A newly discovered species of feathered, bipedal dinosaur with "exceptionally preserved and atypical hands" had just two fingers, each one tipped with a gigantic, claw-like talon.
A remarkable fossil specimen found in southeastern Mongolia gave researchers a clear idea of what the claws would have looked like in real life, with their keratin sheath still clearly visible, extending the bones into wicked talons.
In their published study, a team led by paleontologist Yoshitsugu Kobayashi from Hokkaido University in Japan analyzed the fossils' appearance, similarities to relatives, and geological context to determine where to place this dinosaur in our records.
They've named it Duonychus tsogtbaatari, with a genus that means 'two claws' in Greek, and a species name honoring Mongolian paleontologist Khishigjav Tsogtbaatar.
The fossil was dug out of a Cretaceous sedimentary deposit in the Gobi Desert by a team of researchers from the Mongolian Academy of Sciences in 2012.
Known as the Bayanshiree Formation, this area is a hotspot for dinosaur fossils, thanks to its arid conditions and layers of lithified mud, clay, and sand. It's turned up giant footprints, plane-sized pterosaurs, and the famous 'fighting dinosaurs'.
Uranium-lead dating of the mineral layers adjacent to the find suggests this newly discovered fossil could be around 90 to 95 million years old. Most of the bones are from its upper limbs and torso: There are some vertebrae, a bit of pelvis and ribs, part of one shoulder blade, and a whole lot of arm and hand bones.
And, of course, a set of nails that would make Cardi B jealous. Duonychus's claws measure almost 30 centimeters (12 inches) long.
It joins the therizinosaurs, a group of feathered, bipedal dinosaurs found in Cretaceous-era fossil deposits across Asia and North America. Until now, this clade was defined by its members' three-fingered hands, also studded with massive claws.
"The discovery of Duonychus tsogtbaatari is a big deal because it's the first known therizinosaur with only two fingers," Kobayashi told James Woodford of New Scientist.
"Most theropods, including other therizinosaurs, kept three functional fingers, so finding one that lost a digit is pretty unexpected."
Among theropods, of which Tyrannosaurus is the most notable, only five are known to have reduced or totally absent third fingers, all independently of each other, and seemingly for totally different reasons. We won't know why Duonychus settled with two fingers rather than three without extra specimens or analysis, but the researchers think it may be related to its method of feeding.
The menacing curves and 90-degree range of motion suggest the dinosaur grasped onto vegetation, like a sloth or chameleon. This may have helped Duonychus pull leafy food towards its mouth, in true therizinosaur fashion.
"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," the authors write in a paper describing the find.
"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."
The researchers say these are the largest three-dimensional dinosaur claws preserved with the sheath and underlying finger bone intact.
"Keratin usually doesn't fossilize. It decays long before bones do," Kobayashi told Woodford.
"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."
The research was published in iScience.