The ornithomimds (also sometimes called 'ostrich dinosaurs’) were small dinosaurs that usually ranged between three and four meters long, although some were slightly larger. During the late Cretaceous ornithomimds seem to have been particularly widespread across North America and Asia where they relied upon speed for survival in habitats that were dominated by tyrannosaurs, and shared with horned and armoured dinosaurs like ceratopsians and ankylosaurs.
Amongst the dinosaurs Ornithomimus has one of the most complicated taxonomic histories that continue to cause much confusion amongst researchers. In a nut shell, because it was one of the first ornithomimds discovered, Ornithomimus along with Struthiomimus were effectively used as wastebasket taxons where any remains remotely resembling the earlier material were almost automatically attributed to one of these genera as a new species. This also happened for the first two dinosaur genera ever named, Megalosaurus and Iguanodon, although perhaps a better analogy would come from the pterosaurs Pterodactylus and Rhamphorhynchus. Both of these genera once had an incredible number of species attributed to them, just for most of them to be found to be the same as the type species or actually different genera completely.
It is actually a lot easier to talk about Ornithomimus as a dinosaur rather than its taxonomic history. Like with all other ornithomimids, Ornithomimus is noted for having long legs, the lower portions of which were considerably longer than the femur. This is a sure sign that Ornithomimus was a dinosaur that was built for speed and with the exception of other ornithomimids, was probably the fastest dinosaur in its ecosystem. This speed would have probably been enough to keep it out of the way of big predators like Tyrannosaurus and Albertosaurus, although it would be interesting to see how Ornithomimus faired against smaller and faster predatory dinosaurs, including juveniles of the big tyrannosaurs. To help reduce weight the bones of Ornithomimus, like other ornithomimids, were hollow. Another adaptation to fast running is the tail which in life would have been carried off the ground and supported with tendons so that it could be used as a stiff balancing aid.
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