Deinocheirus: The dinosaur that consisted of nothing but arms for 50 years
A dinosaur, eleven meters long. Yet for almost five decades, all that was known of it was a pair of enormous arms. No face, no legs, no body. Only in 2014 did the truth come to light: by far the largest known representative of the "ostrich dinosaurs".
At a glance
| feature | Value |
|---|---|
| length | approx. 11 m |
| Weight | approx. 6.4 t (range of other estimates: 5–7.3 t) |
| Location | Nemegt Formation, Gobi Desert, Mongolia |
| age | Late Cretaceous, early Maastrichtian, approximately 70 million years ago |
| Food | omnivore |
| Special feature | Largest known ornithomimosaurus, 2.4 m long arms |
At eleven meters long, it was about the same length as a city bus. Its weight of around 6.4 tons was comparable to that of a large bull elephant. Its arms measured 2.4 meters. That's more than most basketball hoops hang above the ground. The claws on them were about 20 centimeters long: roughly the length of a banana.
Why did Deinocheirus remain a mystery for almost 50 years?
On July 9, 1965, during the Polish-Mongolian expeditions, Polish paleontologist Zofia Kielan-Jaworowska unearthed a pair of 2.4-meter-long arms, along with shoulder girdles, individual vertebrae, and ribs, at the Altan Uul III site in the Gobi Desert. The skull, legs, and torso were completely missing. Nevertheless, in 1970, Halszka Osmólska and Ewa Roniewicz gave the find a name: Deinocheirus mirificus, the “unusual terrible hand”.
Without a body, there was plenty of room for wild ideas. Was it a predator, larger than Tyrannosaurus? A relative of the clawed therizinosaurs? Or a climbing, sloth-like creature? As early as 1971, John Ostrom suggested a different solution: a close relationship to the ostrich dinosaurs. He couldn't prove it, however; the necessary material was lacking. Thus, the find remained one of paleontology's great unsolved mysteries for almost five decades.
What does a fossil theft have to do with the solution?
Die Antwort kam erst mit zwei weiteren Skeletten. 2006 fand die Korea-Mongolia International Dinosaur Expedition an der Fundstelle Altan Uul IV ein rund 74 Prozent großes Jungtier (MPC-D 100/128). 2009 folgte in Bugiin Tsav das bislang größte bekannte Exemplar (MPC-D 100/127) – sechs Prozent größer als der ursprüngliche Holotyp.
Both skeletons had already been plundered before their scientific recovery: the larger specimen was missing its skull, hands, and feet. These parts resurfaced in a European private collection in 2011. The French fossil dealer François Escuillié acquired them and handed them over to the Royal Belgian Institute of Natural Sciences.
Only with the complete material could a team led by Yuong-Nam Lee 2014 in Nature presenting the solution – accompanied by a commentary from paleontologist Thomas R. Holtz entitled “Mystery of the horrible hands solved”.
A body as if composed of several animals
The only known skull is just over a meter long. Behind the eyes, however, it measures only 23 centimeters. It has a toothless, broad beak like a duck's – curved downwards, similar to a hadrosaur. The jaw muscles were only weakly attached. Deinocheirus probably had a rather weak bite.
A sail-like structure rises above the back. The spinous processes of the posterior vertebrae become progressively longer towards the back. At the highest vertebra, they are 8.5 times as long as the vertebra itself. Only in Therizinosaurus and at the more well-known Spinosaurus There is a similarly extreme relationship. At the end of the tail is a pygostyle – a fused remnant of the last tail vertebrae, otherwise only known from birds – and a U-shaped furcula, or wishbone. Both of these demonstrate that the relationship to birds was close.
Within the ostrich dinosaurs, Deinocheirus forms a group together with the significantly smaller Garudimimus and the genus Beishanlong, which until now has only been known from specialist literature, the family Deinocheiridae.
The arms remain its trademark feature. 2.4 meters long, with three fingers almost equally long. Only Therizinosaurus had similarly long arms – no other bipedal dinosaur had longer ones. However, the hands were hardly suitable for grasping. They were more likely used for digging and pulling plants along the shore.
What was on the menu?
In the stomach area of the largest skeleton, researchers found over 1,400 gastroliths. These are stones that helped grind up food. Fish vertebrae and scales were also found. This suggests an omnivore: aquatic plants and fish were likely both on the menu. Similar to modern-day ducks or flamingos – only on a gigantic scale.
The Nemegt Formation, where Deinocheirus lived, was significantly wetter than the older desert strata of the Gobi. River channels, mudflats, shallow lakes – often compared to the present-day Okavango Delta in Botswana. Nevertheless, he wasn't safe there. Bite marks on his ribs are attributed to the large predator. Tarbosaurus attributed to it. Even a six-ton giant was not entirely spared.
Why did he weigh ten times as much as his closest relatives?
Deinocheirus belongs to a small family called Deinocheiridae. This family includes only two other, significantly smaller genera: Garudimimus and Beishanlong. The sister group is the slender, fast-running Ornithomimidae. The size difference is enormous. Gallimimus, the largest "classical" representative of this group from the same geological formation, weighed only 285 to 640 kilograms, depending on the calculation. Deinocheirus, at 6.4 tons, weighed at least ten times as much.
A study published as a preprint in 2022, which has not yet undergone final peer review, by Chinzorig, Cullen, Phillips, Rolke and Zanno This describes the foot bones of an ornithomimosaurus from the Eutaw Formation in Mississippi (USA). Their size is more consistent with Deinocheirus than with other known species from North America. If this is confirmed, such giants among the ostrich-like dinosaurs may not have existed only in Mongolia.
From museum piece to streaming star
In the Apple TV+ documentary "Prehistoric Planet" (Season 1, Episode 3, "Freshwater"), a feathered Deinocheirus wades through an Asian wetland. In the game "Jurassic World Evolution," it is one of the playable dinosaurs. However, it is officially absent from "ARK: Survival Evolved." There, it is only available through a popular fan mod, "ARK Additions"—nicknamed "Duck King" in the community.
More information on this topic can be found at urzeittiere.net
- Therizinosaurus – the only other dinosaur with similarly long arms and claws, but from a completely different group of carnivores.
- Tyrannosaurus Rex – for comparison, the most famous predator of the late Cretaceous period: much smaller arms, but a stronger bite.
Image sources: FunkMonk (Michael B. H.), CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons
Size comparison: Conty, CC BY 3.0 https://creativecommons.org/licenses/by/3.0, via Wikimedia Commons
Skull: Derdadort, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons
Deinocheirus arms: Eduard Solà, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons