A twist in the armoured tail tale


US research uncovers why weaponised rear ends have proved evolutionarily unfashionable. Andrew Masterson reports.


The spikes on the end of a Stegosaurus tail.
JUSTIN TALLIS/AFP/Getty Images

In evolutionary terms, weaponised tails are a bit like a Mini fitted with rotor-blades: not entirely pointless, but requiring a level of engineering that far outstrips their value.

That’s the conclusion reached by palaeontologists Victoria Arbour and Lindsay Zanno, both from the North Carolina Museum of Natural Sciences in the US.

Arbour and Zanno decided to see if the fossil record could provide answers to a curious question: if extinct animals ranging from dinosaurs to rodents had tails evidently adapted as formidable defensive weapons, why don’t any species living today?

Picture books about dinosaurs routinely depict creatures with weaponised tails – Stegosaurus with its vicious pairs of spikes, for instance, or Ankylosaurus, with its massive bony club. Tail armour was also present in several other species, including giant armadillos and even some types of turtle.

Tail weapons can be broadly divided into three categories, marked by stiffness, bony spikes, or clubs. From one perspective, these appendages all brought an apparent advantage to the animals that possessed them. Other types of weapons, such as antlers, head spikes and tusks, are positioned near critical organs – eyes, brains, and so on – so combat automatically put life-dependent systems in danger.

Weapons at the other end of the body, well away from essential organs, reduced the risk of serious damage. And a tail is much more expendable than a head.

And yet the evidence is indisputable: across the board evolution has selected against them.

Arbour and Zanno decided to find out why. Using a data set of 286 mammals, reptiles and birds, living and extinct, they looked for characteristics shared by tail-weaponised species, spread across phyla and time.

The results were informative. First, almost all the examples in the fossil record were large creatures, topping 100 kilograms. Second, they were all covered in armour. And third, they were all vegetarians.

In a paper published in the journal Proceedings of the Royal Society B, the scientists suggest that the first two conditions were necessary for weaponised tails to evolve. The organs would have been very heavy, so only a large animal would have the strength to carry one around.

The armour played a secondary role, as well as its obvious defensive one. By being rigid it would have imparted a rigorous stiffness to an animal’s body, allowing it to absorb and counteract the comparatively immense forces required to swing the heavy tail.

And all of this – the spikes, the armour, the musculature and the weight – would have been a significant burden to any species that only ate fruit, vegetables or grass.

“It's rare for large herbivores to have lots of bony armour to begin with," Arbour says, “and even rarer to see armoured species with elaborate head or tail ornamentation because of the energy cost to the animal.

“The evolution of tail weaponry in Ankylosaurus and Stegosaurus required a 'perfect storm' of traits that aren't seen in living animals, and this unique combination explains why tail weaponry is rare even in the fossil record.”

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Andrew Masterson is news editor of Cosmos.
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