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.
Archaeornithomimus
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.
Shenzhousaurus. Courtesy of Steve Brusatte.
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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Dave Hone's Archosaur Musings 
I pushed for the authors to give Shenzhousaurus a -mimus name (even suggested a few), but there were political reasons to go with the one they did.
In other ornithomimosaur news: where are the Jurassic ornithomimosaurs? After all:
Compsognathids? Check
Tyrannosauroids? Check
Therizinosaurians? Well, if Eshanosaurus is one and is Jurassic, then check
Deinonychosaurs? Check
Avialians? Check
So Jurassic Ornithomimosauria and Oviraptorosauria remain conspicuous by their absence.
And alvarezsaurs now of course Tom.
Especially given their relatively basal status among coleurosaurs. Are they still considered to be a sister group to tyrannosaurs?
“In other ornithomimosaur news: where are the Jurassic ornithomimosaurs?”
Well, basically everyone now agrees tyrannosauroids are not maniraptoriforms, and many agree compsognathids aren’t, so ornithomimosaurs should be expected to equal them in age. Given Falcarius and other factors, Eshanosaurus is probably a sauropodomorph. Most Jurassic maniraptoran remains are teeth, which would be a good reason for not finding oviraptorosaurs or ornithomimosaurs. Sure Jurassic members were probably toothed, but who would recognize isolated Pelecanimimus, Incisivosaurus or Caudipteryx teeth? If Haplocheirus is any indication, Jurassic members might have generic ‘coelurosaur/maniraptoran/paravian’ teeth anyway, recurved, compressed, serrated posteriorly. So many of the unidentified teeth of that morphology could belong. And ignoring teeth, which Jurassic maniraptorans do we have? Haplocheirus (if alvarezsaurs are maniraptoran), Lori, a Morrison proximal femur, “Paleopteryx”, Anchiornis, a Tendaguru carpometacarpus, and Archaeopteryx. Maybe Pedopenna and three scansoriopterygid specimens, depending on their age. So I’m not surprised we haven’t found body fossils yet.
“Are they still considered to be a sister group to tyrannosaurs?”
No, that idea hasn’t been seriously considered for a decade.
Eshanosaurus is definitely not a sauropodomorph. It’s either a therizinosaurid or from another archosaur group that has independently acquired therizinosaur-like features (there would be a precedent for this as numerous crurotarsans now convergence on various dinosaurs in morphology). Personally, I think it’s a therizinosaur, but that the date is incorrect.
The relatively large forelimbs and claws make me think of kangaroos, which use the hands not only for some grasping but also plantigrade in quadrupedal (or pentapedal) low-feeding postures or slow-moving gait, but also in fighting (thus unconnected with diet). Male kangaroos do most of the fighting, and (in the large Macropus species at least) have more robust upper bodies and bigger arms than females.
So: are there sufficient specimens of any ornithomimosaurs to test for sexual dimorphism in relative forelimb size? Or, can the hand be examined for evidence of a quadrupedal feeding role? It certainly doesn’t look designed for weight-bearing (in compression) in Shenzhousaurus; elongate penultimate phalanx usually indicates a grasping and sometimes suspensory function.
“Gal- Gallimimus. They’re brachiating this way!”
Ohhh there’s a lot there John and as I noted above, I’m no ornithomimid expert.
There are certainly some big collections of specimens but to my knowledge, most of these are not adult, so any test of dimorphism etc. might be problematic.
I’d rule out quadrupedality as while the arms are long, the legs are even more huge and the tail lacks the real mass required either for balance or pentapediality. If they were quadrupedal, I’d also expect to see much more hoof-like manual claws (as with hadrosaurs say) but then, the pedal claws are not that flat admittedly.
I’d also rule out fighting as, while not a predatory action as such, the claws don’t have the curve and structure I associate with ‘fighting’ claws. I wouldn’t rule it out, but I’d want to look at them in more detail, and they are somewhat flat and narrow.
In short, I don’t know and while it’s probably possible to test all these ideas, I’m not aware of anyone seriously having considered them or examined them in detail. Off the top of my head though, I’d say none of them look particularly likely I’m afraid.