One of the most impressive weapons to appear during the dinosaur arms race of the Cretaceous Period was the big bony tail club wielded by some members of a group of tank-like plant-eaters.

A new study provides a step-by-step account of the evolution of this distinctive feature possessed by the heavily armored dinosaur Ankylosaurus and its cousins, a bludgeon that may have given even the ferocious Tyrannosaurus rex reason to worry.

The researchers studied fossils of the group called ankylosaurs including early, primitive species with no tail club and later species with a fully developed one.