Where Did Animals With Tail Weapons Go? Here’s a Back Story

With a nearly impenetrable hide covered in spikes, the ankylosaurus was like a dinosaur version of an armored tank. And like any battlefield behemoth, it boasted a fearsome weapon: a bone-crushing clubbed tail.

The ankylosaurus was not the only prehistoric beast to have an intimidating backside. Stegosaurus sported spear-like spikes on its tail. Some sauropods flailed fused clumps of bones from their posteriors toward predators.

But in living animals today, formidable tail weaponry is nearly absent. Although porcupines have quills and some lizards lash their tails when threatened, neither animal has the bony armaments seen millions of years ago. To help figure out why, a pair of paleontologists has pieced together a series of traits shared among extinct species that had weaponized their fifth extremity. Their study may help explain why tail weaponry has gone missing since dinosaurs and some ice age animals went extinct.

In a paper published Wednesday in the journal Proceeding of the Royal Society B, the team has identified three characteristics in land-dwelling mammals, reptiles and nonavian dinosaurs that may be linked with evolving bony tail weapons. They include being large — about the size of a mountain goat or bigger — eating plants and already having an armored body.

“That’s a really rare combination no matter what time period you’re looking at,” said Victoria Arbour, a paleontologist at the University of Toronto and the Royal Ontario Museum and an author on the study.

Arbour and her colleague Lindsay Zanno, a paleontologist from North Carolina State University, were sure to note that the three traits they identified are correlated with animals that have tail weapons and do not drive the development of these dangerous appendages.

For the study Arbour and Zanno compiled a data set of nearly 300 extinct and living species of mammals, reptiles, birds and dinosaurs. They plugged in characteristics associated with each — like “is herbivorous” or “has a prehensile tail” — into a computer program. After sorting through hundreds of characteristics, they identified the handful most closely shared between species that had tail weapons.

Included in the anatomical arsenal were tails that evolved to act like flails, spikes, bats and clubs.

The flail was common among long-necked sauropods like the Shunosaurus. On the tip of its tail were large fused bones that could be whipped in self-defense.

Stegosaurus and its relatives had spiky, conical bones.

And the bat and club were iconic features seen in both ankylosaurus and the glyptodons, which were ancient boulder-sized armadillos.

The authors hypothesized that developing bony clubs like the ankylosaurus and the glyptodon was a gradual, evolutionary process.

“They start with a stiffening of the tail to make it stronger and able to better project force,” Zanno said. “You’re swinging with essentially bowling balls at the end of the tails, you’re going to rip it off, so it makes sense that it’s going to happen in that order.”

These differed from defenses seen in tails of modern animals because they were made of bone. Living animals tend to have tails with weapons that are made of keratin, like the quills of a porcupine or the scales of a pangolin. Also, modern lizards like iguanas and komodos, which can lash their flexible tails, lack spikes. The exception, the authors noted, was a lizard known as Smaug which does have a smaller bony, spiky tail, they said.

Andrew Farke, a paleontologist from the Raymond M. Alf Museum of Paleontology at The Webb Schools in California, who was not involved in the study, said the study made a good case.

“Lots of animals were big and herbivorous, but only a few groups had bony skin in place already.” he said. “I’m intrigued by the fact that once an animal gets skin armor, there are so many evolutionary pathways toward supporting dangerous tail tips.”