Deinocheirus

Deinocheirus (/ˌdaɪnɵˈkaɪərəs/ DY-no-KY-rəs; Greek: 'terrible hand') is a genus of very large ornithomimosaur theropod dinosaur, which lived in what is now southern Mongolia, during the late Cretaceous Period (Nemegt Formation, dating to around 70 million years ago).

Discovery and naming
The first 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. Kielan-Jaworowska published the find in 1966.[3]

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 deinos (δεινός), meaning "horrible", and cheir (χείρ), meaning "hand". The specific name, mirificus, comes from Latin, meaning "unusual" or "peculiar," and was chosen for the "unusual structure of the forelimbs."[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, disarticulated 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]

Additional fossils, including fragments of gastralia (belly ribs) belonging to the same specimen, were later found by teams re-examining the original fossil site. Some of these bones contained bite marks made by the contemporary tyrannosaurid species, Tarbosaurus bataar, and showed evidence consistent with scavenging. The possibility that the carcass was scavenged by tyrannosaurs may explain why the specimen was preserved in a scattered, disassociated state.[4]

In 2014, palaeontologists described two new specimens of Deinocheirus discovered in Mongolia in 2006 and 2009. One specimen, MPC-D 100/127, is even larger than the holotype, with a left forelimb 6% longer.[1] The other specimen, MPC-D 100/128, is smaller, and the two together provide a nearly complete skeleton.[5] Both skeletons had been poached by illegal fossil hunters. The skull and foot bones of MPC-D 100/127 were sold to Japan and resold to Germany. They were in 2011 obtained by the French fossil trader François Escuillé who donated them to the Royal Belgian Institute of Natural Sciences: they were repatriated to Mongolia in May 2014.

Description
The best-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: each half of the paired ceratobranchialia measures 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] In 1988 its weight was estimated by Gregory S. Paul to have been between six and twelve tonnes.[8] Later its weight was confirmed as roughly 9,000 kg (20,000 lb).[9] In 2010 Paul revised this to a length of ten metres and a weight of two tonnes.

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 concluded 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 established by comparing a range of theropods, Senter and Robins determined that Deinocheirus likely measured 3.3 m (11 ft) to 3.6 m (12 ft) tall at the hip. This would place it as possibly the tallest known theropod, taller than any contemporary predators such as Tarbosaurus.

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 good mobility relative to the lower arm but was capable of only 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]

Reports of the new specimens, when they were still formally undescribed, showed that the animal had a hump on its back and a skull that superficially resembled a cross between an ornithomimosaur and a hadrosaur.[5][7]

In 2014, a new reconstruction based on the two nearly complete specimens was published in Nature.[1][12] It showed a bipedal, relatively upright-walking animal, sporting on its back tall neural spines up to 8.5 times the centrum height, almost attaining the highest ratio seen in Spinosaurus. The spines were probably specialized to support the abdomen from the hips in view of the probable presence of an intricate system of interspinous ligaments. The back vertebrae were strongly pneumatic. The tail was relatively short and possibly featured a tail fan of feathers. The toothless skull was elongated with a low snout, transversely expanded at its tip. The lower jaws were extremely deep.[1] The specimen MPC-D 100/127 had an estimated body length of 11 metres (36 ft) and an estimated body mass of 6,358 kilograms (14,017 lb), and is the largest known ornithomimosaur.[1] More than 1400 gastroliths were found inside the ribs and gastralia of Deinocheirus, along with fish remains. The stomach contents and ecomorphological features suggest that Deinocheirus was an omnivore.