Newly unearthed dinosaur evolved 'large tail weapon' unlike any other

Choppin' it up with the dog-sized Stegouros.

Monisha Ravisetti Former Science Writer
Monisha Ravisetti was a science writer at CNET. She covered climate change, space rockets, mathematical puzzles, dinosaur bones, black holes, supernovas, and sometimes, the drama of philosophical thought experiments. Previously, she was a science reporter with a startup publication called The Academic Times, and before that, was an immunology researcher at Weill Cornell Medical Center in New York. She graduated from New York University in 2018 with a B.A. in philosophy, physics and chemistry. When she's not at her desk, she's trying (and failing) to raise her online chess rating. Her favorite movies are Dunkirk and Marcel the Shell with Shoes On.
Monisha Ravisetti
2 min read

Fossils found in Chile are from the bizarre dog-sized dinosaur species called Stegouros that had a unique slashing tail weapon.

Lucas Jaymez

In a southern and sparsely populated region of Chile, scientists excavated the skeletal remains of a naturally armored dinosaur that lived over 70 million years ago, during the late Cretaceous period. Much to the team's surprise, they found it possessed a rather bizarre feature: a knife-like artillery in place of a tail.

Although they echo beings straight out of fantasy novels, armored dinosaurs are a well-known crew. Ranging from the sharply adorned Kentosaurus to the curvy backed Hesperosaurus, paleontologists have already studied a long list of the physically shielded animals. But this new member of the warrior-like troop of beings piqued researchers' interest because of its specialized armament that could've once sliced through enemies. 

The ancient herbivore "evolved a large tail weapon unlike any dinosaur," the team said about the discovery in a report published Wednesday in the journal Nature. The dinosaur's oddly shaped backside is decorated with a whopping seven pairs of bony deposits fused together, emulating actual blades. 


A reconstruction of the newly unearthed dinosaur's tail.

Lucas Jaymez

"It was an animal with a proportionally large head and a narrow snout with a beak," Sergio Soto Acuña, lead author of the study and a doctoral student at the University of Chile said. "However, the most notable feature is the caudal weapon: the posterior half of the tail is enclosed in a structure made up of fused bony plates that give the tail a very strange appearance."

The team dubbed the 2-meter (about 6-foot-6-inch) long species Stegouros elengassen due to the rest of its body resembling the Stegosaurus genus -- aka Spike from The Land Before Time. Later, extensive DNA analysis and cranial examination revealed the animal to be more closely related to a dinosaur group called Ankylosaurs, but the team decided to keep the initial name.

"I think this finding radically changes what we thought about the evolution of armored dinosaurs in the southern hemisphere," Acuńa said. "Our results show that they were not simple dispersal events of northern Ankylosaurs, but rather that they were a very ancient branch of primitive Ankylosaurs that evolved in isolation from other armored dinosaurs."


The hips, legs and tails of the Chilean dinosaur's fossilized skeleton.

University of Chile

He said that one of the most surprising outcomes about this discovery was the revelation of an entirely new lineage of Southern Hemisphere armored dinosaurs that had evolved its own posterior weaponry -- independently of plated dinosaurs, or Stegosaurs, and densely armored dinosaurs, or Euankylosaurs. 

Presumably, the dangerous appendage was used to defend against predators. But either way, Acuña adds, "This shows us that the fossil record of the Gondwanan continents can still have unexpected surprises for us."


A stegouros chomping on some leaves.

Lucas Jaymez