Linhenykus' hand does have a leftover bone for a second finger, but the nub of a digit wouldn't have worked at all, Xu added.
That makes Linhenykus the only known one-fingered dinosaur, he said.
The new dinosaur was discovered in a fossil-rich rock formation that dates to the late Cretaceous period, between 84 and 75 million years ago. The site is near the Inner Mongolian town of Linhe (map), which helped inspire the dinosaur's name.
Alvarezsauroid fossils have been found in North and South America as well as Asia, dating from the late Jurassic to the late Cretaceous. Finding an alvarezsauroid in Asia dated to the late Cretaceous adds to the story of this group's dispersal around the world, Xu said.
"[They] probably originated in Asia, then dispersed from Asia to [the ancient supercontinent] Gondwana, and then back to Asia, and finally from Asia to North America," he said.
Most theropods had three fingers on each hand. But Linhenykus belongs to a family known as the alvarezsauroids: small, long-legged dinosaurs that had one big finger alongside two barely functional nub fingers.
"Some researchers speculate that these dinosaurs used their hands to dig [up] termite nests," said study leader Xu Xing of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. This was probably the case for Linhenykus as well, he said.