The mystery has haunted paleontologists for decades: What explains a fossilized mass death in the rocks of Nevada’s West Union Canyon? Even more mysterious: why the victims are all large predatory marine reptiles called ichthyosaurs that swam the seas while dinosaurs walked the earth?
A research team says in a published paper Monday in the journal Current Biology that this riddle set in stone has finally been solved.
The reptile graveyard was once home to the whale-sized ichthyosaur Shonisaurus popularis. About 230 million years ago, it was “this shallow tropical ocean,” said Randall Irmis, a paleontologist at the Natural History Museum of Utah 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 corpse cluster, a grouping of at least seven skeletons known as Quarry 2, can be visited at Berlin-Ichthyosaur State Park. Scientists have speculated about the ichthyosaurs of Quarry 2 since they were first excavated in the 1950s by Charles Camp, a Berkeley paleontologist. But dr. Irmis and colleagues found in 2014 that other gruesome groupings had occurred in the canyon at various points in history, spanning hundreds of thousands of years. This was not just one case of mass death, but many, and all of the victims appeared to be adult ichthyosaurs.
The researchers used different techniques, including 3D scans that digitally reconstructed the fossils, to methodically eliminate hypotheses about how the giant animals met their end. The first suspect: mass strandings, such as those seen in whales today. “It doesn’t match the sedimentology we find,” said Nicholas Pyenson, a paleontologist at the Smithsonian National Museum of Natural History in Washington, D.C., and an author of the study. “There are no beach deposits, there are no mudflats.” Instead, he said, the rocks suggest the area would have been submerged in more than 300 feet of water.
To rule out poisoning or suffocation from volcanic eruptions, for example, the team assessed the mercury and oxygen levels 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 ichthyosaur species. Dr. Pyenson said they found only Shonisaurus and nothing else, not even prey for the carnivorous reptiles to eat.
They finally found a smoking gun where the fossil detectives least expected it – hidden in the dusty cupboards of museum collections. Searching ancient specimens for clues, the team found the tiny Shonisaurus bones from the canyon that could not have belonged to adults, including a fossil embryo tucked into an adult ribcage that Charles Camp himself first excavated but never formally described. Further fieldwork at the site yielded more bone material from embryonic and newborn ichthyosaurs. Rather than a graveyard of mass death, the canyon seems instead to have served as a source of mass life: a birthplace.
“I think this was a place where giant ichthyosaurs came to give birth,” said Dr. Pyenson, similar to how today’s ocean giants – whales and sharks – routinely migrate from one place where they feed to another where they give birth.
Although the oceans differed 230 million years ago, ichthyosaurs “probably showed the same behavior,” he said. The animals didn’t die from some mass catastrophe, but died at a normal rate from various causes, their skeletons grouped together in death because that’s how they behaved in life.
The idea that ichthyosaurs lived and traveled together like whales isn’t new, but “this is the first study that really shows it in a credible and supported way,” said Erin Maxwell, a paleontologist at the State Museum of Natural History Stuttgart in Germany. , who was not involved in the study. Dr. Maxwell hopes the team will then look at the bones of the new specimens, especially the embryos and newborns, in more detail.
Although this case is now closed, like any good hairdresser, it is ripe for follow-up. “There’s a lot about the biology of whale-sized reptiles that we still don’t know,” said Dr. Pyenson. This encompasses the greatest mystery of all: why they went extinct 88 million years ago, tens of millions of years before dinosaurs arrived on land. Stay tuned to see if meddlesome paleontologists (and their nasty scientific method) solve it next!