New ankylosaur species had a war axe for a tail

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Ankylosaurs are the armored dinosaurs most closely related to stegosaurs. They are most famous for their broad feet, armed tails, and for being the most muscular of all the herbivorous dinosaurs. Now, a newly discovered species called Stegouros elengassen is rewriting what we know about these ancient animals.

Alexandre Vargas of the University of Chile’s Department of Biology and colleagues identified the new species based on an 80% complete skeleton discovered in the Patagonia region of Chile. It sheds light on the evolution of the so-called broad-footed dinosaur, and it has a vicious fighting axe of tail, to boot. The results of her study were published in the journal temper nature.

Stegouros elengassen’s unique tail structure consists of a flat surface with a series of sharp, pointed blades along each side, reminiscent of the macuahuitl, the Aztec war club. However, the tail only marks the beginning of the ways in which it differs from other ankylosaurs.

“The skull does not have any bony skin, and there are no plates attached to the skull which are common in ankylosaurs in the north,” Vargas told SYFY WIRE. “The beak is very narrow and is slightly curved, resembling an eagle type, while the beaks of northern ankylosaurus are duck-like. The limbs are slender, and the broad-footed state that would be expected for ankylosaurus is absent.”

When the fossil was first discovered, researchers thought they might have found a completely different type of dinosaur because the limbs weren’t what they expected at all. Animals such as stegosaurs and ankylosaurs are commonly called broad-footed dinosaurs precisely because they shared this trait, but Stegouros challenges that idea.

This actually means that stegosaurs and ankylosauruses independently evolved their broad feet. And the funny thing is that the first broad-footed dinosaurs weren’t broad-footed at all,” Vargas said.

There has been some debate among paleontologists about how closely related ankylosaurs and stegosaurs are in fact. Stegouros’ discovery does not completely solve this question but supports the view that they are related, given the semi-hybrid nature of this species.

“It contains a mixture of stegosaurus and ankylosaurus traits. Many traits we think of as stegosaurian are not actually stegosaurian but are from a common ancestor. Stegouros is hardly an ankylosaurus, but it’s much closer to them than Stegosaurs. That wasn’t a conclusion that was immediately obvious to us,” Vargas said.

The team had to analyze the numbers in five different analyzes of evolution in order to properly place Stegouros in the dinosaur evolution story. Fortunately, all five of these analyzes consistently showed that Stegouros was related, even though it likely took a different evolutionary path than its known relatives. They lived between 72 and 75 million years ago, but the origins of the species are likely much deeper, beginning about 100 million years ago.

“There are actually two major lineages that split very early in their evolution, in the middle of the Jurassic,” Vargas said. “One leads to true ankylosaurs and the other leads to South American ankylosaurs. It’s not your usual flavor and didn’t split for long after their last common ancestor with stegosaurs. That’s why they have such a mixture of traits.”

Vargas and colleagues propose a new clade–Parankylosauria, meaning alongside or alongside–to harbor this species as well as its first ancestor and descendant. Although Vargas lovingly refers to them as “weird” ankylosaurs. There is nothing stranger in them than their tails.

The bones were placed under CT and CT scans in order to get a clear view of what was going on inside and how the tail pieces fit together. Five large osteoderms (bone deposits that form scales and plates) are completely fused. Furthermore, the tail is shorter than typical ankylosaurs by about 10 vertebrae, which likely allowed Stegouros to prevent its tail from dragging and aided in its use as a weapon.

“It’s almost impossible that they didn’t use it for defense,” Vargas said. “Many reptiles use their tails for defense, even those without sophisticated tail weapons. Even an iguana will whip you with its tails. There are studies that say that any animal that has a hard torso, is a herbivore, and has bony skin extending into its tail, will use it as a weapon. In Sometimes these bone skins get fanciful and evolve into truly specialized weapons.”

Stegouros is a dino sticker of being cute but deadly, and it’s positive proof that if we could imagine a way to build a weapon, nature figured out how to do it better millions of years ago. It is also evidence that the story of our planet’s history is only partly told. There are still wonderful things to be found, and if there are rad dinosaurs waiting to be discovered, the child within all of us is asking to be found.

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