As dinosaur names go, ornithomimids (Greek for "bird mimics") is a bit misleading: these small-to-medium-sized theropods weren't named for their similarity to flying birds like pigeons and sparrows, but to large, flightless birds like ostriches and emus. In fact, the typical ornithomimid body plan looked a lot like that of an ostrich: long legs and tail, a thick, rounded trunk, and a small head perched atop a slender neck. (See a gallery of ornithomimid pictures.)
Because ornithomimids like Ornithomimus and Struthiomimus bear such a marked resemblance to modern ratites (as ostriches and emus are technically known), there's a strong temptation to infer similarities in the behavior of these two very different types of animals. Paleontologists believe that ornithomimids were the fastest dinosaurs that ever lived, some long-legged varieties (such as Dromiceiomimus) capable of hitting speeds of 50 miles per hour. There's also a strong temptation to picture ornithomimids as covered with feathers, though the evidence for this isn't as strong as for other families of theropods, such as raptors and therizinosaurs.
Ornithomimid Behavior and Habitats
Like a few other dinosaur families that prospered during the Cretaceous period--such as raptors, pachycephalosaurs and ceratopsians--ornithomimids seem to have been confined mainly to North America and Asia, although some specimens have been dug up in Europe, and one controversial genus (Timimus, which was found in Australia) may not have been a true ornithomimid at all. In keeping with the theory that they were fast runners, these theropods most likely inhabited ancient plains and lowlands, where their pursuit of prey (or retreat from predators) wouldn't be impeded by thick vegetation.
The most unusual characteristic of ornithomimids was their omnivorous diets. These were the only theropods we yet know of, besides the therizinosaurs, that evolved the ability to eat vegetation, as evidenced by the gastroliths found in the fossilized guts of some specimens. Since later ornithomimids had weak, toothless beaks, it's believed that these genera fed on insects, small lizards and mammals as well as plants. (Interestingly, the earliest ornithomimids--Pelecanimimus and Harpymimus--possessed teeth, the former over 200 and the latter a mere dozen.)
Despite what you've seen in movies like Jurassic Park, there's no solid evidence that ornithomimids scurried across the North American plains in vast herds (although hundreds of Gallimimus running from a pack of tyrannosaurs at top speed would certainly have been an impressive sight!) As with many types of dinosaurs, we know frustratingly little about the daily life of ornithomimids, a state of affairs that may well change with further fossil discoveries.
Here's a list of the most notable ornithomimids; just click on the links for more information.
Anserimimus This "goose mimic" didn’t bear much of a resemblance.
Archaeornithomimus A likely ancestor of Ornithomimus.
Deinocheirus All we know for sure about this carnivore is the shape of its arms.
Dromiceiomimus Possibly the fastest dinosaur that ever lived.
Gallimimus This "chicken mimic" roamed the plains of the late Cretaceous.
Garudimimus A relative slowpoke compared to other ornithomimids.
Harpymimus Named after the winged creature of Greek myth.
Ornithomimus This "bird mimic" resembled a modern ostrich.
Pelecanimimus This "pelican mimic" sported over 200 teeth.
Sinornithomimus This ornithomimid is known from over a dozen skeletons.
Struthiomimus This "ostrich mimic" roamed the plains of North America.
Timimus The only ornithomimid ever discovered in Australia.