Bizarre tail weaponry in a transitional ankylosaur from subantarctic Chile

Paleontologists at the University of Chile in Santiago have discovered the remains of a ‘extremely bizarre’ ankylosaurus with a deadly armored tail unlike any other known dinosaur.

This discovery was presented in a new research paper published in the journal Nature on the first of December, the study showed that this “very strange” dinosaur that was newly discovered, and roamed Chile 75 million years ago, was frightening and armed with an overwhelming weapon consisting of a deadly and dangerous armored tail.

“The tail would have looked like a sword; it’s so flat,” study co-lead researcher Alexander Vargas, a vertebrate paleontologist in the Department of Biology at the University of Chile, told Live Science. It would have looked “a bit like an Aztec sword, or the Aztec club called the macuahuitl.”

In addition to revealing its weaponized tail, the dinosaur’s remains tell a previously unknown tale about ankylosaur evolution: The breaking apart of the supercontinent Pangaea during the Jurassic period (201.3 million to 145 million years ago) led to extreme differences between ankylosaurs on the northern supercontinent Laurasia and those on the southern supercontinent Gondwana, like this newfound species, named Stegouros elengassen.

The dinosaur’s genus name, Stegouros, comes from the Greek words for “roof” (stego) and “tail” (uros) — a nod to its covered tail — and its species name, elengassen, refers to an armored beast in the mythology of the local Aónik’enk people.

Paleontologists found S. elengassen in Cretaceous period rocks dating to between 71.7 million and 74.9 million years ago in February 2018. The well-preserved skeleton was about 80% complete, and “it’s weird, because it’s articulated [the bones are in order] from the waist down, and everything from the waist up was kind of scattered,” Vargas said. The beast died by a river, perhaps in quicksand, which would explain why its bottom half is so well preserved, although this is just speculation, he said.

S. elengassen is strikingly different from Laurasian ankylosaurs; it’s lightly armored with a few rows of osteoderms, or bony plates, and has a “rather large head with a narrow, curved beak, which is not common for ankylosaurs,” Vargas said. “It has slender limbs. … It doesn’t have pointed claws; it has rounded, hoof-like claws on both hands and feet.”

What’s more, the ankylosaur’s pelvis is wide and stegosaur-like. “If you had only the pelvis, you would think you had the first Stegosaurus of the Cretaceous,” he said. (Stegosaurus lived earlier, during the Jurassic period.)

According to Matt Lamanna, a vertebrate paleontologist at the Carnegie Museum of Natural History in Pittsburgh, who wasn’t involved with the study, “It’s just exceptional material and just a really unexpected dinosaur.”