Reconstruction of Deinocheirus mirificus. (Credit: Yuong-Nam Lee, KIGAM) We all know you can’t judge a book by its cover, but now paleontologists are learning you can’t judge a dinosaur by its forearms. For over 50 years, everything archaeologists knew about the ostrich-like dinosaur Deinocheirus mirificus was based on just two giant forearms unearthed in 1965. Understandably, the dinosaur represented one of the great unsolved mysteries in paleontology. However, two recent fossil discoveries have exposed the true identity of D. mirificus, and, boy, was it a big surprise.
Based on the forearms found in 1965, archaeologists determined that D. mirificus belonged to a family of medium-sized, ostrich-like dinosaurs known as ornithomimidae – picture the Gallimimus herd from the first Jurassic Park movie. D. mirificus had such large arms that it would certainly be a large and unusual member of this particular family. But that didn’t stop researchers from theorizing about its body type and where it fit in the lineage of dinosaurs. Then, in 2006 and 2009, two nearly complete D. mirificus skeletons were discovered in Mongolia. The analysis of these two skeletons, published this week in Nature, proves that this dinosaur was the biggest member of the ornithomimosaurs. One of the specimens had an estimated body weight of 14,000 pounds and was 36 feet long. Although D. mirificus was large, it wasn’t what you would call a ferocious dinosaur. It had a duck-like, toothless bill and flexible tongue that it used in combination to sift the bottoms of lakes and ponds for fish and plants. Rather than munch its food with teeth, it swallowed thousands of tiny stones to grind organic matter in its digestive system, like a chicken does. Unlike other ornithomimosaurs, D. mirificus was heavy-footed and slow. Researchers believe its only defense was being big.
The two new fossil finds revealed insights about D. mirificus, such as its humped back, that would’ve been impossible to surmise with two forearm specimens. Researchers wrote that D. mirificus serves as a cautionary tale for predicting the body forms of dinosaurs from partial skeletons, even if the animals are closely related. It’s easy to see why paleontologists may give an arm and a leg, to unearth more than just an arm or a leg.