Deinocheirus mirificus: Mysterious Arms and High-Browsing Habits
Size: An estimated size of 30 feet (10 m)
Time Period: The Early Maastrichtian age, about 70 million years ago.
Locale: The Nemegt Formation of Mongolia.
Name: The name Deinocheirus means “terrible hand,” in reference to the only part of the creature that has ever been discovered.
In some cases, it is not known what the entirety of an extinct animal looks like. Scientists are forced to speculate. In the case of the strange Deinocheirus, only a pair of arms is known. These arms are very remarkable, measuring at 8 feet (2.4 meters) each. The owner of these arms is generally accepted to have been a huge ornithomimosaur like Gallimimus, who it lived alongside. But what did it use these arms for?
It was originally proposed that this dinosaur’s arms were used as offensive weapons, powerful tools that it could use to kill smaller animals. Estimates have calculated that this creature was as big as a Tyrannosaurus, and it would have been a fearsome sight. However, the Nemegt Formation was already home to Tarbosaurus bataar, a large predator who filled multiple niches, both as a growing youngster and a fearsome adult. Deinocheirus was probably a high-browsing theropod, a term used for large herbivorous theropods that used long claws to retrieve food from trees. This would put it in a similar niche to that of the contemporaneous Therizinosaurus, if not a more specialized one. If Deinocheirus really was an ornithomimosaur as big as its arms suggest, it would have been an immense 12 feet (3.6 m) at the hip! This suggests that it would have used its long neck to feed higher than Therizinosaurus, placing it at the same browsing level as contemporaneous titanosaurs such as Nemegtosaurus. In fact, this measurement suggests that Deinocheirus was the tallest theropod, taller even than its main predator, Tarbosaurus, who it probably used the claws on its hands to defend against.
The intriguing thing about Deinocheirus is that it seems to represent yet another gigantic coelurosaur, similar to Gigantoraptor, Therizinosaurus, and Tyrannosaurus itself. There was a pattern among coelurosaurs to grow to gigantic size, either to avoid predation or become superpredators themselves. While this trend was especially prevalent among tyrannosaurids, we now know that it occurred among many groups of coelurosaur. The three largest nonpredatory coelurosaurs, all from Mongolia, represent a radiation of high-browsing theropods, who presumably had a similar lifestyle to the giant ground sloths of the Pliestocene, who used elongated hand claws to pull branches closer to them.
While we may never know what Deinocheirus looked like apart from its arms, we can infer how it used them. By making educated guesses, we can see that Deinocheirus was a fantastic creature who was a strange sight in an equally strange world.