A large duck-billed dinosaur with a camel-like hump and the neck of an ostrich lived in what is today Mongolia during the Cretaceous, about 70 million years ago, say Dr Tsogtbaatar Chinzorig of the Mongolian Academy of Sciences’ Paleontological Center and his colleagues who analyzed and pieced together dinosaur fossils recently uncovered in the Gobi desert.

For almost five decades this theropod dinosaur, named Deinocheirus mirificus, has remained one of the most mysterious prehistoric creatures.

It was known only from a pair of 2.4-m-long forelimbs discovered in the Nemegt Formation, Mongolia, in 1965.

The size of the limbs led some paleontologists to believe the dinosaur would be much larger than Tyrannosaurus rex, with its notoriously puny front limbs.

Now, with the discovery of two Deinocheirus mirificus’ skeletons in the Gobi desert, Dr Chinzorig’s team has produced the first accurate reconstruction of the dinosaur.

According to the scientists’ paper in the journal Nature, Deinocheirus mirificus was approximately 11 meters long and had an estimated weight of 6.4 tons.

“It was a behemoth to be sure – but hardly the giant tyrannosaur its massive arms may have suggested. Rather, the apparently disproportionately large forearms were more likely used for digging and gathering plants in freshwater habitats, or for fishing. Among its other unusual attributes are tall dorsal spines, truncated hoof-like claws on the feet to prevent sinking into muddy ground, and bulky hind legs that indicate it was a slow mover,” said co-author Prof Philip Currie of the University of Alberta’s Department of Biological Sciences.

“It almost appears to be a chimera, with its ornithomimid-like arms, its tyrannosaurid-like legs, its Spinosaurus-like vertebral spines, its sauropod-like hips, and its hadrosaur-like duckbill and foot-hooves.”

Deinocheirus mirificus is a descendant of ostrich-like dinosaurs that were only slightly larger than humans, so its evolution into a giant creature is almost certainly responsible for most of its unusual characteristics.

“Its great size probably gave it some protection from the tyrannosaurid Tarbosaurus, which appears to have been relatively common in that part of Mongolia 70 million years ago,” Prof Currie said.

To feed its great bulk, the dinosaur was apparently an omnivore that ingested both plants and fish, as evident from fish remains found in its stomach contents.

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Yuong-Nam Lee et al. Resolving the long-standing enigmas of a giant ornithomimosaur Deinocheirus mirificus. Nature, published online October 22, 2014; doi: 10.1038/nature13874