What tennis rackets tell us about giant extinct armadillos

When sportsmen use rackets or bats, their best bet is to hit a ball on the "sweet spot", the point where various forces balance out to deliver powerful blows with only very small forces on the wielder's wrist. Engineers have the right tools and models to work out where this spot lies on their instruments. Now, palaeontologists have used the same techniques to study biological hammers that adorn the tails of giant prehistoric armadillos called glyptodonts.

At first glance, glyptodonts have little in common with the likes of Andy Murray and Roger Federer. These armoured beasts lived in the Americas several million years ago and the largest of them weighed up to two tons. Much like their modern armadillo relatives, they were clad in large suits of bony armour. Their long tails were similarly protected by bony rings and in some species, they were topped with large clubs, or spiky weapons that resembled medieval morning stars.

Uruguayan scientist Rudemar Ernesto Blanco was set about studying the tail clubs of the most formidable of the glyptodonts, by using the same approaches used to analyse sports tools. The analogy is particularly appropriate for species like Doedicurus, where the rings at the end of the tail were completely fused, meaning that the animal's rear end was defended by a single metre-long piece of solid bone - a biological hammer, indeed.

When wielding this weapon, whether against a predator or a fellow glyptodont, it would be in the animal's interest to strike at the sweet spot of its own tail to reduce the forces acting on the part of the tail where the bony tip met the more flexible base. Otherwise, it might have risked severe strain and damage. To find the locations of these spots, Blanco applied sports modelling techniques to the tails of nine species of glyptodonts.

He found that the spots almost always sat near the largest of the many disc-shaped dimples found on glyptodont clubs. Scientists think that on the living animal, these depressions marked the locations of horny spikes and in fact, almost every drawing or painting of glyptodonts with spikes at these discs. The location of the largest spike over the tail's sweet spot would have allowed the animal to concentrate the force of its strike in a small area, maximising the damage it inflicted on others while minimising the stress to its own body.

These results support the idea that the clubs were used in combat, but Blanco suggests that they were more useful for duelling with rivals rather than warding off hunters. He views the club as a precision weapon, used to deliver blows near the sweet spot during highly ritualised fights, much like those that modern plant-eaters engage in. Hitting a fast mobile predator, like a sabre-tooth cat, would have been far trickier.

However, smaller glyptodonts may have used their tails in more varied ways. While larger species could really only swing them from side to side, the smaller ones had much more flexible tails able to move in a variety of directions. Perhaps predators were more likely to feel the smash of these smaller tails. 

Humans aside, glyptodonts are one of the few species to use a biological hammer to fight, kill or defend. The awe-inspiring mantis shrimp can break aquarium glass with a forearm that hits with the force of a rifle bullet. And as with the glyptodonts, the dinosaur group known as the ankylosaurs wielded bony tail clubs. The location of their sweet spots are next up for Blanco but another scientist - Victoria Arbour from the University of Alberta - is ahead of the game on ankylosaur clubs. 

Reference: Proc Roy Soc B 10.1098/rspb.2009.1144

Image: by Wilson Dias

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