For the study Dr. Arbour and Dr. 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,” said Dr. Zanno. “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.”