It took decades of struggling with the weather on a small, desolate island off the Antarctic Peninsula. But now, scientists have finally unearthed the heaviest known elasmosaur, an ancient aquatic reptile that swam the seas of the Cretaceous period alongside the dinosaurs. The animal would have weighed as much as 15 tons, and it is now one of the most complete ancient reptile fossils ever discovered in Antarctica.

Elasmosaurs make up a family of the plesiosaurs, which represent some of the largest sea creatures of the Cretaceous. Plesiosaurs generally look a little like large manatees with giraffe necks and snake-like heads, though they have four flippers rather than a manatee’s three. (Find out about a plesiosaur fossil found with a baby preserved in its body.)

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The team thinks the newly described heavyweight belongs to the genus Aristonectes, a group whose species have been seen as outliers to other elasmosaurs, since they differed so much from fossilized specimens discovered in the U.S. This genus, found in the Southern Hemisphere, is characterized by shorter necks and larger skulls.

“For years it was a mystery ... we didn’t know if they were elasmosaurs or not,” says José O’Gorman, a paleontologist with the National Scientific and Technical Research Council of Argentina (CONICET) who is based at the Museum of La Plata near Buenos Aires. “They were some kind of weird plesiosaurs that nobody knew.”

Researchers needed a more complete specimen, and as it happened, William Zinsmeister of Purdue University had discovered a potential candidate on Seymour Island—just south of the northern tip of the Antarctic Peninsula—during a 1989 expedition. At the time, though, he didn’t have the resources to excavate the fossil find, but he informed researchers in Argentina about the discovery.

Glacial excavations

The Argentina Antarctic Institute got involved and started excavating the fossil as part of its annual summer research expeditions, but the giant reptile was uncovered at a glacial pace due to weather and logistics.

O’Gorman, who was five years old when the fossil was discovered, went on the first of these trips starting in 2012. Work could only happen for a few weeks in January and early February, and some years the dig didn’t happen at all because of conditions and limited resources. On active days, the team had to wait for the sun to defrost the soil before they could excavate, and every piece wrested from the dirt would then need to be shipped by helicopter to the Argentine Marambio Base a few miles away.

“The weather is one of the problems. The weather controls all. Maybe one day you can work, and the next day you cannot because you have a snowstorm,” O’Gorman says.

“It takes a little more effort and logistics in the first place, and not just everyone stumbles into those fossils,” agrees Anne Schulp, vertebrate paleontologist at Utrecht University in the Netherlands and the Naturalis Biodiversity Center who was not involved in the research.

A colossus among giants

The excavation finally finished in 2017, yielding a substantial portion of the animal’s skeleton, which O’Gorman and his colleagues describe in their recent paper in Cretaceous Research.

“We don’t have a skull, but we have a lot of pieces of the specimen,” O’Gorman says.

They estimate that the as-yet-unnamed elasmosaur weighed between 11.8 tons and 14.8 tons, with a head-to-tail length of nearly 40 feet. While some previously known Aristonectes have weighed about 11 tons or so, most other elasmosaurs only come in at around five tons.

“That guy is big!” Schulp says from looking at photos of the bones.

He thinks the work is well done, and he’s happy that the team hasn’t jumped to hasty conclusions—O’Gorman even hesitates to say whether the species is definitely from the Aristonectes genus, since further evidence may put the species in a new genus entirely.

Last call of the Cretaceous

Schulp has worked on some plesiosaurs from the Netherlands, but he says the aquatic reptiles are very different in the Southern Hemisphere. The new specimen is also very interesting because it dates so close to the end of the Cretaceous—just 30,000 years before the mass extinction event that wiped out the non-avian dinosaurs about 66 million years ago.

A lot of marine life would have needed to thrive there to satisfy the appetite of such a large creature, so the fact that these animals continued to exist so late in the Cretaceous adds to the evidence that the aquatic world, at least, was doing just fine right up until the sudden mass extinction. (Would the dinosaurs have died out if not for that asteroid? Here’s the science.)

“Even in Antarctica, there were lots of happy elasmosaurs,” Schulp says. The different morphology of this species also shows that specialization was still happening at this late point in the existence of plesiosaurs. “It’s definitely an indication that toward the end of the Cretaceous, [plesiosaurs] managed to expand their feeding repertoire,” Schulp says.

While the animal’s exact diet can’t be known without fossilized stomach contents or other evidence, O’Gorman says that it likely fed on crustaceans and small fish, based on the small size of its teeth. And work on the bones unearthed over the past few decades has just begun; now that they are housed in a museum, O’Gorman says there is a lot of other research that can be done on this ancient specimen.

Schulp adds that the work moves our knowledge of plesiosaurs forward, and he is excited to see Argentine paleontologists go back out there and find more fossils.

“The Southern Hemisphere—at least the plesiosaurs—could definitely use some attention,” he says.