Duonychus tsogtbaatari: Giant Two-Clawed Therizinosaur from Mongolia Rewrites Digit Evolution in Bizarre Herbivorous Dinosaurs

In a landmark iScience paper published March 2025, paleontologists led by Yosнιтsugu Kobayashi (Hokkaido University) have described Duonychus tsogtbaatari, the first didactyl (two-fingered) therizinosaur ever discovered. Unearthed from the ~95–90-million-year-old Bayanshiree Formation in Mongolia’s Gobi Desert, this medium-sized therizinosaurid—roughly 3 meters long and 260 kg—challenges long-held ᴀssumptions about hand evolution in these enigmatic, giant-clawed herbivores.

The holotype (MPC-D 100/85) preserves an exceptionally complete left and right manus, including a mᴀssive, keratin-sheathed claw on digit I and a second equally large ungual on digit II. Digit III is completely absent, reduced to a mere splint-like metacarpal. This configuration is unique among therizinosaurs, which typically retain three functional fingers with outsized claws thought to aid in foliage harvesting.

Phylogenetic analysis places Duonychus within Therizinosauridae, revealing that extreme digit reduction evolved independently in this lineage. The preserved keratin sheath offers rare direct evidence of claw function, supporting a mix of scansorial (climbing), tensorial (grappling), and amplectorial (grasping) behaviors—far more versatile than simple leaf-stripping.

“This specimen demonstrates that therizinosaur hands were far more plastic than previously realized,” Kobayashi noted. The discovery forces a re-evaluation of feeding ecology and forelimb evolution across Avetheropoda, showing that digit loss was not confined to tyrannosauroids or birds.

Named in honor of Mongolian paleontologist Khishigjav Tsogtbaatar, Duonychus (“two claws”) adds a striking new branch to the therizinosaur tree and underscores the Gobi’s unmatched fossil wealth. After decades of three-fingered giants, the two-clawed “odd man out” proves these bizarre dinosaurs continued to experiment with their anatomy deep into the Late Cretaceous.