| Deinocheirus Temporal range: Late Cretaceous, 70 Ma |
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|---|---|
| Holotype fossil of Deinocheirus | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | Dinosauria |
| Order: | Saurischia |
| Suborder: | Theropoda |
| Infraorder: | †Ornithomimosauria |
| Family: | †Deinocheiridae Osmólska & Roniewicz, 1970 |
| Genus: | †Deinocheirus Osmólska & Roniewicz, 1970 |
| Species: | †D. mirificus |
| Binomial name | |
| Deinocheirus mirificus Osmólska & Roniewicz, 1970 |
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Deinocheirus ( /ˌdaɪnɵˈkaɪərəs/ dy-no-ky-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).
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The only known fossil remains are a single pair of massive forelimbs and the remains of some ribs and vertebrae. They were found on 9 July 1965 during a Polish-Mongolian expedition to the Gobi by Professor Zofia Kielan-Jaworowska at the Altan Ula III site in Ömnögovi Province. The find was reported by her in 1966.[1] Deinocheirus was named by Halszka Osmólska and Ewa Roniewicz in 1970.[2] The type species and only named species is Deinocheirus mirificus. The generic name is derived from Greek δεινός, deinos, "terrible", "horrible", and χείρ, cheir, "hand". The specific name is Latin for 'unusual', 'peculiar'.[2]
The holotype specimen, ZPal MgD-I/6, was discovered on the desert surface in sandstone dating to the early Maastrichtian. It consists of a partial skeleton, most parts of which had already weathered away at the moment of discovery. Both forelimbs excluding the right claws, the complete shoulder girdle, centra of three dorsal vertebrae, five ribs, gastralia and two ceratobranchialia, supporting neck bones, could still be recovered.[2]
The most well-preserved parts of Deinocheirus are its forelimbs, which measured 2.4 m (8 ft) long — a 938 mm humerus, 688 mm ulna and 770 mm hand — including up to 19.6 cm (8 in) long recurved claws. Each scapulocoracoid of the shoulder girdle has a length of 153 centimetres. The neck must also have been massive with each half of the paired ceratobranchialia measuring 42 centimetres.[2]
The gigantic size of these skeletal elements has generated much speculation about the magnitude of the animal as a whole. Osmólska and Roniewicz thought it could be best compared with the Ornithomimosauria, as the structure of its arms is similar to the members of this group.[2] Should Deinocheirus itself be a member this would make it by far the largest ornithomimosaur, indeed, one of the largest theropods. The describers estimated its size to be equal to the largest specimens of Tyrannosaurus.[2] Its weight was in 1988 estimated by Gregory S. Paul to have been between six and twelve tonnes.[3] Later estimates confirmed a number of roughly 9,000 kg (20,000 lb).[4] In 2010 Paul revised this to a length of ten metres and a weight of two tonnes.[5]
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 scapula (shoulder blade), better than that of the humerus (upper arm bone), could be used to accurately predict hip height. Using the equation found over a range of theropods, Senter and Robins determined that Deinocheirus likely measured 3.3 m (11 ft) -3.6 m (12 ft) tall at the hip. This places it as possibly the tallest known theropod, taller than any contemporary predators such as Tarbosaurus.[6]
Though the arms of Deinocheirus have a considerable absolute size, being the longest of any known theropod with the exception of Therizinosaurus, they are not very long relative to the shoulder girdle, the ratio being less than that with most ornithomimosaurs. The shoulder-blade is long and narrow. The humerus is relatively slender. The ulna and radius too are elongated and not very firmly connected to each other in a syndesmosis. The metacarpus is long compared to the fingers. The hand had a good mobility relative to the lower arm but was only capable of a limited flexing motion, unable to close in grasping. The fingers are about equal in length to each other, the first being the stoutest and the second the longest. Only the claw of the left second finger has been preserved in its entirety; it has a diameter of 196 millimetres and a length along its outer curvature of 323 millimetres.[2]
Deinocheirus is now considered by most paleontologists to be an ornithomimosaur, however, over the decades, scientists have not always agreed about the placement of Deinocheirus within Dinosauria. While Kielan-Jaworowska had initially assigned the find to the Megalosauridae, 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, "faute de mieux", assigned to the superfamily Megalosauroidea, basically because it was obviously not a tyrannosauroid — tyrannosaurids having greatly reduced forelimbs endowed with only two extremely small fingers.[2]
Peter 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.[7] Basal traits include the recurved claws, the low humerus/scapula ratio, the lack of syndesmosis, a short posterior process of the coracoid and the combined length of the second and third phalanx of the third finger being greater than the length of the third phalanx. Yoshitsugu 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. Derived traits include the short hand, a flat metacarpus and a relative elongation of the claws.[8]
Early work generally envisioned Deinocheirus as a carnivore that used its long forelimbs "in tearing dead or weakly agile prey asunder" (Osmólska & Roniewicz 1970: 15).[2] David 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".[9] G.S. Paul disagreed, suggesting that the claws are too blunt for killing but would have been good defensive weapons.[3] 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 hindlimbs no longer than the forelimbs,[10] but there is no hard evidence for this and the climbing hypothesis has not received much support from other scientists.
Paul in 1988 suggested a herbivorous lifestyle for Deinocheirus. It shared its habitat with several other giant, nine to thirteen meter long, herbivores uncovered in the Nemegt Formation, like the small titanosaur sauropod Opisthocoelicaudia, the lambeosaurine hadrosaurs Saurolophus and Barsboldia, and the bizarre even longer-clawed Therizinosaurus. Trying to determine the respective niches for these animals Senter & Robins concluded that Deinocheirus due to its hip height had the largest vertical feeding range of them all, specialising in eating high foliage.[6] Paul in 1988 had proposed it would have used its long neck to reach into trees and rip leaves, the arms assisting in pulling down branches.[3]
ZPALNo.Mgd-I/6, the holotype of Deinocheirus mirificus, has abnormal pits, grooves and tubercles on the first and second phalanx of the left second finger that may be the result of injuries to the joint between the two bones.[2][11]