Why extinct creatures fought with their tails, while today animals use their heads

Why weaponized tails are so rare in the modern animal kingdom
A member of staff from London's Natural History Museum poses next to a nearly complete Stegosaurus skeleton in 2014. Large, armoured dinosaurs like Stegosaurus sported weaponized tails. (Paul Hackett/Reuters)
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Beware of the tail

To use your tail as an effective weapon, you likely have to be a large, armoured herbivore. That's the conclusion of a new study by Canadian researchers. 

Today when we see animals with powerful weapons, they are often found on the head. Think of rams smashing together their massive, coiled horns, or the crocodile's powerful jaws and teeth or a hawk's sharp beak.

Nowadays, the list of animals that pack their weapons in the posterior is much shorter. But it wasn't always that way. Large, armoured dinosaurs like Stegosaurus and other prehistoric animals used to maim and kill with their tails.

This week, researchers in Ontario and North Carolina published a study looking into why weaponized tails became such a rare sight in the modern animal kingdom.

Victoria Arbour holds an NSERC postdoctoral fellowship at the Royal Ontario Museum and the University of Toronto. She and her team focused on amniotes — reptiles, birds and mammals that lay eggs or give live birth on land.

Today, the examples of amniotes that use their tails as weapons are limited to monitor lizards and other small, spiky lizards that will sometimes thrash their tail from side to side to get predators to leave them alone.

Sledgehammer tails

The scientists looked at a data set of 286 amniote species, both living and extinct, to see if there were patterns that pointed to the evolution of certain types of tail weapons, such as spikes or a bony knob at the tip of the tail.

"There's examples that are probably familiar to lots of people like stegosaurus, which has large spikes at the tip of its tail. There's my favourite group of dinosaurs, the ankylosaurs, which have those really amazing tail clubs where the back half of their tail is stiff and then they sort of expand to the tip of it with armour to make a big knob of bone. And there's also some weird examples of long-necked dinosaurs, the sauropods, that sort of fuse and inflate some of their tail bones as well to make kind of a flail like weapon," Arbour said.

Arbour previously showed how the ankylosaur tail clubs are effective sledgehammers, strong enough to deliver forceful blows to break the bone if it impacted something else, but well adapted to absorbing those impact stresses so they themselves would not break under those impact forces.

Competitors or predators?

Without a time machine, there's no way to know what extinct animals used the weapons for, Arbour said. But by looking at modern animals such as bighorn sheep and deer antlers, they can make a guess.

"We've wondered a little bit if some of the structures we see in the fossil record might also be used for fighting members of your own species, but they could also be used as defensive weapons like what we see in say those monitor lizards that thrash their tails today."

When the researchers used statistical methods to look for common features between extinct animals with elaborate, bony tail weapons and those with weaponized tails today, they found three factors best predicted bony tails:

  • Large body size.
  • Body armour.
  • Herbivory. 

"There aren't very many animals today that are really large, armoured herbivores or even just large armoured animals at all. And I think that's partly because armour is relatively expensive to maintain. So if you have bony armour, bones take a lot of energy and resources for your body to grow and keep healthy."

Being large is usually a good enough defence in and of itself, Arbour said. For instance, most predators don't go after adult elephants because they're too big. 

Next, Arbour hopes to continue to study armoured dinosaurs and extinct armadillos to see whether or not tail weapons evolved in the presence of predators or to fight members of their own species.