Ornithomimosauria, often called “ostrich-mimic dinosaurs,” were a group of theropod dinosaurs. They resembled modern-day ostriches, known for their swift, bipedal movement. These dinosaurs thrived during the Cretaceous Period across ancient Laurasia, including what is now Asia and North America.
Defining Physical Characteristics
Ornithomimosaurs possessed distinctive anatomical features contributing to their bird-like appearance. Their slender bodies were supported by long, powerful legs adapted for rapid movement, making them among the fastest dinosaurs. The hind limbs featured a long foot with short, strong toes ending in hoof-like claws.
A long, slender neck supported a relatively small skull with large eyes. While some early members like Pelecanimimus had small teeth, most later species developed a toothless beak, likely covered by a keratinous sheath similar to modern birds.
These dinosaurs were extensively feathered. Specimens of Ornithomimus edmontonicus, for instance, show traces of hair-like filaments covering the body, rump, legs, and neck. Some feathered forms also exhibited longer, complex arm feathers, giving them a wing-like appearance. These arm feathers were likely used for display rather than flight.
Lifestyle and Behavior
Ornithomimosaur diet was primarily herbivorous or omnivorous. Many specimens, including primitive forms, have been found with gastroliths (stomach stones) to aid in digesting tough plant material. Some paleontologists propose that their long, slender arms and specialized hands may have been used to pull down branches for feeding.
Fossil discoveries support a plant-based diet. While early theories suggested filter-feeding, anatomical and paleoecological evidence indicates suspension-feeding and strict carnivory were unlikely for derived ornithomimosaurs. These dinosaurs were widespread, with most fossils discovered in Late Cretaceous strata across East Asia and North America. Their adaptations for speed suggest running was a primary defense mechanism against predators.
Notable Ornithomimosaur Species
Ornithomimosauria includes diverse species. Gallimimus, a well-known example, was one of the largest ornithomimids, reaching about 6 meters (20 feet) long and weighing 400–490 kilograms (880–1,080 pounds). Its long legs and streamlined body made it a swift runner, with estimated speeds of 42–56 km/h (29–34 mph). Discovered in Mongolia’s Gobi Desert, Gallimimus had a small head with large, side-facing eyes and a toothless, keratinous beak.
Deinocheirus mirificus was an unusual member of the clade. This massive dinosaur, the largest ornithomimosaur, measured about 11 meters (36 feet) long and weighed around 6.5 metric tons (7.2 short tons). Its distinctive features included disproportionately long forelimbs, reaching 2.4 meters (7.9 feet), equipped with large, blunt claws. Deinocheirus also possessed tall neural spines along its back, forming a prominent sail-like structure. Unlike other ornithomimosaurs, Deinocheirus was not adapted for speed and is believed to have been omnivorous, consuming plants and possibly fish.
The “Bird-Mimic” Connection
The name Ornithomimosauria, meaning “bird-mimic lizards,” highlights their resemblance to modern flightless birds like ostriches and emus. This similarity does not mean they are direct ancestors of birds. Birds evolved from a different theropod lineage, the Maniraptora group.
Bird-like features in ornithomimosaurs, such as feathers, beaks, and a fast-running bipedal stance, resulted from convergent evolution. This biological phenomenon occurs when unrelated species independently evolve similar traits because they adapt to similar environments or lifestyles. For example, both ostriches and ornithomimosaurs developed long legs and a body plan suited for speed in open habitats. These traits developed independently along separate evolutionary paths.