Ornithomimus ("bird mimic") is a genus of ornithomimid dinosaur from the Late Cretaceous Period of what is now North America.
In 1890 Ornithomimus velox was named by Othniel Charles Marsh on the basis of a foot and partial hand from the Maastrichtian Denver Formation. Another seventeen species have been named since. Most of these have subsequently been assigned to new genera or shown to be not directly related to Ornithomimus. The best material of species still considered part of the genus has been found in Canada, representing the earlier Edmontonian-age Ornithomimus edmontonicus Sternberg 1933, known from several skeletons. However, on some of these the new genus Dromiceiomimus including Dromiceiomimus brevitertius (Parks 1926) has been based, causing taxonomic problems of priority and identity that are still unresolved.
Ornithomimus was a relatively small swift bipedal animal, equipped with a small toothless beaked head, that may indicate an omnivorous diet.
Description
Like other ornithomimids, Ornithomimus is characterized by a foot with three weight-bearing toes, long slender arms and a long neck with a birdlike, elongated, toothless, beaked skull. It was bipedal and superficially resembled an ostrich, except for its long tail. It would have been a swift runner. It had very long limbs, hollow bones, and a large brain and eyes. The brains of ornithomimids were large for dinosaurs, but this may not necessarily be a sign of greater intelligence; some paleontologists think that the enlarged portions of the brain were dedicated to kinesthetic coordination. Their hands are remarkably sloth-like in appearance, which led Henry Fairfield Osborn to suggest that they were used to hook branches during feeding.
Ornithomimus differs from other ornithomimids, such as Struthiomimus, in having a short back, long slender forearms, very slender, straight hand and foot claws and in having metacarpals and fingers of similar lengths.
The three Ornithomimus species today seen as possibly valid, differ rather in size. In 2010 Gregory S. Paul estimated the length of O. edmontonicus at 3.8 metres, its weight at 170 kilograms (370 lb). One of its specimens, CMN 12228, preserves a femur (thigh bone) 46.8 centimetres (18.4 in) long. O. sedens was by Paul estimated at 4.8 metres and 350 kilograms (770 lb). O. velox, the type species of Ornithomimus, is based on material of a much smaller animal. Whereas the holotype of O. edmontonicus, CMN 8632, preserves a second metacarpal eighty-four millimetres long, the same element with O. velox measures only fifty-three millimetres.
Paleobiology
The diet of Ornithomimus is still debated. As theropods, ornithomimids might have been carnivorous but their body shape would also have been suited for a partly or largely herbivorous lifestyle. Suggested food includes insects, crustaceans, fruit, leaves, branches, eggs, and the meat of lizards and small mammals.
Ornithomimus had legs that seem clearly suited for rapid locomotion, with the tibia about 20% longer than the femur. The large eye sockets suggest a keen visual sense, and also suggest the possibility that they were nocturnal.
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