Deinocheirus ( ) is a genus of large ornithomimosaur that lived during the Late Cretaceous around 70 million years ago. In 1965, a pair of large arms, shoulder girdles, and a few other bones of a new dinosaur were first discovered in the Nemegt Formation of Mongolia. In 1970, this specimen became the holotype of the only species within the genus, Deinocheirus mirificus; the genus name is Greek for "horrible hand". No further remains were discovered for almost fifty years, and its nature remained a mystery. Two more complete specimens were described in 2014, which shed light on many aspects of the animal. Parts of these new specimens had been looted from Mongolia some years before, but were repatriated in 2014.
Deinocheirus was an unusual ornithomimosaur, the largest of the clade at long, and weighing . Though it was a bulky animal, it had many hollow bones which saved weight. The arms were among the largest of any bipedal dinosaur at long, with large, blunt claws on its three-fingered hands. The legs were relatively short, and bore blunt claws. Its vertebrae had tall neural spines that formed a "sail" along its back. Most of the vertebrae and some other bones were highly pneumatised by invading air sacs. The tail ended in pygostyle-like vertebrae, which indicate the presence of a fan of feathers. The skull was long, with a wide bill and a deep lower jaw, similar to those of hadrosaurs.
The classification of Deinocheirus was long uncertain, and it was initially placed in the theropod group Carnosauria, but similarities with ornithomimosaurians were soon noted. After more complete remains were found, Deinocheirus was shown to be a primitive ornithomimosaurian, most closely related to the smaller genera Garudimimus and Beishanlong, together forming the family Deinocheiridae. Members of this group were not adapted for speed, unlike other ornithomimosaurs. Deinocheirus is thought to have been omnivorous; its skull shape indicates a diet of plants, fish scales were found in association with one specimen and gastroliths were also present in the stomach region of the specimen. The large claws may have been used for digging and gathering plants. Bite marks on Deinocheirus bones have been attributed to the tyrannosaurid Tarbosaurus.
Discovery
upright=1.2|left|thumb|[[Cretaceous-aged dinosaur fossil localities of Mongolia; Deinocheirus fossils have been collected in the Altan Ula III, IV, and Bugiin Tsav localities of area A (left)]]
The first known fossil remains of Deinocheirus were discovered by Polish palaeontologist Zofia Kielan-Jaworowska on July 9, 1965, at the Altan Ula III site (coordinates: ) in the Nemegt Basin of the Gobi Desert. She was part of a Polish group accompanied by Mongolian palaeontologist Rinchen Barsbold during the 1963–1965 Polish-Mongolian palaeontological expeditions, which were organised by the Polish Academy of Sciences and the Mongolian Academy of Sciences. The crew spent July 9–11 excavating the specimen and loading it onto a vehicle. A 1968 report by Kielan-Jaworowska and Naydin Dovchin, which summarised the accomplishments of the expeditions, announced that the remains represented a new family of theropod dinosaur. Currie stated in an interview that it was a policy of their team to investigate quarries after they had been looted and recover anything of significance, and that finding any new Deinocheirus fossils was cause for celebration, even without the poached parts. A virtual model of Deinocheirus revealed at the SVP presentation brought applause from the crowd of attending palaeontologists, and the American palaeontologist Stephen L. Brusatte stated he had never been as surprised by a SVP talk, though new fossils are routinely presented at the conference. On May 1, 2014, the fossils were repatriated to Mongolia by a delegation from the Belgian Museum, during a ceremony held at the Mongolian Academy of Sciences.
Description
thumb|left|alt=Size comparison|Size of three specimens (holotype in green) compared to a human
Deinocheirus is the largest ornithomimosaurian (ostrich dinosaur) discovered; according to the 2014 description, the largest known specimen measured about long, with an estimated weight of . Also in 2016, Asier Larramendi and Molina-Pérez presented a higher length estimate of and a higher mass estimate of , and an estimated hip height of . In 2020, Campione and Evans gave a body mass estimate of approximately or depending on calculation method they used. When only the incomplete holotype arms were known, various sizes were extrapolated from them by different methods. A 2010 study estimated the hip height of Deinocheirus to be .
The wishbone (furcula), an element not known from any other ornithomimosaurs, was U-shaped. The hindlimbs were relatively short, and the thigh bone (femur) was longer than the shin bone (tibia), as is common for large animals. The metatarsus was short and not arctometatarsalian, as in most other theropods. The claw bones of the feet were blunt and broad-tipped instead of tapered, unlike other theropods, but resembled the unguals of large ornithischian dinosaurs. The proportions of the toe bones resembled those of tyrannosaurs, due to the large weight they had to bear.
Palaeobiology
thumb|alt=Cast of the arms, London|left|Reconstructed cast of the holotype arms in [[Natural History Museum, London|Natural History Museum]]
The blunt and short hand-claws of Deinocheirus were similar to those of the therizinosaur Alxasaurus, which indicates the long arms and claws were used for digging and gathering plants. The blunt claws of the feet could have helped the animal from sinking into substrate when wading. The robust hind limbs and hip region indicates the animal moved slowly. The large size of the animal may have protected it against predators such as Tarbosaurus, but in turn it lost the running ability of other ornithomimosaurs. The long neural spines and possible tail fan may have been used for display behaviour. Deinocheirus was likely diurnal (active during the day), since the scleral rings of the eyes were relatively small in comparison with its skull length.
Diet
thumb|left|alt=Claw cast|Cast of a hand-claw, showing its blunt, recurved shape
The distinct shape of the skull shows that Deinocheirus had a more specialised diet than other ornithomimosaurs. The beak was similar to that of ducks, which indicates it may have likewise foraged in water, or browsed near the ground like some sauropods and hadrosaurs. The attachment sites for the muscles that open and close the jaws were very small in comparison to the size of the skull, which indicates Deinocheirus had a weak bite force. The skull was likely adapted for cropping soft understorey or water vegetation. The depth of the lower jaw indicates the presence of a large tongue, which could have assisted the animal in sucking in food material obtained with the broad beak when foraging on the bottom of freshwater bodies.
A 2022 article by Waisum Ma and colleagues examined how feeding mechanics varied between different non-bird coelurosaurian groups through finite element analysis, revealing that they all underwent reduction of feeding-related stress in their jaws. They found that Deinocheirus showed different patterns of stress and strain distribution than other ornithomimisaurs, indicating it was a specialized feeder. They suspected Deinocheirus may have reverted to omnivory/carnivory.
Various feeding behaviours were proposed before more complete remains of Deinocheirus were known, and it was early on envisioned as a predatory, allosaur-like animal with giant arms.
Palaeopathology
thumb|alt=Tarbosaurus illustration showing feeding behavior|Restoration of [[Tarbosaurus feeding on Deinocheirus; bite marks of this tyrannosaur have been identified on Deinocheirus bones]]
Osmólska and Roniewicz reported palaeopathologies in the holotype specimen such as abnormal pits, grooves and tubercles on the first and second phalanx of the left second finger that may have been the result of injuries to the joint between the two bones. The damage may have caused changes to the arrangement of ligaments of muscles. The two coracoids are also differently developed.. This geologic formation has never been dated radiometrically, but the fauna present in the fossil record indicate it was probably deposited during the early Maastrichtian age, at the end of the Late Cretaceous about 70 million years ago. Mammal fossils are rare in the Nemegt Formation, but many birds have been found, including the enantiornithine Gurilynia, the hesperornithiform Judinornis, as well as Teviornis, a possible Anseriform. Herbivorous dinosaurs of the Nemegt Formation include ankylosaurids such as Tarchia, the pachycephalosaurian Prenocephale, large hadrosaurids such as Saurolophus and Barsboldia, and sauropods such as Nemegtosaurus, and Opisthocoelicaudia.