This Dinosaur Found in Chile Had a Battle Ax for a Tail

It’s not every day you find a dinosaur that defended itself from predators with a completely unique weapon.

In a study published Wednesday in Nature, Chilean researchers announced the discovery of a new species of ankylosaur, a family of dinosaurs known for their heavy armor, from subantarctic Chile. The animal, which they named Stegouros elengassen, offers new clues about where these tank-like dinosaurs came from — and features a bizarre, bony tail shaped like a club that was wielded by Aztec warriors.

“It’s lacking most of the traits we’d expect from an ankylosaur and has a completely different tail weapon which shows there’s something very idiosyncratic happening here in South America,” said Alexander Vargas, a professor at the University of Chile and a co-author on the study.

A diverse collection of ankylosaurs once roamed in great numbers across Laurasia — the northern supercontinent that once contained North America and Asia. Even in a group of animals famous for its inventive approach to defense, the ankylosaur family stands out. Splitting from their closest relatives, the stegosaurs, in the mid-Jurassic, ankylosaurs developed hides covered in bone deposits called osteoderms, which formed lattices of tooth-breaking armor. The most famous species of ankylosaur evolved shin-shattering tail clubs like the maces of ancient warriors.

But their relatives from the southern continent of Gondwana — now South America and Antarctica — are less well studied, Dr. Vargas said. Since these are believed to include the earliest members of the group, the origins and early evolution of the family have been an enduring mystery.

In February 2018, a team of paleontologists from the University of Texas stumbled across a set of bones in the frigid, wind-blasted valley of Río Las Chinas, on the southernmost tip of Chile. Despite its forbidding nature, the site is a beacon for paleontologists: Dr. Vargas has spent the last decade working there with researchers including Marcelo Leppe from the Chilean Antarctic Institute, dating rocks and looking for fossil hot spots.

There were only five days left in the field season when the Texas paleontologists alerted Dr. Vargas and Dr. Leppe to the find. Working at night under very cold conditions, they hauled the block of fossils downhill to the campsite. One person got a sprained ankle and another broke a rib. Many people came close to hypothermia.

Delivering the fossil from the field in Rio Las Chinas in 2018.Credit…via Alexander Vargas
Fossils of Stegouros entered a medical scanner for computerized tomography.Credit…via Alexander Vargas
Sergio Soto-Acuña, one of the two leading authors of the team.Credit…via Alexander Vargas

But what came out of the block was worth it. Preparation revealed an unusually complete ankylosaur: 80 percent of a skeleton, including a largely articulated back half, as well as vertebrae, shoulders, forelimbs and scraps of skull.

In life, Stegouros would have been about six feet long, with a proportionally large head, slender limbs and a strange short tail, tipped with seven pairs of flattened, bony osteoderms that form a single structure.

That tail weapon — which Dr. Vargas compared to a macuahuitl, the obsidian-studded bladed club of Mesoamerican warriors — seems to have evolved independently of other ankylosaurs. Early ankylosaurs from the north have no tail clubs, and later ones developed them through the evolution of stiffened vertebrae, forming the “handle” of the blunt tail club.

But the tail club of Stegouros is stiffened through osteoderms fusing over the vertebrae, forming the distinctive wedged shape. The fused osteoderms may have been covered in sharp sheaths of keratin, the material that covers horns and claws, said James Kirkland, state paleontologist with the Utah Geological Survey who was not involved in the study. A blow from the tail would have been like being “whacked in the shins by a battle ax,” he said.

Victoria Arbour, paleontology curator at Canada’s Royal British Columbia Museum, said the tail resembled those of giant extinct armadillos called glyptodonts. “It’s another interesting example of the evolution of bony tail weapons, which have only evolved a couple of times ever but seem to have evolved multiple times in ankylosaurs,” she said.

By crunching anatomical data, Dr. Vargas and his colleagues concluded that Stegouros was closely related to southern ankylosaurs found in Antarctica and Australia.

Digital reconstruction of the unique tail weapon of the new species of armored dinosaur Stegouros elengassen. The tail was encased in pairs of dermal bones; a portion of the dermal bones has been digitally sliced away, to reveal the tail vertebrae within. Different colors signal physically separate bones; many dermal bones have fused into a single unit (liliac).CreditCredit…José Palma and Joao Francisco Botelho

After the final separation of Laurasia and Gondwana in the late Jurassic, Dr. Vargas said, the two northern and southern ankylosaurs pursued different evolutionary trajectories, suggesting the possibility that an entire lineage of strange ankylosaurs in Gondwana are waiting to be discovered.

Dr. Kirkland agrees that Stegouros is closely related to Antarctica’s Antarctopelta, and suggests it may even be the same animal. But it’s possible that Gondwana hosted multiple lineages of ankylosaur, including some more closely related to northern animals. “It’s not often that a new ‘family’ of dinosaurs is discovered,” Dr. Kirkland said. “The record of armored dinosaurs in the Southern Hemisphere has been pretty poor and this beast hints at what we have been missing.”

Stegouros also represents a breakthrough for Chilean paleontology, Dr. Vargas said. Paleontologists are discussing and debating how to make their field less dependent on North American and European institutions. The paper, led by Chilean paleontologists and published in Nature, a top journal, was funded by Chilean grants rather than outside institutions.

“This is very rare for Chilean science,” Dr. Vargas said. “And it’s just the beginning. In terms of academic achievement, the fossil record of Chile is tremendously important.”

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