Resolving the long-standing enigmas of a giant ornithomimosaur Deinocheirus mirificus

Two almost complete skeletons are presented for the theropod dinosaur Deinocheirus mirificus, revealing a humpbacked form with a duckbill-like skull. For almost 50 years, the strange theropod dinosaur Deinocheirus mirificus has been regarded as one of the most intriguing puzzles in palaeontology. Known only from two gigantic forelimbs unearthed in Mongolia in 1965, it had been variously described as an ornithomimid, a therizinosaur or a member of a completely new theropod clade. New excavations have now revealed two almost entire skeletons of the creature, enabling Yuong-Nam Lee et al. to offer a detailed picture of D. mirificus. What emerges is a gigantic creature with a duckbill-like skull and a humped back, the largest known member of the ornithomimosaurs, living in a damp environment, with plants and fish as part of the diet of D. mirificus. The holotype of Deinocheirus mirificus was collected by the 1965 Polish–Mongolian Palaeontological Expedition at Altan Uul III in the southern Gobi of Mongolia1. Because the holotype consists mostly of giant forelimbs (2.4 m in length) with scapulocoracoids2, for almost 50 years Deinocheirus has remained one of the most mysterious dinosaurs. The mosaic of ornithomimosaur and non-ornithomimosaur characters in the holotype has made it difficult to resolve the phylogenetic status of Deinocheirus3,4. Here we describe two new specimens of Deinocheirus that were discovered in the Nemegt Formation of Altan Uul IV in 2006 and Bugiin Tsav in 2009. The Bugiin Tsav specimen (MPC-D 100/127) includes a left forelimb clearly identifiable as Deinocheirus and is 6% longer than the holotype. The Altan Uul IV specimen (MPC-D 100/128) is approximately 74% the size of MPC-D 100/127. Cladistic analysis indicates that Deinocheirus is the largest member of the Ornithomimosauria; however, it has many unique skeletal features unknown in other ornithomimosaurs, indicating that Deinocheirus was a heavily built, non-cursorial animal with an elongate snout, a deep jaw, tall neural spines, a pygostyle, a U-shaped furcula, an expanded pelvis for strong muscle attachments, a relatively short hind limb and broad-tipped pedal unguals. Ecomorphological features in the skull, more than a thousand gastroliths, and stomach contents (fish remains) suggest that Deinocheirus was a megaomnivore that lived in mesic environments.