Two recently discovered skeletons, described in the journal Nature, show the appendages belonged to Deinocheirus mirificus, a massive, bizarre-looking omnivore that lived 70 million years ago.

“Now we have two specimens that are complete enough, that we can confirm that these were ornithomimids,” says one of the study’s authors Dr Philip Currie of the University of Alberta.

Ornithomimosaurians are a group of theropod dinosaurs that bore a superficial resemblance to modern ostriches.

But Deinocheirus mirificus (Greek for horrible hand) were strange looking creatures compared to other ornithomimids.

“Ornithomimids usually have small heads like ostriches, however Deinocheirus had a skull over a metre long, with a beak spread out like a duck-bill dinosaur,” says Currie.

“This animal had feathers and that duck-bill face, this was a gigantic chicken!”

Massive mystery

Debate has raged among palaeontologists about the identity of massive 2.4-metre-long forearms and curved claws since they were discovered together with the remains of some ribs and vertebrae, at Altan Uul III in the southern Gobi desert of Mongolia in 1965.

The new almost-complete specimens were unearthed at separate sites in Mongolia’s Nemegt formation.

A small specimen, thought to be a juvenile, was found at Altan Uul IV in 2006, while a specimen even larger than the original 1965 discovery, was uncovered at Bugiin Tsav in 2009.

The larger of the two specimens is about 11-metres-long and would have weighed almost six-and-a half tonnes.

“It’s unusually big — as big as Tarbosaurus, the Asian cousin of Tyrannosaurus rex — and one of the biggest of the meat-eating dinosaurs,” says Currie.

“This animal [Deinocheirus] is far bigger than other ornithomimus dinosaurs which were about the size of a human.

And they were probably slow movers, he adds.

“The legs were stocky and looked a lot more like a Tyrannosaurus, and the hips are totally strange, they are awfully broad because they’re supporting a lot of weight.

“The vertebrae were also really peculiar, with neural spines that stick way up almost like a sail back, but in this case the sail is probably used to anchor powerful tendons to support the animal’s weight, like a cable-stayed bridge.”

The authors believe Deinocheirus sacrificed speed for size as part of a dinosaur arms race.

“Speed worked for a long time as a survival technique for these dinosaurs, but young predators like Tarbosaurs, would have been able to catch them, and so getting bigger made Deinocheirus too big to take on,” says Currie.

Walking with Deinocheirus mirificus Credit: Yuong-Nam Lee (KIGAM)

Ancient paradise

The world Deinocheirus lived in during the Cretaceous Period was reminiscent of the Nile River valley today.

“It was a very rich riverine environment with a broad band of vegetation close to the river, surrounded by an arid desert,” says Currie.

Deinocheirus would have been well suited to its river habitat, with its duck-like bill used to forage for food at the bottom of streams.

“Ornithomimids don’t have teeth, so it wasn’t surprising that previous ornithomimid discoveries included gastroliths (stomach stones) which usually indicates the animals were herbivores,” says Currie.

“Many modern day birds swallow stones to help them grind up and digest seeds and other plant material, as a proxy for teeth.

“Deinocheirus also has gastroliths, but mixed up with them were fish vertebrae and scales, so this guy had fairly broad culinary tastes, a true omnivore.”

Source: abc.net