Ornithomimus (/ˌɔːrnɪθəˈmaɪməs, -θoʊ-/;[2] "bird mimic") is a genus of ornithomimid theropod dinosaurs from the Campanian and Maastrichtian ages of Late Cretaceous Western North America.

O. velox was named in 1890 by Othniel Charles Marsh on the basis of a foot and partial hand from the Denver Formation of Colorado.

Both consist of fragmentary fossils found by John Bell Hatcher in Montana, which is today understood as tyrannosauroid material.

At first, Marsh assumed Ornithomimus was an ornithopod, but this changed when Hatcher found specimen USNM 4736, a partial ornithomimid skeleton, in Wyoming.

On that occasion, Ornithomimus minutus was also created based on specimen YPM 1049 (a metatarsus),[4] but it has since been recognized as belonging to an alvarezsaurid.

[5] A sixth species, Ornithomimus altus, was named in 1902 by Lawrence Lambe and was based on specimen CMN 930 (hindlimbs found in 1901 in Alberta),[6] but this was renamed to a separate genus in 1916: Struthiomimus, by Henry Fairfield Osborn.

[7] In 1920, Charles Whitney Gilmore named Ornithomimus affinis for Dryosaurus grandis (Lull 1911),[8] based on indeterminate material.

[12] Also in 1933, Charles Mortram Sternberg named the species Ornithomimus edmontonicus for a nearly complete skeleton from the Horseshoe Canyon Formation (specimen CMN 8632).

[16] To solve this confusion by scientifically testing the separation between Ornithomimus and Struthiomimus, Dale Russell in 1972 published a morphometric study.

[17] Two tibiae from the Navesink Formation of New Jersey were named Coelosaurus antiquus ("antique hollow lizard") by Joseph Leidy in 1865.

[19] An SVP 2012 abstract agreed with Weishampel by noting that Coelosaurus differs from Gallimimus and Ornithomimus in the features of the tibiae.

In 2004, Peter Makovicky, Yoshitsugu Kobayashi, and Phil Currie studied Russell's 1972 proportional statistics to re-analyze ornithomimid relationships in light of newly discovered specimens.

[25] Makovicky's team also considered Dromiceiomimus samueli to be a junior synonym of O. edmontonicus, though Longrich later suggested it may belong to a distinct, unnamed species from the Dinosaur Park Formation which has yet to be described.

O. sedens was named by Marsh in 1892 from partial remains found in the Lance Formation of Wyoming a year after the description of O. velox.

Dale Russell, in his 1972 revision of ornithomimids, could not determine which genus it actually belonged to, though he speculated that it may be intermediate between Struthiomimus and Dromiceiomimus.

[30] The bones of the hands are remarkably sloth-like in appearance, which led Henry Fairfield Osborn to suggest that they were used to hook branches during feeding.

The fact that the feather imprints were found in sandstone, previously thought to not be able to support such impressions, raised the possibility of finding similar structures with more careful preparation of future specimens.

A study describing the fossils in 2012 concluded that O. edmontonicus was covered in plumaceous feathers at all growth stages and that only adults had pennaceous wing-like structures, suggesting that wings may have evolved for mating displays.

[34] A fourth feathered specimen of Ornithomimus, this time from the lower portion of the Dinosaur Park Formation, was described in October of 2015 by Aaron van der Reest, Alex Wolfe, and Phil Currie.

Suggested food in its diet includes insects, crustaceans, fruit, leaves, branches, eggs, lizards, and small mammals.

[37] In a 2001 study conducted by Bruce Rothschild and other paleontologists, 178 foot bones referred to Ornithomimus were examined for signs of stress fracture, but none were found.