The ornithomimosaurs (members of the clade Ornithomimosauria, or “bird mimics”) were a theropod dinosaur that—from general superficial appearances—resembled modern-day ratite birds such as ostriches. These dinosaurs were slender, with flexible necks, small heads, toothless beaks, long slim hind limbs, elongated arms, and grasping hands with three powerful fingers. Ornithomimosaurs arose in the Late Jurassic period and died out at the end of the Cretaceous.

These dinosaurs were not limited to small sizes; some could reach up to 20 feet (6 meters) in length. Because of their lower leg bones and foot structure, some scientists think that these dinosaurs, like the ostrich, could run very fast. Estimates for one species, the Struthiomimus, range from 22 to 37 miles (35 to 60 kilometer) per hour. Other ornithomimosaurs include Gallimimus, Anserimimus, and Ornithomimus.