A new species of dinosaur found at a Mongolian building site has the largest fully preserved claw ever found. The bipedal, herbivorous animal had only two fingers on each hand, which it may have used to grasp branches and pull them towards its mouth.

The 90-million-year-old fossil – which included parts of the pelvis, both arms and hands, and numerous vertebrae – was found near Khanbogd in the Gobi desert in 2012, but it has only now been properly studied and given the scientific name Duonychus tsogtbaatari. The genus name means “two claws” and the species name honours Mongolian palaeontologist Khishigjav Tsogtbaatar.

Duonychus is a smaller relative of Therizinosaurus, which features in the film Jurassic World Dominion. Yoshitsugu Kobayashi at Hokkaido University in Japan and his colleagues estimate the dinosaur would have been around 3 metres long and weighed approximately 270 kilograms.

“The discovery of Duonychus tsogtbaatari is a big deal because it’s the first known therizinosaur with only two fingers,” says Kobayashi. “Most theropods, including other therizinosaurs, kept three functional fingers, so finding one that lost a digit is pretty unexpected.”

Altogether, five groups of theropods evolved to have only two fingers, the most famous being Tyrannosaurus.

Duonychus’s claws are nearly 30 centimetres long. Unusually, the claw sheaths – made of keratin, the same material as our fingernails – have been preserved.

“Keratin usually doesn’t fossilise. It decays long before bones do,” says Kobayashi. “Most of the time, when we find dinosaur claws, we’re only looking at the bony core. But in life, the actual claw would have been covered in a thick keratin sheath, making it longer and more curved.”

Team member Darla Zelenitsky at the University of Calgary, Canada, says finding a two-fingered therizinosaur was extremely surprising: “We were like, wow, this is truly bizarre for one of these dinosaurs.”

“This is the largest fully preserved 3D dinosaur claw found, by far,” she says. “This claw is not preserved as an outline on the rock surrounding the bone, which is pretty typical for fossils that have the true claws preserved. This claw is actually three-dimensional and forms a sheath around the underlying finger bone, similar to what we see in the paws of dogs and cats.”

The researchers think the claws were an adaptation to grabbing and pulling down vegetation. With fewer fingers, each one could be stronger and more flexible, allowing for a tighter, more controlled grip, says Kobayashi.

“The curved claws and extreme flexion suggest it could hook onto branches or clusters of leaves more easily,” he says. “Three fingers might have just gotten in the way, while two provided a more precise and efficient grasp.”