Ornithomimosaurs were a group of theropod dinosaurs that roamed the Earth during the Mesozoic Era, specifically throughout the Cretaceous Period, from approximately 140 to 66 million years ago. Their name, meaning “bird-mimic lizards,” aptly describes their appearance, as they bore a striking resemblance to modern-day ostriches. This unique group of dinosaurs developed long legs and necks, along with small heads, giving them a distinct bird-like silhouette.
Physical Traits and Bird-Mimicry
Their physical characteristics earned them their “bird-mimic” title. These dinosaurs generally possessed a slender, lightweight body plan, with relatively small skulls in proportion to their overall size. Their heads were perched atop long, flexible necks.
A notable feature of ornithomimosaurs was their beaks, or rhamphotheca, which were toothless in more derived species. Earlier, more primitive members of the group, such as Pelecanimimus and Harpymimus, did possess small teeth in their jaws. They also had large eye sockets, suggesting good vision.
Their forelimbs were long and slender, ending in hands with three fingers that bore powerful, straight claws. These hands, while not adapted for grasping like some other theropods, may have been used to hook branches for feeding or to assist with balance while running. Fossil evidence has also shown that many ornithomimosaurs, including Ornithomimus edmontonicus, were covered in feathers.
Diet, Habitat, and Speed
Ornithomimosaurs were likely omnivorous, meaning their diet consisted of both plant matter and small animals. Evidence supporting this includes the presence of gastroliths, or gizzard stones, found in some fossilized specimens, which would have helped grind tough plant material in their digestive systems. Their toothless beaks also suggest a diet that did not require extensive chewing, possibly including insects, small lizards, and mammals, in addition to plants.
These dinosaurs inhabited various Late Cretaceous environments across different continents, including North America and Asia, with some reports from Australia. They were commonly found in mesic (moderately wet) environments, suggesting a possible reliance on water-borne food sources, potentially even filter-feeding. Their abundance in certain regions, particularly North America, is consistent with a diet that included plants.
Ornithomimosaurs were built for speed and agility. Their long, powerful hind limbs, with elongated foot bones and strong, short toes, were well-adapted for running. Some estimates suggest they could reach speeds of up to 60 kilometers per hour (about 37 miles per hour), with some paleontologists proposing speeds as high as 80 kilometers per hour (50 miles per hour) for certain long-legged varieties. This speed would have been a primary defense mechanism against predators.
Notable Ornithomimosaur Species
Gallimimus bullatus, meaning “chicken mimic,” is one of the largest ornithomimids. Adults could reach lengths of about 6 meters (20 feet) and weigh between 400 to 490 kilograms (880 to 1,080 pounds). Fossils of Gallimimus were discovered in the Gobi Desert of Mongolia, particularly in the Nemegt Formation.
Struthiomimus altus, meaning “ostrich-mimic,” is another North American ornithomimid. It shared many similarities with Ornithomimus but differed in proportions, such as having a more robust build. Fossils of Struthiomimus are common in Dinosaur Provincial Park in Alberta, Canada, with specimens ranging in size, some weighing around 233.8 kilograms (515 pounds). Its overall abundance and toothless beak suggest it was primarily herbivorous or omnivorous.
Ornithomimus velox and Ornithomimus edmontonicus represent the namesake genus of the group. Ornithomimus was a swift, bipedal dinosaur from Late Cretaceous Western North America. Ornithomimus was about 4.3 meters (14 feet) long and weighed around 170 kilograms (375 pounds), distinguished from Struthiomimus by its shorter torso and more slender limbs.