Ornithomimus

MEANING: Bird mimic

PERIOD: Late Cretaceous

CONTINENT: North America

Ornithomimus was a swift runner that superficially resembled an ostrich. It had very long limbs, hollow bones, and a large brain and eyes. Several specimens have been found preserving evidence of feathers, and it had with a small toothless beak that may indicate an omnivorous diet. Ornithomimus was about 4 m in length and 170 kg in weight.

Ornithomimus is from the Late Cretaceous. The Cretaceous is the third and final geological period of the Mesozoic Era, with the Late Cretaceous making up roughly the second half of it, lasting from about 100 to 66 million years ago. It was a time of significant evolutionary change, with dinosaurs reaching their greatest diversity before the mass extinction at the end of the Cretaceous.

The Cretaceous was a period with a relatively warm climate, though the Late Cretaceous experienced a global cooling trend, caused by falling levels of carbon dioxide in the atmosphere. The continents were nearing their present positions, but high sea levels flooded low-lying regions, turning Europe into an archipelago, and forming the Western Interior Seaway in North America. These seas were home to a variety of marine reptiles, including mosasaurs and plesiosaurs, while pterosaurs and birds shared the skies.

On land, dinosaurs continued to thrive and diversify during the Late Cretaceous, producing many of the most well-known goups, including tyrannosaurs, hadrosaurs, and pachycephalosaurs. Established Cretaceous dinosaur clades like the ceratopsians, ankylosaurs, and dromaeosaurs continued to flourish. Sauropod species consisted almost exclusively of titanosaurs, which seemed to be confined to the Southern Hemisphere for much of the Late Cretaceous. Flowering plants and grasses diversified and spread, becoming the dominant flora similar to what we see today.

The Cretaceous (along with the Mesozoic) ended with the Cretaceous-Paleogene (K-Pg) extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out. This event, likely triggered by an asteroid impact, is marked by the abrupt K-Pg boundary, a distinct geologic layer separating the Mesozoic and Cenozoic Eras. In its aftermath, mammals and avian dinosaurs rapidly diversified, becoming the dominant land animals of the Cenozoic Era.

Ornithomimus is an ornithomimosaur. Ornithomimosauria was a unique group of theropod dinosaurs whose name means "bird mimics," as their general body plan resembles modern ostriches. Part of the larger group Coelurosauria, they were characterized by their small heads, long necks, lightly built bodies, and long, powerful legs built for fast running. Most ornithomimosaurs had toothless beaks, and some evidence suggests they may have been omnivorous or herbivorous rather than strict predators like many other theropods. As they evolved, the derived ornithomimids became some of the most specialized runners of the Mesozoic Era.

Ornithomimosaur fossils have been discovered in North America, Asia, and possibly other regions, showing that they had a wide distribution by the Late Cretaceous. Their success in multiple environments suggests they were highly adaptable, capable of thriving in a variety of ecosystems ranging from floodplains to deserts. Though ornithomimosaurs were not top predators, their speed and agility likely helped them evade the larger carnivores of their time.