Duonychus tsogtbaatari

Duonychus tsogtbaatari is a therizinosaurid from the Bayanshiree Formation of Mongolia's eastern Gobi Desert. Its partial articulated skeleton preserves both arms and hands well enough to show that the third finger had been reduced to a splint, leaving two functional claw-bearing digits. Even more unusually, the first finger retains much of its keratinous claw sheath. This soft covering lengthened and increased the curvature of the bony claw, giving direct evidence about a structure usually reconstructed only from bone.

Its fossils occur between approximately 95.9 and 89.6 million years ago. Values shown here are approximate and reflect the current curated seed dataset.

The holotype preserves six dorsal vertebrae, six sacrals, the first tail vertebra, ribs, part of the shoulder girdle, both upper and lower arms, wrists and hands, and portions of the pelvis. Metacarpal III is reduced and does not support a normal functional finger. The nearly complete sheath on left manual digit I brings that claw to about 30 centimetres along its outer curve. Joint mobility and claw curvature indicate a powerful grasp capable of pulling branches toward the body.

• Only two functional fingers on each hand
• Extremely reduced splint-like third metacarpal
• Nearly complete three-dimensional keratin sheath on the first manual claw
• Approximately 30-centimetre sheathed claw with strong curvature
• Highly mobile finger joints suited to flexing around vegetation

MPC-D 100/85 was collected at Urlibe Khudak in the Bayanshiree Formation of the eastern Gobi Desert and was publicly shown before receiving a formal name. Yoshitsugu Kobayashi, Darla Zelenitsky, Anthony Fiorillo and Tsogtbaatar Chinzorig described it in March 2025. The genus combines duo, two, with onyx, claw, while the species honours Mongolian palaeontologist and former institute director Khishigjav Tsogtbaatar.

Naming authors: Yoshitsugu Kobayashi, Darla K. Zelenitsky, Anthony R. Fiorillo, Tsogtbaatar Chinzorig.

The Bayanshiree Formation records alluvial and river deposits in a seasonally dry eastern Gobi setting with meandering channels and wetter local habitats. Duonychus shared this ecosystem with other therizinosaurs, hadrosauroids, armoured dinosaurs, tyrannosauroids and large dromaeosaurids.

Biomechanical analysis supports using the two large fingers together as a grasp, probably to hook and pull vegetation within reach of the head. This is more specific than simply calling the claws weapons, but it does not exclude display or defence. Diet is reconstructed as predominantly plant-based from therizinosaur anatomy; no skull, gut contents or direct feeding trace is preserved with this specimen.

• It is the first known therizinosaur with only two functional fingers.
• Its preserved keratin sheath is exceptionally rare among non-bird theropods.
• Keratin made the claw longer and more strongly curved than the underlying bone alone suggests.
• Digit loss evolved independently in several unrelated theropod lineages.

A research repository is not necessarily a public exhibit. Loan and display status can change, so check with the institution before visiting.

1. 01
   Didactyl therizinosaur with a preserved keratinous claw from the Late Cretaceous of Mongolia

   Yoshitsugu Kobayashi, Darla K. Zelenitsky, Anthony R. Fiorillo, Tsogtbaatar Chinzorig · iScience 28(4) · 2025

   Open source
2. 02
   Press release: Duonychus tsogtbaatari

   Hokkaido University · 2025

   Open source