Deinocheirus mirificus, meaning “terrible hand,” was a unique and fascinating dinosaur from the Late Cretaceous period, approximately 70 million years ago. Its name refers to its most distinctive feature: unusually large forelimbs. This massive ornithomimosaur, or “ostrich-mimic” dinosaur, remained an enigma to paleontologists for decades, known only from these striking arm bones.
The Mystery of the Giant Arms
The initial discovery of Deinocheirus mirificus occurred in 1965 during a Polish-Mongolian paleontological expedition in the Nemegt Basin of Mongolia. Researchers uncovered a pair of enormous forelimbs, measuring approximately 2.4 meters (7.9 feet) in length, along with shoulder girdles and some rib fragments. These remains were formally described in 1970, giving the dinosaur its memorable name. For nearly five decades, these isolated arms were the only known evidence of Deinocheirus, leading to speculation about the creature’s full anatomy.
The mystery began to unravel with subsequent discoveries. In 2006 and 2009, a joint Korean and Mongolian team unearthed two more complete Deinocheirus specimens in the Nemegt Formation. These new skeletons, although initially missing the head and feet due to illicit fossil trade, provided substantial new information. The missing skull and foot bones were eventually located, repatriated to Mongolia, and reunited with the rest of the skeleton. An international team’s comprehensive description of these specimens in 2014 finally revealed the full, peculiar nature of this long-mysterious dinosaur.
A Peculiar Appearance
Complete skeletons of Deinocheirus revealed a dinosaur of immense proportions, making it the largest known ornithomimosaur. It reached lengths of about 11 meters (36 feet) and weighed an estimated 6.5 metric tons (7.2 short tons). Despite its bulk, it possessed numerous hollow bones, which likely helped reduce its overall weight. Its most striking features included tall neural spines along its back vertebrae, which likely supported a prominent sail-like structure or a fleshy hump.
The skull of Deinocheirus was also distinctive, measuring over 1 meter (3.3 feet) long. It featured a long, toothless, duck-like snout that flared outwards, resembling the wide bill of a hadrosaur. The lower jaw was notably deep and robust. Evidence for feathers, typical of many ornithomimosaurs, included pygostyle-like vertebrae at the end of its tail, suggesting it may have had a tail fan.
An Omnivorous Diet
Unlike many of its close relatives, which were primarily herbivorous, Deinocheirus exhibited an omnivorous diet. Direct evidence for its diverse food consumption comes from the contents found within its abdominal cavity. Paleontologists discovered over 1,400 gastroliths, or stomach stones, within one specimen. These rounded stones, commonly found in plant-eating animals, would have aided in grinding tough plant material in its digestive system.
Along with the gastroliths, preserved fish remains, including scales and vertebrae, were also found in the stomach region. This combination of stomach stones and fish fragments provides strong evidence that Deinocheirus consumed both plants and aquatic animals. Its specialized duck-like bill and blunt claws on its hands and feet were well-suited for foraging in a watery environment, suggesting it likely fed on vegetation and small aquatic life.
Habitat and Extinction
Deinocheirus mirificus inhabited the Nemegt Formation in what is now the Gobi Desert of Mongolia during the Late Cretaceous period. This ancient environment was characterized by wet, river-dominated landscapes, often compared to the modern-day Okavango Delta in Botswana. The presence of stream channels, mudflats, and shallow lakes provided a suitable niche for this large omnivore.
Within this rich ecosystem, Deinocheirus coexisted with various other dinosaurs. Predatory theropods like the formidable Tarbosaurus shared its habitat, as did the large hadrosaur Saurolophus. The Nemegt Formation also hosted other large herbivores such as the long-necked theropod Therizinosaurus. Deinocheirus lived during the final stages of the Mesozoic Era, nearing the end of the age of dinosaurs.