Scientists try to understand mass death of whale-sized reptiles 230 million years ago

The mystery had haunted paleontologists for decades: what would explain a case of fossilized mass death in the rocks of West Union Canyon, in Nevada (central east USA)? Even more mysterious: why are all the victims large predatory marine reptiles called ichthyosaurs, which swam the seas while dinosaurs walked the Earth?

A team of researchers says in an article published on Monday (2) in the journal Current Biology that this enigma in stone has finally been solved.

The reptile graveyard was once a habitat for the ichthyosaur Shonisaurus popularis, which was the size of a whale. About 230 million years ago, it was “this shallow tropical ocean,” said Randall Irmis, a paleontologist at the Utah Museum of Natural History and author of the study. “Now we are in the middle of the Great Basin, in the high desert, at the foot of the Shoshone Mountains.”

The canyon’s most famous cluster of corpses, a cluster of at least seven skeletons known as Quarry 2, can be visited at Ichthyosaur State Park Berlin.

Scientists have speculated about the Quarry 2 ichthyosaurs since they were first excavated in the 1950s by Charles Camp, a paleontologist at the University of California, Berkeley. But Irmis and her colleagues discovered in 2014 that other spooky clusters had occurred in the canyon at different points in history, spanning hundreds of thousands of years. It wasn’t just one case of mass death, but many, and all the victims appeared to be adult ichthyosaurs.

The researchers used a variety of techniques, including 3D scans that digitally reconstructed the fossils, to methodically rule out how the gigantic animals met their end. The first suspect: cases of mass strandings, as we see happening today with whales.

“This doesn’t match the sedimentology we found,” said Nicholas Pyenson, a paleontologist at the Smithsonian National Museum of Natural History in Washington, DC, and one of the study’s authors. “There are no beach deposits, there are no tidal flats.” On the contrary, he said, the rocks suggest that the area would be submerged in more than 90 meters of water.

To rule out poisoning or asphyxiation due to, for example, volcanic eruptions, the team assessed the levels of mercury and oxygen preserved in the rock and found nothing suspicious. The only other strange thing about the canyon was the lack of other preserved life forms such as fish or other types of ichthyosaurs. Pyenson said they found only Shonisaurus and nothing else, not even prey that carnivorous reptiles could have eaten.

They finally found a hypothesis where fossil detectives least expected it – hidden in the dusty closets of the museum’s collections.

While combing through ancient specimens for clues, the team found small bones of Shonisaurus from the canyon that could not have belonged to adults, including a fossil embryo trapped in an adult ribcage that Camp himself first excavated but never formally described.

Further fieldwork at the site revealed more bone material from embryonic and newborn ichthyosaurs. Rather than a mass graveyard of death, the canyon appears to have served as a source of mass life: a maternity ward.

“I think it was a place where giant ichthyosaurs went to give birth,” Pyenson said, similar to how today’s ocean giants — whales and sharks — routinely migrate from one place where they feed to one where they spawn.

Although the oceans were different 230 million years ago, ichthyosaurs “probably behaved the same way,” he said.

The animals didn’t die from some mass catastrophe, but rather at a normal rate from various causes, their skeletons clump together in death because that’s how they behaved in life.

The idea that ichthyosaurs lived and traveled in groups like whales is not new, but “this is the first study to really demonstrate this in a credible and proven way,” said Erin Maxwell, a paleontologist at the State Museum of Natural History in Stuttgart. , in Germany, who did not participate in the study. Maxwell hopes the team will examine the bones of the new specimens, especially the embryos and newborns, more thoroughly.

Although this case is now closed, like any mystery story there can be sequels. “There’s a lot about the biology of whale-sized reptiles that we still don’t know,” Pyenson said. That includes the biggest mystery of all: why they went extinct 88 million years ago, tens of millions of years before land dinosaurs. Stay tuned to see if some nosy paleontologist (and his annoying scientific method) solves this next.