In zoology, a club is a bony mass at the end of the tail

Tail

The tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, reptiles, and birds...

 of some dinosaur

Dinosaur

Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

s and of some mammals, most notably the ankylosaurids

Ankylosauridae

An ankylosaurid is a member of the Ankylosauridae family of armored dinosaurs that evolved 125 million years ago and became extinct 65 million years ago during the Cretaceous-Tertiary extinction event...

 and the glyptodonts. It is thought that this was a form of defensive armour

Armour (zoology)

Armour in animals is external or superficial protection against attack by predators, formed as part of the body , usually through the hardening of body tissues, outgrowths or secretions. It has therefore mostly developed in 'prey' species...

 or weapon that was used to defend against predators, much in the same way as a thagomizer

Thagomizer

Thagomizer is an informal name for the distinctive arrangement of four to ten spikes on the tails of stegosaurid dinosaurs. These spikes are believed to have been a defensive measure against predators.-Paleobiology:...

, possessed by stegosaurids, though at least in glyptodonts it is hypothesized it was used in fighting for mating rights

Display (zoology)

Display is a form of animal behaviour, linked to survival of the species in various ways. One example of display used by some species can be found in the form of courtship, with the male usually having a striking feature that is distinguished by colour, shape or size, used to attract a female...

. Among dinosaurs, the club was present mainly in ankylosaurids, although the sauropod Shunosaurus

Shunosaurus

Shunosaurus, meaning "Shu Lizard", is a genus of sauropod dinosaur from Middle Jurassic beds in Sichuan Province in China, 170 million years ago...

also possessed a tail club. The tail club is most often depicted on Ankylosaurus

Ankylosaurus

Ankylosaurus is a genus of ankylosaurid dinosaur, containing one species, A. magniventris...

, especially in encounters with larger predators such as Tyrannosaurus

Tyrannosaurus

Tyrannosaurus meaning "tyrant," and sauros meaning "lizard") is a genus of coelurosaurian theropod dinosaur. The species Tyrannosaurus rex , commonly abbreviated to T. rex, is a fixture in popular culture. It lived throughout what is now western North America, with a much wider range than other...

. Whether Ankylosaurus was actually able to swing its tail with enough force to prove unassailable or not however is yet to be proven.