New dinosaur species with massive claws, missing finger rewrites history
An artistic reconstruction of Duonychus tsogtbaatari, the newly discovered two-fingered therizinosaur, using its large claws to pull vegetation closer in a Cretaceous forest in what is now Mongolia, Mar. 25, 2025. (Photo via X / @dgdt2425)
A new species of dinosaur with unusually large claws and only two fingers on each hand has been discovered in Mongolia’s Gobi Desert.
The fossil, named Duonychus tsogtbaatari, offers rare insight into the evolution of theropod dinosaurs and challenges the previous understanding of a group known as therizinosaurs.
The fossil was uncovered during construction work on a water pipeline in the Bayanshiree Formation in southern Mongolia. Researchers say the area is one of the richest dinosaur fossil sites in the world, especially for the Late Cretaceous period. The Gobi Desert is considered by UNESCO to be the largest dinosaur fossil reservoir globally.
The new species belongs to the therizinosaurs, a group of two-legged, plant-eating dinosaurs that lived in Asia and North America during the Cretaceous Period, which began around 145 million years ago and ended 66 million years ago.
While most therizinosaurs had three clawed fingers, Duonychus tsogtbaatari had only two. This unique feature inspired its name—”Duonychus” means “two claws” in Greek—and the species honors Mongolian paleontologist Khishigjav Tsogtbaatar.
This newly discovered species from Mongolia breaks the mold with just two fingers instead of the typical three, offering a rare glimpse into how theropod hands evolved and adapted.
Lead author of the study, Yoshitsugu Kobayashi, a paleontologist at Hokkaido University in Japan
The specimen was exceptionally well preserved. It included parts of the arms, spine, hips, and most notably, a fully intact keratinous sheath on one of the claws—the first of its kind found in a medium- to large-sized theropod dinosaur. Keratinous sheaths, similar to human fingernails, help indicate the actual size and function of the claws.
“The hands, a hallmark of therizinosaurs, are exquisitely preserved with all the finger and wrist bones intact in this specimen,” said co-author Darla Zelenitsky, associate professor at the University of Calgary. “Even the keratinous sheath of the claw is preserved, revealing how big and sharp its claws really were.”
One claw measured about 30 centimeters (11.8 inches), and researchers believe it was much larger than the underlying bone. Kobayashi said the shape of the claw suggests it was used to grip and pull vegetation closer, much like how modern chameleons use their limbs.
The loss of the third finger in Duonychus tsogtbaatari might actually have made its remaining two fingers even better at what they were designed for—grasping. Based on the shape of its well-preserved claw and how it curved, Duonychus was likely using its hands to grab onto branches and pull vegetation closer.
Kobayashi
The dinosaur weighed about 260 kilograms (573 pounds) and had long necks, small heads, and rotund bodies, earning it the nickname “sloths of the Cretaceous,” according to Zelenitsky. The researchers believe the claws may have served other purposes as well, including defense, digging, display, or even courtship.
According to the study, Duonychus tsogtbaatari represents the fifth known example of a theropod lineage evolving to lose its third finger, and the only known therizinosaur to do so. The researchers suggest this change was likely an evolutionary adaptation rather than an isolated anomaly.
“While more fossils would help confirm this, all signs point to Duonychus representing a true evolutionary shift, not just a fluke,” said Kobayashi.
The discovery provides new insight into how theropods evolved and diversified. It also adds to the growing body of evidence showing that therizinosaurs were among the most unusual dinosaurs to have ever lived.
“They had rotund bodies, long necks, small heads and unusually long arms with big hands,” Zelenitsky told the Calgary Herald. “They call them the sloths of the Cretaceous period—totally bizarre-looking things.”
While Duonychus tsogtbaatari is currently known from a single specimen, scientists believe further discoveries could shed more light on how and why some theropods developed two-fingered hands. The researchers also speculate that the species may have lived in both Asia and North America, although further evidence is needed to confirm that.
