Deinocheirus

In 2013, the discovery of two new Deinocheirus specimens was announced before the annual Society of Vertebrate Paleontology (SVP) conference by Lee, Barsbold, Currie, and colleagues. Housed at the Mongolian Academy of Sciences, these two headless individuals were given the specimen numbers MPC-D 100/127 and MPC-D 100/128. MPC-D 100/128, a subadult specimen, was found by scientists in the Altan Ula IV locality (coordinates: 43°36.091′N 100°27.066′E) of the Nemegt Formation during the Korea-Mongolia International Dinosaur Expedition in 2006, but had already been damaged by 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) 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 black market.[10] The poached elements were spotted in a private European collection by the French fossil trader François Escuillé, who notified Belgian palaeontologist Pascal Godefroit about them in 2011. They suspected the remains belonged to Deinocheirus, and contacted the Korean-Mongolian team. Escuillé subsequently acquired the fossils and donated them to the Royal Belgian Institute of Natural Sciences.[11] The recovered material consisted of a skull, a left hand, and feet, which had been collected in Mongolia, sold to a Japanese buyer, and resold to a German party (the fossils also passed through China and France). The team concluded that these elements belonged to specimen MPC-D 100/127, as the single leftover toe bone fit perfectly into the unprepared matrix of a poached foot, the bone and matrix matched in colour, and because the elements belonged to an individual of the same size, with no overlap in skeletal elements.[1][12] 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.[13] The reunited skeleton was deposited at the Central Museum of Mongolian Dinosaurs in Ulaanbaatar, along with a Tarbosaurus skeleton which had also been brought back after being stolen. American palaeontologist 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]

Cast of specimen MPC-D 100/127's skull (the first known skull of this genus), on exhibit at 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 caudal vertebrae, and the right forelimb; MPC-D 100/128 fills in most gaps of the other skeleton, with nearly all dorsal and caudal vertebrae, the ilium, a partial ischium, and most of the left hindlimb. In 2014, the specimens were described by Lee, Barsbold, Currie, Yoshitsugu Kobayashi, Hang-Jae Lee Lee, Godefroit, Escuillié, and Tsogtbaatar Chinzorig.[1] A similar series of events was reported earlier in 2014 with Spinosaurus, another sail-backed theropod which had only been known from few remains since 1912. Poached remains were reunited with specimens obtained by scientists, and Spinosaurus was shown to have been quite different from other spinosaurids. The two cases showed that the lifestyle and appearance of incompletely known extinct animals cannot always be safely inferred from close relatives.[14] By 2017, the Mongolian government had increased its effort to seize poached fossils from collectors and repatriate them, but proving their provenance had become a scientific and political concern. Therefore, a study tested the possibility of identifying poached fossils by geochemical methods, using Deinocheirus and other Nemegt dinosaurs as examples.[9] In 2018, numerous large, tridactyl (three-toed) tracks were reported from the Nemegt locality (discovered in 2007 alongside sauropod tracks). Though the tracks were similar to those of hadrosaurs, no tracks of 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]

Labelled diagram of the left arm and shoulder blade of Deinocheirus

Deinocheirus and Therizinosaurus possessed the longest forelimbs known for any bipedal dinosaurs.[19] The holotype forelimbs measure 2.4 m (7.9 ft) long—the humerus (upper arm bone) is 93.8 cm (36.9 in), the ulna 68.8 cm (27.1 in), and the hand is 77 cm (30 in)—including the 19.6-centimetre-long (7.7 in) recurved claws. Each scapulocoracoid of the shoulder girdle has a length of 1.53 m (5 ft). 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 upper arm (humerus) was relatively slender, and only slightly longer than the hand. The ulna and radius (lower arm bones) were elongate and not 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 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.[8] The ten neck vertebrae were low and long, and progressively shorter backwards from the skull. This resulted in a more S-curved neck than seen in other ornithomimosaurs, 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 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]

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

All the vertebrae were highly pneumatised by invading air sacs, except for the atlas bone and the hindmost tail vertebrae, and were thereby connected to the respiratory system. 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, as they reduce weight. 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.[1][24]

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.[1]

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]

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

More than 1,400 gastroliths (stomach stones, 8 to 87 mm 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 the theory that these gastroliths helped the toothless animals in grinding their food. Features such as the presence of a beak and a U-shaped, downturned jaw, are indicators of facultative (optional) herbivory among 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, diplodocoid and titanosaur sauropods, Segnosaurus, and 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]

Restoration of two Tarbosaurus attacking a Deinocheirus; bite marks show that this 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]