The boney bludgeons carried by Xenicibis xympithecus are unlike anything else known in the bird world – or in mammals, reptiles or amphibians.
Writing in the Royal Society journal Proceedings B, the scientists report finding bones that had apparently been broken by another bird’s club.
The species may have survived until less than 10,000 years ago.
A member of the ibis family, it was probably about the size of a chicken, but with an infinitely more robust armoury.
Fossils show that the metacarpus – one of the “hand” bones – was elongated and much bigger than in related species, with very thick walls.
This allowed the wings to function “in combat as a jointed club or flail”, the researchers write.