Deinocheirus (pronounced /ˌdaɪnɵˈkaɪrəs/ DYE-no-KYE-rəs, Greek: 'terrible hand') is a genus of large theropod dinosaur, possibly an ornithomimosaurian, which lived in what is now southern Mongolia, during the late Cretaceous Period (Nemegt Formation, dating to around 70 million years ago). The only known fossil remains are a single pair of massive, forelimbs and the remains of some ribs and vertebrae. Deinocheirus was named by Halszka Osmólska and Ewa Roniewicz in 1970. The type species and only named species is D. mirificus (Latin: 'unusual', 'peculiar').
Holotype fossil of Deinocheirus exhibited at the Experimentarium in Denmark, 1998Deinocheirus is considered by most paleontologists to be an ornithomimosaur, as the structure of its arms is similar to other dinosaurs of this group. This would make Deinocheirus by far the largest ornithomimosaur (indeed, one of the largest theropods) weighing roughly 9,000 kg. Makovicky et al. pointed out that if Deinocheirus is an ornithomimosaur, it is a fairly primitive one, since it lacks some of the features typically seen in ornithomimosaurs. Kobayashi and Rinchen Barsbold added Deinocheirus to several recent cladistic analyses of theropods and were unable to resolve its exact relationships but noted some support for it as a possible ornithomimosaur.
The most well-preserved parts of Deinocheirus are its forelimbs, which measured 2.4 m (8 ft) long, with 25 cm (10 in) long claws. In 2010, Phil Senter and H.J. Robins attempted to estimate the total height at the hip of Deinocheirus. By studying more completely known theropods, they found that the length of the humerus (upper arm bone) and scapula (shoulder blade) could be used to accurately predict hip height. Using this equation, Senter and Robins found that Deinocheirus likely measured 3.3 m (10 ft) -3.6 m (10 ft) tall at the hip. This places it as possibly the tallest known theropod, taller than any contemporary predators such as Tarbosaurus.
While Deinocheirus is now considered by most paleontologists to be an ornithomimosaur, over the decades, scientists have not always agreed about the placement of Deinocheirus within Dinosauria. Osmólska and Roniewicz created a new family for Deinocheirus, the Deinocheiridae. The family Deinocheiridae was initially placed in the infraorder Carnosauria, owing to the "gigantic size and thick-walled limb bones" but Osmólska and Roniewicz also speculated that it possibly "constitutes a link between Carnosauria and Coelurosauria". Within Carnosauria, the family Deinocheiridae was tentatively assigned to the superfamily Megalosauroidea, basically because it was obviously not a tyrannosauroid (tyrannosaurids having greatly reduced forelimbs).
Hypothetical restoration of Deinocheirus as a giant ornithomimosaurEarly work generally envisioned Deinocheirus as a carnivore that used its long forelimbs "in tearing dead or weakly agile prey asunder" (Osmólska & Roniewicz 1970: 15). Lambert supported this view, describing the clawed hands of Deinocheirus as "horrifying weapons for attacking dinosaurs of almost any size ... capable of ripping open a sauropod's soft underbelly". Gregory S. Paul disagreed, suggesting that the claws are too blunt for killing but would have been good defensive weapons. The Russian paleontologist Rozhdestvensky compared the forelimbs of Deinocheirus to sloths, leading him to hypothesize that Deinocheirus was a specialized climbing dinosaur, that fed on fruits and leaves and perhaps also eggs and any small animals found in trees. Rozhdestvensky imagined Deinocheirus with the trunk and hind limbs no longer than the fore limbs, but there is no hard evidence for this and the climbing hypothesis has not received much support from other scientists.
Replicas of the fossilized 'arms' are currently on display at the Paleontological Museum of the University of Oslo, Norway, the American Museum of Natural History, New York, the Natural History Museum, London, The Dinosaur Discovery Museum in Kenosha, Wisconsin and The Dinosaur Museum, Blanding, Utah.