Paleontologists have unearthed the fossilized remains of a new and unusual therizinosaurid dinosaur with atypical hands in Mongolia.
Life reconstruction of Duonychus tsogtbaatari. Image credit: Masato Hattori.
Duonychus tsogtbaatari lived in what is now Mongolia during the Late Cretaceous epoch, between 95 and 90 million years ago.
The new species belongs to Therizinosauria, a group of herbivorous or omnivorous theropod dinosaurs that lived in Asia and North America during the Cretaceous.
“Therizinosauria is a clade of unusual herbivorous or omnivorous theropod dinosaurs known from Cretaceous deposits of Asia and North America,” Hokkaido University Museum vertebrate paleontologist Yoshitsugu Kobayashi and his colleagues wrote in their paper.
“This clade is most recognizable for their tridactyl (three-fingered) hands sporting three large claw-like unguals, as exemplified by the large-bodied Therizinosaurus from the latest Cretaceous of Mongolia.”
“More primitive members of the clade like Falcarius, Beipiaosaurus, and Jianchangosaurus had relatively smaller unguals compared to more derived forms, such as Erliansaurus, Nothronychus, and especially Therizinosaurus.”
“As herbivorous or omnivorous theropods with long necks and small leaf-shaped teeth, the evolution of their unusual hands likely played an important role in the feeding ecology of this clade.”
According to the paleontologists, Duonychus tsogtbaatari is a medium-sized therizinosaur, with an estimated body mass of approximately 260 kg.
The fossils of this dinosaur were recovered from the Urlibe Khudak locality of the Bayanshiree Formation in the Gobi Desert, Ömnögovi province, southeastern Mongolia.
“The specimen consists of a partial skeleton, including: six articulated dorsal vertebrae, six articulated sacral vertebrae with sacral ribs, the anterior-most caudal vertebra, some dorsal ribs, partial left scapula and coracoid, humeri, ulnae, radii, carpals, metacarpals, the left and right manus, the right ilium, both pubes, and the proximal end of the left ischium,” the researchers wrote.
Reconstructed skeleton and selected elements of Duonychus tsogtbaatari. Image credit: Kobayashi et al., doi: 10.1016/j.isci.2025.112141.
Duonychus tsogtbaatari is different from other therizinosaurs in that the hand possesses only two fingers, rather than three fingers.
“With respect to other therizinosaurs, this new species is unique in possessing a definitive didactyl (two-fingered) hand,” the scientists wrote.
“The specimen also novelly — among non-paravian theropods — preserves a complete, three-dimensional sheath representing the keratinous claw.”
The well-preserved specimen of Duonychus tsogtbaatari provides insight into functional aspects of its hands.
“Based on ventral curvature and angle of attack parameters of the claw, the preserved keratinous claw of Duonychus tsogtbaatari reveals functions within the range of scansorial (climbing), tenasorial (grappling) to amplectorial (grasping),” the authors wrote.
“Of these functions, the herbivorous or omnivorous diet and body size of Duonychus tsogtbaatari, as in other therizinosaurids, would suggest an amplectorial usage where the claws serve to grasp branches as in chameleons and some mammals (e.g., southern tamandua, anteater), rather than to climb trees (e.g., squirrels), or to grasp, seize, or manipulate prey (e.g., birds of prey, felids).”
“Despite having only two functional digits, Duonychus tsogtbaatari was likely an effective grasper, considering the extreme flexion at the ungual joint and the strong curvature of the keratinous claw, features unknown for other therizinosaurs.”
“Based on the shape of the ungual and the dimensions of the keratinous claw, Duonychus tsogtbaatari could have grasped branches or swaths of vegetation up to approximately 10 cm in diameter, which is less than the diameter of those grasped by Therizinosaurus, suggesting that Duonychus tsogtbaatari may have been more selective in its foraging behavior.”
“The manus of Duonychus tsogtbaatari, with its strong ungual flexion and claw curvature, further supports that the manus of derived therizinosaurs likely served in a rake or hook-and-pull function to bring vegetation to the mouth during feeding as previously suggested.”
“Although claws usually have a dominant function, likely for hook-and-pull foraging in most derived therizinosaurs, these structures could also have been utilized for other purposes, such as territoriality, defense, courtship, play, etc.”
“In addition to an unexpected morphological diversity of the therizinosaur manus (i.e., didactyly), Duonychus tsogtbaatari reveals a greater species richness of therizinosaurs in ecosystems of the Bayanshiree Formation than previously realized,” they concluded.
The team’s paper was published on March 25 in the journal iScience.
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Yoshitsugu Kobayashi et al. Didactyl therizinosaur with a preserved keratinous claw from the Late Cretaceous of Mongolia. iScience, published online March 25, 2025; doi: 10.1016/j.isci.2025.112141
