ORNITHOMIMUS

genus of ostrichlike dinosaurs (infraorder Tetanurae) found as fossils in Tibetan and North American deposits from the Late Cretaceous Period (97.5 to 66.4 million years ago). Ornithomimus was about 3.5 m (11.5 feet) long. With its slender, muscular rear limbs, it was well adapted for running and is believed to have used its long, outstretched tail for balance. Like other ostrich dinosaurs, Ornithomimus was toothless, having beaklike jaws. Its small, thin-boned skull had an unusually large brain cavity. For a time Ornithomimus was identified with Struthiomimus, but authorities have differentiated the two by the forelimbs (Struthiomimus' were much stronger) and by the age of the rocks bearing fossils (Ornithomimus is found in younger rocks). They are also believed to have inhabited different environments. See also Struthiomimus.