Ornithiomimosaurs (or more commonly, though less correctly, ornithomimids) first came to the attention of the general public with the appearance of Gallimimus in Jurassic Park, when before then they’d been largely overlooked. One can see why – they’re not especially big, lack big teeth, crests, horns or big display structures and they’re probably not predatory either. For a theropod in competition with Allosaurus, Tyrannosaurs, Deinonychus and the rest for a bit of media, or even museum, coverage you can see why they might loose out. Still, like pretty much any living organism (or previously living organism) they have their interesting features once you start to dig down and their share of unresolved mysteries.
In gross morphology they are quite big by human standards, but pretty small by theropod standards with a typical animal being 3-4 m long and standing about 2 high or so to the top of the head. They are pretty gracile with relatively small bodies, long arms, legs, and a long neck with a small (and usually toothless) head that housed large eyes and a fairly large brain. The legs are, in fact, especially long and the group are justifiably considered likely to be the fastest of dinosaurs.
The hands and arms are also quite big with relatively large claws on the fingers, which is a bit of a surprise (photo is of Shenzhousaurus). If these animals are indeed herbivorous as has been suggested (though filter feeding for some has also been supported) then it’s hard to see why they might need such arms and claws (though the arms do not appear to be especially strong and the claws are far less curved that those of definitively predatory theropods). The idea of herbviory for most, if not all, ornithomimids comes in part from the fact that with a couple of early exceptions, they lack teeth but they also lack any obviously powerful crushing / cutting beak that say many oviraptorosaurs have that could get at eggs, small animals or nuts. They are also known in several cases with gastroliths which as noted before are indicators (though far from absolute ones) of herbivory.
The last thing worth mentioning here is the naming of the group. Ornithomimids basically translates as bird mimic and this is kept up throughout almost the entire group with Gallimimus (chicken mimic), Struthiomimus (ostrich mimic), Pelicannimimus (pelican mimic), Anserimimus (goose mimic) and others. This is something I really like from taxonomists as it does help you remember the names and keep things sorted out in your head. It used to be really rather common (rhynchosaurs at least are still pretty much all ‘rhynchus’s or ‘onyx’s) but is dying out as people strive to get more inventive with their names, which is rather ironic given the plethora of unoriginal and unhelpful names that abound these days.
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