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| Duonychus tsogtbaatari Kobayashi, Zelenitsky, Fiorillo & Chinzorig, 2025 illustration by Masato Hattori |
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. Phylogenetically, Duonychus reveals a broader but still limited appearance of didactyly among avetheropodans. The manual ungual of Duonychus tsogtbaatari also preserves a nearly complete three-dimensional structure covering the ungual of the left manual digit I, which represents a keratinous manual claw. The strong curvature of this large claw and high ungual flexion indicate that Duonychus was likely amplectorial (grasping) and an effective grasper of vegetation, despite having only two functional fingers.
Systematic Paleontology
Theropoda
Coelurosauria
Therizinosauria
Therizinosauridae
Duonychus tsogtbaatari gen. et sp. nov.
Horizon and locality: The holotype specimen was collected from the Urlibe Khudak Locality (N43°27.092′, E107°23.476′) within the Bayanshiree Formation (Figure 1), which primarily consists of conglomerates, sandstones, and mudstones deposited in alluvial and fluvial settings, in the eastern Gobi Desert, Mongolia. This formation, dated to the Late Cretaceous, spans the Cenomanian to Santonian stages, as indicated by radiometric dating and biostratigraphic data. ...
Diagnosis: A derived therizinosaur exhibiting the following unique combination of characters: dorsal vertebrae with a long neural spine; a thick centrodiapophyseal lamina without additional laminae; dorsal vertebrae lacking pleurocoels; sacral vertebrae (2–4) with flat ventral surfaces; dorsal vertebrae possessing a weak ventral keel; pneumatic dorsal ribs; fused distal carpals 1 and 2; distal carpals 1 and 2 semilunate-shaped with a mediodorsal process; a small angle between the articular surfaces of the radiale for the semilunate carpal and the radius; a reduced metacarpal III forming a splint; absence of digit III; equally sized manual unguals I-2 and II-3; absence or reduction of the flexor fossa in manual phalanges; and a reduced dorsal process at the proximal end of manual phalanges. Duonychus can be distinguished from other therizinosaurs of the Bayanshiree Formation by several features. Its humerus has a more proximally positioned deltopectoral crest compared to Erlikosaurus, and in Duonychus, the entepicondyle is reduced with a markedly narrow groove separating it from the ulnar condyle. Unlike Enigmosaurus, which has a fused pubic boot and coalesced sacral vertebrae, Duonychus has an unfused pubic boot and ventrally flat sacral vertebrae with a shorter second sacral. Duonychus has an anteriorly deflected humeral shaft and a nearly straight ventral border of the pubic boot, distinguishing it from the convex shape seen in Segnosaurus.
Etymology: The generic epithet is derived from duo for two in Greek in reference to the presence of a didactyl hand which is unusual in therizinosaurs, and from onyx for claw in Greek. The species epithet honors Khishigjav Tsogtbaatar, a paleontologist and former director of the Institute of Paleontology, Mongolian Academy of Sciences in Ulaanbaatar, Mongolia.
Yoshitsugu Kobayashi, Darla K. Zelenitsky, Anthony R. Fiorillo and Tsogtbaatar Chinzorig. 2025. Didactyl therizinosaur with a preserved keratinous claw from the Late Cretaceous of Mongolia. iScience. www.cell.com/iscience/fulltext/S2589-0042(25)00401-8
DOI: 10.1016/j.isci.2025.112141 [March 25, 2025]
