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The new dinosaur species lived in what is now Chile during the Upper Cretaceous epoch, some 74 million years ago.

Named Stegouros elengassen, it was between 1.8 and 2 m (5.9-6.6 feet) long including its short tail.

The ancient creature was a type of ankylosaur, a group of herbivorous armored dinosaurs.

“Armored dinosaurs are well known for their evolution of specialized tail weapons — paired tail spikes in stegosaurs and heavy tail clubs in advanced ankylosaurs,” said Universidad de Chile’s Dr. Sergio Soto-Acuña and his colleagues.

“Armored dinosaurs from southern Gondwana are rare and enigmatic, but probably include the earliest branches of Ankylosauria.”

Stegouros elengassen evolved a large tail weapon unlike any dinosaur: a flat, frond-like structure formed by seven pairs of osteoderms encasing the distal half of the tail.

“The weapon that Stegouros elengassen possesses in the tail is different from both that of stegosaurs (composed of pairs of spikes), and that of advanced ankylosaurs (similar to a club),” the paleontologists said.

“It looks like a macuahuitl, the weapon of the ancient Aztecs.”

The mostly complete skeleton of Stegouros elengassen was discovered in the lower section of the Dorotea Formation in Río de las Chinas Valley, Chile’s Magallanes region, a region that is biogeographically related to West Antarctica.

The researchers found that the new species was closely related to two ankylosaur species, Kunbarrasaurus ieversi from Australia and Antarctopelta oliveroi from Antarctica, forming a clade of Gondwanan ankylosaurs that split earliest from all other ankylosaurs.

“The large osteoderms and specialized tail vertebrae in Antarctopelta suggest that it had a tail weapon similar to Stegouros,” they said.

“We propose a new clade, the Parankylosauria, to include the first ancestor of Stegouros — but not Ankylosaurus — and all descendants of that ancestor.”

A paper describing the discovery was published in the journal Nature.

S. Soto-Acuña et al. 2021. Bizarre tail weaponry in a transitional ankylosaur from subantarctic Chile. Nature 600, 259-263; doi: 10.1038/s41586-021-04147-1

Image credit: Luis Pérez López - An artist’s reconstruction of Stegouros elengassen.

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