The other dinosaurs in this group probably didn't make much use of their functional nub fingers, the study authors note. But the extra fingers would have been biologically inexpensive to maintain, so they didn't totally disappear.
By contrast, Linhenykus had no working vestigial nubs, and its one long finger wasn't as specialized for digging as the digits of other alvarezsauroids. This demonstrates that hand evolution in this group "did not follow a simple linear trend," the study authors write.
The new, one-fingered dinosaur is described online today in the journal Proceedings of the National Academy of Sciences.