They are the armor among the dinosaurs: bony plates protected ankylosaurs from predatory dinosaurs such as the T. rex. But they could also dish out powerful tail clubs. But apparently they didn't just use it for defense - sex apparently also played a role.

Ankylosaurs are often seen in prehistoric comics or movies using their bony tail clubs to fend off attacking tyrannosaurs. In fact, they didn't just teach T. rex to fear: The armored dinosaurs may also use their iconic tail clubs to beat up unwelcome fellow species, reports a research team in the journal "Biology Letters".

A 76-million-year-old specimen of the species Zuul crurivastator was examined. Fossil remains of one such animal were found years ago in the US state of Montana. Its tail was about ten feet long and had sharp spikes on the sides. The back half of the tail was stiff, and the tip was encased in huge lumps of bone that formed a massive, hammer-like weapon. In addition, the hardy herbivore bore rows of spikes along its flanks, which were armored with bone plates.

The well-preserved specimen had some spines broken off on either side near the hips - the tips were missing, and the bone and horny sheath had healed to a blunter form, the authors write. The injuries were caused by a blow from another Zuul's massive tail club, not from an attacking predator such as a tyrannosaur, as the team suspects based on the location and pattern of the injuries. It is possible that ankylosaurs would have fought for social and territorial dominance or for females.

In addition to other indications, this is supported by the fact that the largest and sharpest spikes were on the flanks of Zuul - and predatory dinosaurs would therefore hardly have attacked these places in particular. In the case of injuries inflicted by tyrannosaurs, a random distribution is to be expected, especially on the back or in the vulnerable neck area.

But why did the dinosaurs fight with their tails? Today, specialized animal weapons such as deer antlers and antelope horns are typically used to fight conspecifics over mates or territory, the researchers said. The tail clubs of the ankylosaurs could also possibly have been such sexually selected structures and were primarily used for intraspecific fights - whereby the clubs were always ritualisedly swung into the opponent's flanks, similar to the flank thrust of bison or the neck strike of giraffes.