Deinocheirus

Deinocheirus Temporal range: Late Cretaceous,
Arms and shoulder blades of the holotype specimen in CosmoCaixa, Barcelona
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Clade:	†Ornithomimosauria
Family:	†Deinocheiridae
Genus:	†Deinocheirus Osmólska & Roniewicz, 1970
Species:	†D. mirificus
Binomial name
†Deinocheirus mirificus Osmólska & Roniewicz, 1970

Deinocheirus (/ˌdaɪnˈkaɪərəs/ DY-no-KY-rəs; Greek: 'terrible hand') is a genus of very large ornithomimosaurian dinosaur that lived in what is now southern Mongolia during the Late Cretaceous around 70 million years ago. The genus contains one species, Deinocheirus mirificus. The taxon was named in 1970 by Osmólska and Roniweicz, for a pair of complete shoulder girdles and arms.

Deinocheirus was long thought of as a very mysterious dinosaur, known only from a set of gigantic fossil arm bones many meters in length. Speculation was made about many aspects of the biology of Deinocheirus, such as size, diet, and classification. The discovery of more complete skeletons in the late 2000s helped to solve this longstanding mystery, revealing a very strange, giant, bipedal omnivore. It had a skull with a wide bill and a thick lower jaw, high vertebrae spines forming a distinctive feature along its back, and was a gigantic 11 m (36 ft) long and had a weight of 6.36 t (14,000 lb).

Through history, the placement of Deinocheirus was uncertain. It was thought to be a carnosaur, a new group, and an ornithomimosaurian. Deinocheirus was confirmed to represent a truly unique ornithomimosaurian with new discoveries, possessing a wide variety of similar features to those of the group. It was related to Beishanlong and Garudimimus and together the genera formed the family Deinocheiridae.

Description

Size compared to a human

Deinocheirus was the largest ornithomimosaurian dinosaur; the largest known specimen (MPC-D 100/127) measured up to 11 m (36 ft) long, with an estimated weight of 6.36 t (14,000 lb). The two other known specimens are smaller, the holotype (MPC-D100/18) by 6%, and the smallest (MPC-D100/128) by 74%. When only the incomplete holotype specimen was known, various sizes were extrapolated from them by various methods. A 2010 study estimated the hip height of Deinocheirus to be 3.3–3.6 m (11–12 ft). The weight had previously been estimated from 2 tonnes (4,400 lb) to 12 tonnes (26,000 lb).

Deinocheirus and Therizinosaurus possessed the longest forelimbs known for any bipedal dinosaurs. The holotype forelimbs measure 2.4 m (7.9 ft) long — the humerus (upper arm bone) was 938 mm (36.9 in), the ulna 688 mm (27.1 in), and the hand was 770 mm (30 in) — including the 19.6 cm (7.7 in) long recurved claws. Each scapulocoracoid of the shoulder girdle had a length of 153 cm (60 in). Each half of the paired ceratobranchialia measure 42 cm (17 in). The shoulder-blade was long and narrow, and the deltopectoralis crest was pronounced and triangular. The humerus was relatively slender, and only slightly longer than the hand. The ulna and radius were elongate and not very firmly connected to each other in a syndesmosis. The metacarpus was long compared to the fingers. The three fingers were about equal in length, the first being the stoutest and the second the longest. Various rough areas and impressions on the forelimbs indicate the presence of powerful muscles. Most articular surfaces of the arm bones were deeply furrowed, indicating that the animal had thick pads of cartilage between the joints. Though the arms of Deinocheirus were large, the ratio between them and the shoulder girdle was less than that of the smaller Ornithomimus. The arm bones of Deinocheirus also had similar proportions to those of the small theropod Compsognathus. The furcula, an element not known from any other ornithomimosaurs, was U-shaped. The hindlimbs were relatively short, and the thigh bone was longer than the shin bone, 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.

Skull on exhibit in Munich

The only known skull, belonging to the largest specimen, measures 1,024 mm (40.3 in) from the premaxilla at the front to the back of the occipital condyle. The skull was similar to those of other ornithomimosaurs in being low and narrow, but differed in that the snout was more elongated. The skull bone walls were rather thin, about 6 mm (0.24 in). It had a rounded, flattened beak, which would have been covered by keratin in life. The nostrils were turned upwards, and the nasal bone was a narrow strap, that extended up above the eye sockets. The outer diameter of the sclerotic rings in the eyes were small, 84 mm (3.3 in), compared to the size of the skull. The rear of the skull was relatively small. The lower temporal fenestrae, openings behind the eyes, were partially closed off by the jugal bones, similar to Gallimimus. The jaws were toothless and down-turned, and the lower jaw was very massive and deep compared to the slender and low upper jaw. The lower jaw was closer in relative size to that of tyrannosaurids than to other ornithomimosaurs. The snout flared outwards to the sides, which is similar to the duck-billed hadrosaurids.

Diagram of the dorsal and sacral vertebrae that form a "sail"

The ten neck vertebrae were low and long, and become progressively shorter backwards from the skull. This resulted in a more S-curved neck than seen in other ornithomomisaurs, due to the larger skull. The neural spines of the twelve back vertebrae became increasingly longer from front to back, the last one being 8.5 times the height of the centrum part. This is almost the same as the highest ratio in the neural spines of the theropod Spinosaurus. The neural spines had a system of ligaments, which were probably used to support the abdomen by attaching to the hips and hind legs. Together, the neural spines formed a tall "sail" along the lower back, hips, and base of the tail, somewhat similar to that of Spinosaurus.

Life restoration

All the vertebrae were highly pneumatised, except for the atlas bone and the hindmost tail vertebrae. Pneumatised vertebrae have been invaded by the air sacks of the respiratory system. This causes hollows on the sides, as well as a hollowed-out inside. The back vertebrae were as pneumatised as those of sauropod dinosaurs, and had an extensive system of depressions. These adaptations may be correlated with gigantism. The six vertebrae of the sacrum were also tall and pneumatised, and all but the first one were fused together at the top, their neural spines forming a neural plate. The ilium, the top hip bone, was also partially pneumatised close to the sacral vertebrae. Part of the pelvis was hypertrophied (enlarged) compared to other ornithomimosaurs, to support the weight of the animal with strong muscle attachments. The front hip bones tilted upwards in life. The tail of Deinocheirus ended in at least two fused vertebrae, which were described as similar to the pygostyle of oviraptorosaurian and therizinosauroid theropods. Ornithomimosaurs are known to have had pennaceous feathers, so this feature suggests that they might have had a fan of feathers at the tail end.

History of discovery

Holotype on display, Barcelona

The first known fossil remains are a single pair of massive forelimbs and their respective shoulder girdles. They were uncovered on July 9 1965 during a Polish-Mongolian expedition to the Gobi by Professor Zofia Kielan-Jaworowska at the Altan Ula III site in the Nemegt Basin. The original specimen number was ZPal MgD-I/6, but this has since re-catalogued as MPC-D 100/18. The crew of the expedition spent July 9–11 excavating the specimen and loading it onto a vehicle. The remains were briefly published on in the 1968 narrative of the expeditions by Zofia Kielan-Jaworowska and Naydin Dovchin, who had already realised the significance of the remains. The coordinates for the find were 43°33.987′N 100°28.959′E. The holotype specimen 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.

Deinocheirus was named by Halszka Osmólska and Ewa Roniewicz in 1970. The type 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, comes from Latin, meaning "unusual" or "peculiar," and was chosen for the "unusual structure of the forelimbs."

Cast of the holotype arms in American Museum of Natural History

In 2012, Phil R. Bell, Philip J. Currie, and Yuong-Nam Lee announced the discovery of additional elements of the holotype specimen, including fragments of gastralia (belly ribs), found by teams re-examining the original quarry. The possibility that the carcass was scavenged by Tarbosaurus may explain why the specimen was preserved in a scattered, disassociated state.

In the early 2010s, informal reports of new specimens of Deinocheirus surfaced, describing the animal as a gigantic ornithomimosaur with a hump on its back and a skull that resembled a cross between an ornithomimosaur and a hadrosaur. In 2013 these reports were clarified as the findings were presented before the Society of Vertebrate Paleontology. The new specimens are housed in the Paleontological Center of Mongolian Academy of Sciences, were given the specimens numbers MPC-D 100/127 and MPC-D 100/128.

MPC-D 100/128 was first discovered in 2006, during the Korea-Mongolia International Dinosaur Expedition. It was found in the Altan Ula IV locality of the Nemegt Formation, at the specific coordinates 43°36.091′N 100°27.066′E. The first people to uncover it were fossil poachers, who damaged the specimen but did not remove anything as they could not find the head, hands or feet, common elements that could sell for large sums of money. The second specimen, found in 2009 expeditions by palaeontologists, was given the number MPC-D 100/127. It was from the Bugiin Tsav locality at the coordinates 43°54.025′N 99°58.359′E. This specimen included many partially articulated bones and multiple blocks. It too had been damaged by poachers, who found and removed the skull, hands and feet. However, they were apparently rushed as they left behind a single toe bone.

The missing elements were in 2011 obtained by the French fossil trader François Escuillé who donated them to the Royal Belgian Institute of Natural Sciences. She and Pascal Godefroit informed the other authors of the presence of a skull, left hand and feet in a private collection in Europe. The specimens were bought by the authors, who guaranteed that it was their specimen because the single toe bone fit perfectly into the unprepared matrix of the poached feet. In May of 2014 the fossils were finally repatriated to Mongolia. Together, both specimens provide almost the entire skeleton of Deinocheirus, as MPC-D 100/127 includes all material apart from the middle dorsal vertebrae, most caudals, and the right forelimb; MPC-D 100/128 fills in most gaps of the other skeleton, with nearly all post-cervical (dorsal and caudal) vertebrae, ilia, partial ischia, and most of the left hindlimb. In 2014, the specimens were described in the Nature journal by Yuong-Nam Lee, Rinchen Barsbold, Philip J. Currie, Yoshitsugu Kobayashi, Hang-Jae Lee, Pascal Godefroit, François Escuillié and Tsogtbaatar Chinzorig.

Classification

When Deinocheirus was only known from the original forelimbs, its taxonomic relationship was difficult to determine, and several hypotheses were proposed. Osmólska and Roniewicz created a new family for Deinocheirus, the Deinocheiridae, initially placed in the infraorder Carnosauria, owing to the large size and thick-walled limb bones, and specuated that it represented a link between Carnosauria and Coelurosauria. In 1971, John Ostrom first proposed that Deinocheirus had similarities with Ornithomimosauria, while noting that it contained both ornithomimosaurian and non-ornithomimosaurian characters. In 1976, Rhinchen Barsbold named the order Deinocheirosauria, which was to include the supposedly closely related genera Deinocheirus and Therizinosaurus. Peter Makovicky et al. pointed out that Deinocheirus was likely a fairly primitive ornithomimosaurian, since it lacked some of the features typically seen in ornithomimids. Primitive 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.

Diagram of a left arm and shoulder blade

A cladistics analysis accompanying the 2014 description of two relatively complete specimens found that Deinocheirus formed a clade with Garudimimus and Beishanlong, which were therefore included in the Deinocheiridae. The resulting cladogram follows below:

Ornithomimosauria	Nqwebasaurus unnamed Pelecanimimus unnamed Shenzhousaurus unnamed Harpymimus unnamed Deinocheiridae Beishanlong unnamed Garudimimus Deinocheirus Ornithomimidae Anserimimus unnamed Gallimimus unnamed Ornithomimus Struthiomimus

Deinocheiridae was defined to include all taxa with a more recent common ancestor with Deinocheirus mirificus than with Ornithomimus velox. The three members share various anatomical features in the limbs. The cladogram suggested that ornithomimosaurians diverged into two lineages in the Early Cretaceous, Deinocheiridae and Ornithomimidae. Unlike other ornithomimosaurians, deinocheirids were not built for running. The anatomical peculiarities of Deinocheirus when compared to other, much smaller ornithomimosaurs, can largely be explained by its much larger size and weight. Deinocheirids and the smaller ornithomimids did not have teeth, unlike more primitive ornithomimosaurs.

Paleobiology

Cast of a claw in Japan

The blunt and short claws of Deinocheirus are 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 helped it against predators such as Tarbosaurus, but in turn it lost the running ability of other ornithominosaurs. 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 sclerotic rings of the eyes were relatively small in comparison with its skull length. 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 brain of Deinocheirus was reconstructed through CT scans and presented at the 2014 Society of Vertebrate Palaeontology conference. The brain was globular and similar in shape to that of birds and troodontid theropods, the cerebrum was expanded in a way similar to most theropods, and the olfactory tracts were relatively large. The brain was proportionally small and compact, and its Reptile Encephalisation Quotient (brain-body ratio) was estimated as 0.69, which is low for theropods, and similar to sauropods. Other ornithomimosaurs have proportionally large brains, and the small brain of Deinocheirus may reflect its social behaviour or diet. Its coordination and balance would not have been as important as for carnivorous theropods.

Diet

Reconstructed cast of the holotype in Museum of Natural History, London

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 understory 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 various freshwater bodies.

More than 1,400 gastroliths (8 to 87mm) in size were found among the ribs and gastralia of specimen MPC-D100/127. The ratio of gastrolith mass to total weight, 0.0022, supports that these helped these toothless animals in grinding their food. Features such as the presence of a beak and a U-shaped, downturned jaw, are indicators of facultative herbivory among coelurosaurian theropods. Yet fish vertebrae and scales were also found among the gastroliths, which suggests that it was an omnivore. Ornithomimosaurs in general are thought to have fed on both plants and small animals.

Front view of a mounted arm cast, Japan

Osmolska and Roniewicz found that the hands of Deinocheirus were unsuited for grasping, but could instead have been used to tear prey apart. 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". G.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 hypothesise that Deinocheirus was a specialised 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.

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 titanosaurian Opisthocoelicaudia, the hadrosaurs Saurolophus and Barsboldia, and the bizarre even longer-clawed Therizinosaurus. 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. 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.

Palaeopathology

The holotype specimen 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. The damage may have caused changes to the arrangement of ligaments of musces. The two coracoids are differently developed, and another pathology is a healed bone trauma showing the new bone on the seventh rib of MPC-D 100/127.

Bite marks have been found on two fragmentary gastralia (lower ribs) of the holotype specimen. The size and shape of the marks made by teeth match Tarbosaurus more than any other theropod from the Nemegt Formation. Various types of feeding traces were identified; punctures, gouges, striae, fragmentary teeth, and combinations of the above marks. The bite marks probably represent feeding or scavenging behaviour instead of aggression between the species, and the fact that bite marks were not found elsewhere on the body indicates the predator focused on internal organs. Tarbosaurus bite marks have also been identified on hadrosaur and sauropod fossils, but theropod bite marks on bones of other theropods are very rare in the fossil record.

Palaeocology

Tarbosaurus, a predator of Deinocheirus, in Late Cretaceous Mongolian environment

The three known Deinocheirus specimens have been recovered from the Nemegt Formation in the Gobi Desert of southern Mongolia. 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 stage, at the end of the Late Cretaceous about 70 million years ago. The rock facies of the Nemegt formation suggest the presence of stream and river channels, mudflats, and shallow lakes. Such large river channels and soil deposits are evidence of a far more humid climate than those found in the older Barun Goyot and Djadochta formations. However, caliche deposits indicate at least periodic droughts. Sediment was deposited in the channels and floodplains of large rivers. Fossils found in these sediments also indicate a rich ecosystem, offering diverse food in abundant amounts that could sustain large dinosaurs like Deinocheirus.

The river systems of the Nemegt Formation provided a suitable niche for Deinocheirus with its omnivorous habits. The environment was similar to the Okavango Delta of present day Botswana. Within this ecosystem, Deinocheirus would have eaten plants and small animals, including fish. It may have competed for trees with other large herbivorous dinosaurs such as the long-necked theropod Therizinosaurus, various titanosaurian sauropods, and the smaller hadrosaurid Saurolophus. Deinocheirus may have competed with those herbivores for trees, but was also able to feed on material that they could not. Along with Deinocheirus, the discoveries of Therizinosaurus and Gigantoraptor show that three groups of herbivorous theropods (ornithomimosaurs, therizinosaurs and oviraptorosaurs), independently reached their maximum sizes in the late Cretaceous of Asia.

The habitats in and around the Nemegt rivers where Deinocheirus lived provided a home for a wide array of organisms. Occasional mollusc fossils are found, as well as a variety of other aquatic animals like fish and turtles. Nemegt crocodylomorphs included several species of Shamosuchus, a genus with teeth adapted for crushing shells. Mammal fossils are extremely rare in the Nemegt Formation, but many birds have been found, including the enantiornithine Gurilynia and the hesperornithiform Judinornis, as well as Teviornis, a possible early member of the Anseriformes, the bird order that includes ducks and geese. Herbivorous dinosaurs of the Nemegt Formation include ankylosaurids such as Tarchia, and the pachycephalosaurian Prenocephale, large hadrosaurids such as Saurolophus and Barsboldia, and sauropods such as Nemegtosaurus, and Opisthocoelicaudia. Predatory theropods included Tarbosaurus, the small tyrannosaurid Alioramus, troodontids (Borogovia, Tochisaurus, and Saurornithoides), oviraptorosaurians (Elmisaurus, Nemegtomaia, and Rinchenia), and Bagaraatan, sometimes considered a basal tyrannosauroid. Other theropods, like the gigantic Therizinosaurus and ornithomimosaurians such as Anserimimus and Gallimimus from the Nemegt Formation, may have been primarily herbivorous.

External links

• TEDx Talk about Deinocheirus featuring Philip J. Currie
• CGI animation of Deinocheirus by Korea Institute of Geoscience and Mineral Resources