Deinocheirus

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Deinocheirus
Temporal range:
Ma
↓
Large skeleton
Reconstructed skeleton in Japan
Scientific classification Edit this 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 (

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

hadrosaurs
.

The classification of Deinocheirus was long uncertain, and it was initially placed in the

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

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

theropod dinosaur.[1][2]

The specimen was discovered on a small hill in

generic name is derived from Greek deinos (δεινός), meaning "horrible", and cheir (χείρ), meaning "hand", due to the size and strong claws of the forelimbs. The specific name comes from Latin and means "unusual" or "peculiar", chosen for the unusual structure of the forelimbs.[3] The Polish-Mongolian expeditions were notable for being led by women, among the first to name new dinosaurs.[4] The original specimen number of the holotype was ZPal MgD-I/6, but it has since been re-catalogued as MPC-D 100/18.[1]

CosmoCaixa

The paucity of known Deinocheirus remains inhibited a thorough understanding of the animal for almost half a century onwards, and the scientific literature often described it as among the most "enigmatic", "mysterious", and "bizarre" of dinosaurs.[1][5][6] The holotype arms became part of a traveling exhibit of Mongolian dinosaur fossils, touring various countries.[7] 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, found by a Korean-Mongolian team which re-located the original quarry in 2008. Bite marks on two gastralia were identified as belonging to Tarbosaurus, and it was proposed that this accounted for the scattered, disassociated state of the holotype specimen.[6]

Additional specimens

In 2013, the discovery of two new Deinocheirus specimens was announced before the annual

fossil poachers. The second specimen, MPC-D 100/127, was found by scientists in the Bugiin Tsav locality (coordinates: 43°54.025′N 99°58.359′E / 43.900417°N 99.972650°E / 43.900417; 99.972650) in 2009. It is slightly larger than the holotype, and it could be clearly identified as Deinocheirus by its left forelimb, and therefore helped identify the earlier collected specimen as Deinocheirus. The specimen had also been excavated by poachers, who had removed the skull, hands and feet, but left behind a single toe bone. It had probably been looted after 2002, based on money left in the quarry.[1][8] Skulls, claw bones and teeth are often selectively targeted by poachers on the expense of the rest of the skeletons (which are often vandalized), due to their saleability.[9] 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.[10]

After the new specimens were announced, it was rumoured that a looted skull had found its way to a European museum through the

Thomas R. Holtz stated in an interview that the new Deinocheirus remains looked like the "product of a secret love affair between a hadrosaur and Gallimimus".[11]

Munich Fossil Show

Combined with the poached elements, both new specimens represent almost the entire skeleton of Deinocheirus, as MPC-D 100/127 includes all material apart from the middle dorsal vertebrae, most

hadrosaur hands were identified, and since the feet of Deinocheirus are now known to have been similar to those of hadrosaurs, it cannot be ruled out that the tracks were made by this genus.[15]

Description

Size comparison
Size of three specimens (holotype in green) compared to a human

Deinocheirus is the largest

tyrannosaurs, even though members of that group did not have large arms in proportion to their body size.[20][21][22]

Life restoration
Life restoration

The only known skull, belonging to the largest specimen, measures 1.024 m (3.36 ft) from 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 relative size of the lower jaw was closer to that of tyrannosaurids than to other ornithomimosaurs. The snout was spatulate (flared outwards to the sides) and 25 cm (9.8 in) wide, which is wider than the skull roof.[1] This shape was similar to the snout of duck-billed hadrosaurids.[14]

Postcranial skeleton

Arm diagram
Labelled diagram of the left arm and shoulder blade of Deinocheirus

Deinocheirus and

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 ornithomimosaur Ornithomimus.[3] The arm bones of Deinocheirus were similar in proportions to those of the small theropod Compsognathus.[23]

Though Deinocheirus was a bulky animal, its dorsal ribs were tall and relatively straight, indicating that the body was narrow.

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 interconnecting ligaments, which stiffened the vertebral column allowing it to support the abdomen while transmitting the stress to the hips and hindlimbs.[1] Together, the neural spines formed a tall "sail" along the lower back, hips, and base of the tail, somewhat similar to that of Spinosaurus.[14]

Vertebrae
Labelled diagram of some of the vertebrae that form a "sail"

All the vertebrae were highly

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.[1][24]

The wishbone (

ornithischian dinosaurs. The proportions of the toe bones resembled those of tyrannosaurs, due to the large weight they had to bear.[1]

Classification

When Deinocheirus was only known from the original forelimbs, its taxonomic relationship was difficult to determine, and several hypotheses were proposed.

Deinocheirosauria, which was to include the supposedly related genera Deinocheirus and Therizinosaurus. A relationship between Deinocheirus and the long-armed therizinosaurs was supported by some later writers, but they are not considered to be closely related today.[5]

In 2004, Peter Makovicky, Kobayashi and Currie pointed out that Deinocheirus was likely a primitive ornithomimosaurian, since it lacked some of the features typical of the

cladistic analysis accompanying the 2014 description of the two much more complete specimens found that Deinocheirus formed a clade with Garudimimus and Beishanlong, which were therefore included in the Deinocheiridae. The resulting cladogram follows below:[1]

Original arms
Holotype specimen MPC-D 100/18 on exhibit in CosmoCaixa
Ornithomimosauria

The 2014 study defined Deinocheiridae as a clade including 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 2014 cladogram suggested that ornithomimosaurians diverged into two major 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.[1] Deinocheirids and the smaller ornithomimids did not have teeth, unlike more primitive ornithomimosaurs.[14] In 2020, the deinocheirid Paraxenisaurus from Mexico was named, making it the first member of the group known from North America. Its describers suggested deinocheirids originated in Laurasia (the northern supercontinent of the time) or that they dispersed across polar regions in the Northern Hemisphere, and a similar interchange is also known to have occurred in other dinosaur groups with Asian affinities during the Campanian–Maastrichtian ages. This study also found Harpymimus to be a basal deinocheirid, while placing Beishanlong just outside the group, as a basal ornithomimosaur.[26]

Palaeobiology

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

The blunt and short hand-claws of Deinocheirus were similar to those of the therizinosaur

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.[1] The hand had good mobility relative to the lower arm, but was capable of only a limited flexing motion, unable to close in grasping.[3]

The brain of Deinocheirus was reconstructed through

olfactory tracts were relatively large. The brain was proportionally small and compact, and its reptile encephalization quotient (brain-body ratio) was estimated at 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.[27]

Life restoration showing sparse feathering

In 2015, Akinobu Watanabe and colleagues found that together with Archaeornithomimus and Gallimimus, Deinocheirus had the most pneumatised skeleton among ornithomimosaurs. Pneumatisation is thought to be advantageous for flight in modern birds, but its function in non-avian dinosaurs is not known with certainty. It has been proposed that pneumatisation was used to reduce the mass of large bones (associated with gigantic size in the case of Deinocheirus), that it was related to high metabolism, balance during locomotion, or used for thermoregulation.[24]

A bone

histological study of a gastralia fragment from the holotype presented at a 2018 conference showed that its internal structure was similar to that of ossified tendons of other theropods. The osteons contained possible canaliculi, which would be the first-known occurrence of such structures in a basal ornithomimosaur. The structure of the periosteum and lack of growth arrest lines suggests that the holotype was a fully grown adult.[29]

Diet

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.[1]

Claw cast
Cast of a hand-claw, showing its blunt, recurved shape

More than 1,400

coelurosaurian theropods. In spite of these features, fish vertebrae and scales were also found among the gastroliths, which suggests that it was an omnivore.[1] Ornithomimosaurs in general are thought to have fed on both plants and small animals.[14]

David J. Button and Zanno found in 2019 herbivorous dinosaurs mainly followed two distinct modes of feeding, either processing food in the gut—characterized by gracile skulls and low bite forces—or the mouth, characterized by features associated with extensive processing. Deinocheirus, along with ornithomimid ornithomimosaurs,

caenagnathids, was found to be in the former category. These researchers suggested that deinocheirids and ornithomimid ornithomimosaurians such as Gallimimus had invaded these niches separately, convergently achieving relatively large sizes. Advantages from large body mass in herbivores include increased intake rate of food and fasting resistance, and these trends may therefore indicate that deinocheirids and ornithomimids were more herbivorous than other ornithomimosaurians. They cautioned that the correlations between body mass and body mass were not simple, and that there was no directional trend towards increased mass seen in the clade. Furthermore, the diet of most ornithomimosaurians is poorly known, and Deinocheirus appears to have been at least opportunistically omnivorous.[30]

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.[31]

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.[14] In their original description, Osmólska and Roniewicz found that the hands of Deinocheirus were unsuited for grasping, but could instead have been used to tear prey apart.[3] In 1970, the Russian paleontologist Anatoly Konstantinovich Rozhdestvensky compared the forelimbs of Deinocheirus to sloths, leading him to hypothesise that Deinocheirus was a specialised climbing dinosaur, that fed on plants and animals found in trees.[32] In 1988, Paul instead suggested that the claws were too blunt for predatory purposes, but would have been good defensive weapons.[22] While attempting to determine the ecological niches for Deinocheirus and Therizinosaurus in 2010, Phil Senter and James H. Robins suggested that Deinocheirus had the largest vertical feeding range due to its hip height, and specialised in eating high foliage.[19] In 2017, it was suggested that the claws of Deinocheirus were adapted for pulling large quantities of herbaceous plants out of water, and to decrease the resistance of water.[33]

Palaeopathology

tyrannosaur
fed on Deinocheirus

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.[3][34] A rib of specimen MPC-D 100/127 shows a healed trauma which has remodelled the bone.[1] In 2012, bite marks on two gastralia of the holotype specimen were reported. The size and shape of the bite marks match the teeth of Tarbosaurus, the largest known predator 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 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.[6]

Palaeoenvironment

The Okavango Delta, which is similar to the environment Deinocheirus inhabited

The three known Deinocheirus specimens were recovered from the

caliche deposits indicate at least periodic droughts occurred. Sediment was deposited in the channels and floodplains of large rivers.[38]

Deinocheirus is thought to have been widely distributed within the Nemegt Formation, as the only three specimens found have been 50 km (31 mi) apart. The river systems of the Nemegt Formation provided a suitable niche for Deinocheirus with its omnivorous habits.

foliage such as 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.[14]

The habitats in and around the Nemegt rivers where Deinocheirus lived provided a home for a wide array of organisms. Occasional

troodontids such as Borogovia, Tochisaurus, and Zanabazar. Theropod groups with both omnivorous and herbivorous members include therizinosaurs, such as Therizinosaurus, oviraptorosaurians, such as Elmisaurus, Nemegtomaia, and Rinchenia, and other ornithomimosaurians, such as Anserimimus and Gallimimus.[42]

See also

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External links

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