Smithsonian.com

The skull of Tarbosaurus. Photo by Flickr user Jordi Paya.

When I think of Deinocheirus, I think of arms. A few other parts of the dinosaur’s skeleton are known—vertebrae, ribs and most of the hip—but none of those elements are quite as impressive as the immense forelimbs. The arms, tipped with curved claws, measure about eight feet long, and the creature that carried them must have been about as large as the stubby-armed tyrannosaurs that roamed the same habitats in Mongolia around 70 million years ago. The clues from the arms and associated bones hint that Deinocheirus was a gigantic ornithomimid—one of the “ostrich mimic” dinosaurs like Struthiomimus. The trouble is that only the single specimen has been described so far, and so many parts of the skeleton are missing that we don’t wholly know what the gargantuan dinosaur looked like. A new paper, online at Cretaceous Research, suggests that the dining habits of tyrannosaurs might explain why paleontologists didn’t find more of Deinocheirus.

Deinocheirus was discovered in 1965 by the Polish-Mongolian Palaeontological Expedition. To find out more about this dinosaur, in 2008 members of the Korea-Mongolia International Dinosaur Project tracked down the quarry that yielded the single known specimen. The paleontologists hoped that the original excavations had left some bones behind, or that new pieces of the dinosaur’s skeleton might have been exposed in the intervening time.

According to the Cretaceous Research paper by Phil Bell, Philip Currie and Yuong-Nam Lee, the search turned up multiple bone fragments and several gastralia—the “belly ribs” that formed a basket beneath the dinosaur’s ribcage. And those gastralia may explain why so little of Deinocheirus became preserved. Two of the slender, curved bones recorded the bite marks of a large predatory dinosaur. This Deinocheirus was being eaten shortly before burial.

There are many kinds of bite marks. Paleontologists can categorize them, and each pattern of damage corresponds to different biting behavior. The Deinocheirus gastralia exhibited two different kinds of bite marks: tooth scores and parallel striations created as the serrations of the carnivorous dinosaur’s teeth scraped along the bone surface. The minute troughs suggest that a large tyrannosaur, most likely Tarbosaurus, fed on the Deinocheirus. Since the striations record the number and shape of bumps called denticles on the feeding dinosaur’s teeth, they act like a sort of dental fingerprint. Of all the theropod dinosaurs found in the same geologic formation, only Tarbosaurus seems to have had teeth that match the damaged bones.

We can’t know whether the tyrannosaur killed the Deinocheirus or scavenged it. While healed bite wounds record attacks that the victim survived, unhealed bite marks only show that the dinosaur was consumed before burial. In this case, it seems that the tyrannosaur opened up the stomach of the Deinocheirus for access to the viscera inside, but the bite marks record only those brief, violent moments. Whether the tyrannosaur brought down the Deinocheirus or just happened across a rotting carcass is a mystery. But the tyrannosaur also ensured that the particular Deinocheirus would remain an enigma. As the Tarbosaurus feasted, it dismembered the body and scattered the bones of its prey. If paleontologists want a complete look at Deinocheirus, they are going to have to hope for another skeleton elsewhere.

References:

Bell, P.R., Currie, P.J., Lee, Y. (2012). Tyrannosaur feeding traces on Deinocheirus (Theropoda:?Ornithomimosauria) remains from the Nemegt Formation (Late Cretaceous), Mongolia Cretaceous Research : 10.1016/j.cretres.2012.03.018