Deinocheirus mirificus

Deinocheirus mirificus transformed from one of palaeontology's great mysteries into one of its strangest well-known dinosaurs. For nearly half a century it was represented chiefly by a pair of 2.4-metre forelimbs, shoulder bones and a few ribs. Two much more complete skeletons revealed an approximately eleven-metre ornithomimosaur with a deep belly, broad hips, unusually short lower legs, wide feet, tall neural spines and a toothless expanded snout. It was not a giant predator built like Tyrannosaurus: stomach-region fish remains, gastroliths and its beak instead indicate an opportunistic omnivore living in the wet environments of the Nemegt Formation.

Its fossils occur between approximately 72 and 69 million years ago. Values shown here are approximate and reflect the current curated seed dataset.

The long forelimbs ended in three large but relatively blunt claws. Its skull was low and elongated, with a broad, toothless beak and a deep lower jaw; the huge lower-jaw opening helped keep this expanded structure light. Tall neural spines rose above the trunk and pelvis and supported a high-backed soft-tissue profile, although whether that outline resembled a hump or a sail cannot be determined from bone alone. The pelvis was broad, the lower legs were short, and the feet were unusually wide for a theropod, suggesting a heavy animal adapted for steady movement over soft, wet ground. Fused vertebrae at the tail tip formed a pygostyle-like structure that may have supported a fan of feathers, but no feathers are directly preserved with Deinocheirus.

• Forelimbs about 2.4 metres long with three large claws
• Elongated toothless skull ending in a broad beak
• Deep lower jaw and exceptionally large mandibular opening
• Tall neural spines producing a high-backed body profile
• Broad pelvis and deep, heavy torso
• Short lower legs and unusually wide weight-bearing feet
• Pygostyle-like series of fused vertebrae at the tail tip

Zofia Kielan-Jaworowska's Polish-Mongolian expedition found the holotype ZPAL MgD-I/6 at Altan Uul III in 1965. Halszka Osmolska and Ewa Roniewicz named Deinocheirus mirificus in 1970 from its spectacular arms. Decades later, Mongolian-Korean teams recovered the more complete MPC-D 100/127 and MPC-D 100/128 from the Nemegt Basin. Parts of one skeleton had been poached and dispersed through the fossil trade; a skull, hands and feet were identified in Europe, reunited with the Mongolian skeleton and repatriated in 2014. The combined specimens finally allowed Lee and colleagues to publish a near-whole-body reconstruction in Nature.

Discovery credit: Polish-Mongolian Palaeontological Expedition, Zofia Kielan-Jaworowska.

Naming authors: Halszka Osmolska, Ewa Roniewicz.

The Nemegt Formation preserves large river channels, floodplains, ponds and seasonally wet lowlands in the Gobi region. Deinocheirus shared this landscape with the tyrannosaurid Tarbosaurus, the ornithomimosaur Gallimimus, hadrosaurs, titanosaurs, turtles, crocodile-line reptiles and abundant freshwater animals. Wide feet and short lower legs are consistent with moving across soft substrates, but they do not demonstrate a fully aquatic lifestyle.

A cluster of more than 1,400 gastroliths and fish remains in the abdominal region of one specimen provide unusually direct dietary evidence. Deinocheirus probably cropped soft vegetation with its beak, ground food with swallowed stones and also consumed aquatic prey or carrion when available. Its claws may have gathered branches or helped with defence, but their exact function is not preserved. No nest, trackway or mass assemblage currently establishes its social organisation, reproductive behaviour or swimming ability.

• For 44 years, almost the entire animal was reconstructed from little more than its gigantic arms.
• Missing bones from a poached specimen were recognised in a private European collection and returned to Mongolia.
• Its wide feet, duck-like beak and deep body made it look radically different from other familiar theropods.
• Fish remains and more than a thousand gastroliths were preserved together in the stomach region of one skeleton.

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1. 01
   Deinocheiridae, a new family of theropod dinosaurs

   Halszka Osmolska, Ewa Roniewicz · Palaeontologia Polonica 21 · 1970

   Open source
2. 02
   Resolving the long-standing enigmas of a giant ornithomimosaur Deinocheirus mirificus

   Yuong-Nam Lee, Rinchen Barsbold, Philip J. Currie and 5 coauthors · Nature 515 · 2014

   Open source
3. 03
   65-year-old Deinocheirus mirificus mystery resolved

   Hokkaido University · 2014

   Open source