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The Mystery of Deinocheirus Solved

By John Long

With gigantic arms, a beer belly, a humped back and a duck bill, Dinocheirus is one bizarre dinosaur.

Collected in 1965 from an expedition to Mongolia, a pair of gigantic dinosaur arms each measuring 2.4 metres long have long intrigued palaeontologists and dinosaur fans alike. What kind of dinosaur owned such massive arms?

They are a record for the longest arms of any bipedal animal that has ever lived, barring the flying pterosaurs. It was given the monicker Deinocheirus mirificus, meaning “unusual horrible hand” because the arms had sharp claws and were initially thought of as belonging to a large predatory dinosaur of some sort, while others thought it might be one of the slender, ostrich-like ornithomimosaurs. Reconstructions of it abounded as a regular-looking but grossly overblown ostrich-mimic.

Now, new material of two partially articulate skeletons have been found and described in Nature. The results show that no matter how hard palaeontologists try to reconstruct an animal from a few bones, the truth can be very elusive: the animal is simply bizarre and unexpected in every way.

Imagine a beast with monstrous arms, a hump-back like a camel, a massive round beer-gut, and an expanded broad duck-like bill on a toothless mouth. It’s exactly like the first impressions of the platypus: a strange mosaic of many features, each of which, if found in isolation, would have justified classification in different animal families. “This is an entirely new body plan” for such dinosaurs, says dinosaur expert Stephen Brusatte of The University of Edinburgh.

The elongated back vertebrae probably helped support its massive belly, which contained more than 1400 gastroliths – stones ranging 8–87 mm in size that were used to help the animal grind down its food. In addition, the small bones of fish were found in its stomach area, suggesting that the animal was largely omnivorous, using its massive tongue to suck up aquatic plants and fish. Its broad toes attest to its penchant for wading in soft, squishy mud while feeding alongside rivers.

The discovery not only solves a 50-year mystery of what this beast was like, but demonstrates how diverse some of these dinosaurs really were. It stood around 4–5 metres high and was some 11 metres long, possibly weighing as much as 6.3 tonnes – the mass of a large Tyrannosaurus rex.

It lived in the late Cretaceous of Mongolia alongside a great diversity of other dinosaurs, including large predators like Tarbosaurus and many smaller nubile pack-hunters like the Velociraptor. It’s highly likely that its large arms bearing razor-sharp claws were its main defence in such a predator-filled ecosystem.

When the new data of the dinosaur were put through a phylogenetic analysis it came out as a fairly derived member of the Ornithomimosauria that simply evolved into its own bizarre shape by the end of the dinosaur era 70 million years ago.

John Long is Strategic Professor in Palaeontology at Flinders University.