Deinocheirus

MEANING: Horrible hand

PERIOD: Late Cretaceous

CONTINENT: Asia

For half a century, the massive arms were all that was known of Deinocheirus, leading to speculation and confusion in the paleontological community. The largest ornithomimosaur at 12 m long and 6 t in weight, Deinocheirus is one of the few examples of herbivorous theropods. Its skull had a wide bill and a deep lower jaw, similar to those of hadrosaurs.

Deinocheirus 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 groups, 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.

Deinocheirus is a deinocheirid. Deinocheiridae is a rare and enigmatic group within the ornithomimosaurs, a clade of bird-like theropod dinosaurs. While ornithomimosaurs are typically known for their lightweight bodies, long legs, and adaptations for fast running, deinocheirids took a very different evolutionary path. They were large, heavily built animals with broad hips, elongated arms ending in massive claws, and toothless beaks. Unlike their more gracile relatives, deinocheirids were not specialized for speed, suggesting a unique ecological role among Late Cretaceous theropods.

This group is currently only known from a small number of fossil specimens, primarily from Asia. Though the fossil record is limited, these dinosaurs appear to have occupied an unusual niche. Their large size, robust build, and beak morphology suggest they were likely omnivorous, possibly feeding on a mix of vegetation, small animals, and aquatic life. Deinocheirids highlight the surprising diversity that evolved within ornithomimosaurs, their unusual proportions and limited fossil record making them one of the more distinctive yet poorly understood groups.