Something About This Mysterious Fossil Graveyard Was Fishy

The mystery has haunted paleontologists for decades: What explains a fossilized mass death event in the rocks of the West Union Canyon in Nevada? Even more mysterious: Why are the victims all large predatory marine reptiles called ichthyosaurs that swam the seas while dinosaurs walked the earth?

One research team says in a paper published Monday in the journal Current Biology that this enigma in stone has finally been solved.

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

The canyon’s most famous cluster of corpses, 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 discovered in 2014 that other ghastly groupings had occurred in the canyon at different points in history, spanning hundreds of thousands of years. This wasn’t just one case of mass death, but many, and all of the victims seemed to be adult ichthyosaurs.

The researchers used various techniques, including 3-D scans that digitally reconstructed the fossils, to methodically eliminate hypotheses for how the titanic animals met their end. The first suspect: mass stranding events, like those seen in whales today. “It doesn’t match the sedimentology that 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’s no beach deposits, there’s no tidal flats.” Instead, he said, the rocks suggest that the area would have been submerged in over 300 feet of water.

To rule out poisoning or asphyxiation from, say, volcanic eruptions, the team assessed levels of mercury and oxygen preserved in the rock and did not find anything suspicious. The only other strange thing about the canyon was the lack of other preserved life-forms, like fish or other types of ichthyosaur. Dr. Pyenson said they found only Shonisaurus and nothing else, not even prey that the carnivorous reptiles could have dined on.

They finally found a smoking gun where the fossil detectives least expected it — hiding in the dusty cabinets of museum collections. By poking around old specimens for clues, the team found tiny Shonisaurus bones from the canyon that couldn’t belong to adults, including a fossil embryo tucked into an adult rib cage that Charles Camp himself first excavated but never formally described. Further field work at the site turned up more bone material from embryonic and newborn ichthyosaurs. Rather than a graveyard of mass death, the canyon instead seems to have served as a wellspring of mass life: a birthing ground.

“I think this was a place where giant ichthyosaurs came to give birth,” Dr. Pyenson said, 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 undertook the same behavior,” he said. The animals didn’t die from some mass catastrophe, but rather died at a normal rate from various causes, their skeletons grouping 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 to really demonstrate it in a believable and supported way,” said Erin Maxwell, a paleontologist at the State Museum of Natural History Stuttgart in Germany, who was not involved with the study. Dr. Maxwell hopes the team will next look at the bones of the new specimens, especially the embryos and newborns, in more detail.

Though this case is now closed, like any good caper it is ripe for sequels. “There’s a lot about the biology of whale-sized reptiles we still don’t know,” Dr. Pyenson said. This includes the biggest mystery of all: why they went extinct 88 million years ago, tens of millions of years before dinosaurs on land. Stay tuned to see if any meddling paleontologists (and their pesky scientific method) solve it next!