Highlights

• Duonychus tsogtbaatari is described as another therizinosaur from Mongolia

• It is a didactyl therizinosaur, possessing only two manual digits

• A keratinous sheath on digit I provides rare insight into theropod claw structure

• Duonychus reveals convergent digit III loss in at least five theropod clades

Summary

Therizinosaurs were a group of herbivorous/omnivorous theropod dinosaurs that lived in Asia and North America during the Cretaceous Period. These theropods are notable for their three-fingered hands sporting large claw-like unguals. Here, we describe a new and unusual species of the therizinosaurid Duonychus tsogtbaatari, recovered from the lower Upper Cretaceous Bayanshiree Formation of the Gobi Desert of Mongolia. This species is different from other therizinosaurs in that the hand possesses only two fingers, rather than three fingers, the typical condition for Avetheropoda, providing further insight into forelimb/hand reduction among theropods.