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‘Sea monster’ graveyard mystery solved? Fossils offer signs of 230-million-year-old migration.

Adult and newly born ichthyosaurs called Shonisaurus gather in the Triassic seas more than 200 million years ago in this illustration. (Gabriel Ugueto)
5 min

Fossil experts believe they have solved a decades-old mystery: How did at least 37 school-bus-size marine reptiles die and become embedded in stone about 230 million years ago in what is now central Nevada? If the scientists from the Smithsonian National Museum of Natural History and other institutions are correct, the fossil graveyard near an old silver mine represents an early example of migration, one of the most fundamental and deeply ingrained of all animal behaviors.

The bones found at the Nevada site come from the giant ichthyosaur Shonisaurus, which resembled an enormous, out-of-shape dolphin. Shonisaurus glided barge-like thousands of miles through an ocean known as Panthalassa, the ancient version of today’s Pacific, to breed and deliver their offspring, according to a new study in Current Biology.

The finding offers a rare window into the behaviors of prehistoric animals, something that is not always captured by individual fossils. It raises the possibility that further clues embedded in sediment and soil may offer a deeper understanding of marine reptiles that inhabited the planet long before humans.

The earliest known evidence of migration dates back more than 300 million years to ancient Bandringa sharks with long spoon bill-shaped snouts and prehistoric fish with armored plates. Today billions of animals migrate, including species as diverse as hummingbirds and humpback whales, monarch butterflies and blue wildebeests.

Climate change might be playing a role in reports of larger-than-normal fish in unexpected areas. (Video: John Farrell, Brian Monroe/The Washington Post)

Clues from similar fossils found in other regions suggest that Shonisaurus migrated to central Nevada from parts of modern-day California, Alaska and New Mexico.

If so, that behavior could link the prehistoric Shonisaurus, the largest creature to travel the oceans in the Triassic period, with modern giants — the blue whales observed today with their calves in the Gulf of California. Whales tend to migrate to warmer waters to give birth, then to cooler waters that are rich in nutrients.

“One has to wonder if the same ecological rules are at play even though there are over 200 million years between [whales and Shonisauruses],” said Nicholas D. Pyenson, one of the new paper’s authors who works in the department of paleobiology at the National Museum of Natural History.

Not all experts in the field believe Pyenson and his colleagues have solved the mystery surrounding the great abundance of Shonisaurus bones at the site and the absolute absence of any other ichthyosaurs.

“This study is probably not the final word, but it’s a good step forward,” cautioned Martin Sander, a professor of paleontology at the University of Bonn in Germany and research associate at the Natural History Museum of Los Angeles County.

Sander, who was not involved in the study, added “I’m not entirely convinced. It’s a good idea but it’s awfully difficult to prove.”

The skeletons at Berlin-Ichthyosaur State Park in West Union Canyon show that the Shonisaurus grew up to 50 feet, five times the length of a modern dolphin, and weighed about 22 tons, the equivalent of three large elephants. Their offspring were only a few feet long.

Charles L. Camp, a University of California at Berkeley paleontologist, was first to excavate the alternating layers of limestone and mudstone at the site in the 1950s. He immediately wondered what might account for the large cluster of Shonisaurus skeletons.

“He thought it might be a mass stranding,” like those involving whales, said Neil P. Kelley, another of the paper’s authors and an assistant professor in Vanderbilt University’s department of earth and environmental sciences.

But the fossil evidence disproves that hypothesis, showing that the skeletons had settled underwater far from shore.

The effort to explain why Shonisaurus bones have so far been the only ichthyosaur fossils discovered at the Nevada site became a feat of scientific detective work. Researchers combined 3D scanning and geochemistry with more traditional tools such as museum collections, field notes, photographs and archival materials.

They came to view migration as the most likely scenario after eliminating other possibilities. Testing the sediment revealed an absence of the mercury levels that would have signaled volcanic activity, which is believed to have caused the largest mass extinction 252 million years ago.

Researchers were also able to eliminate the possibility that a deadly algal bloom poisoned the marine reptiles.

In the end, only the migration scenario appeared to make sense.

“Shonisaurus definitely occurs at other locations so the genus had a broad geographic range, and it is very reasonable that these large individuals traveled long distances, as most large marine vertebrates do today,” Kelley said. “It should be possible to gather additional data in the future which could test the hypotheses we present in the paper, including migration.”

At least two other mysteries remain surrounding the ancient marine reptiles called ichthyosaurs.

Sander at the University of Bonn said ichthyosaurs, like sea turtles, were originally land animals of some kind, “but they appear in the fossil record as fully blown open ocean animals. We don’t have the right rocks to show how the ichthyosaurs went to the sea.”

Also, while Shonisaurus went extinct about 200 million years ago at the end of the Triassic, “smaller ichthyosaurs survived into the Jurassic and beyond with the entire group going extinct around 88 million years ago in the Cretaceous,” Kelley said. Why the small ichthyosaurs survived and the giants didn’t is not clear.

Pyenson cannot help but think that the ultimate fate of Shonisaurus carries a lesson for modern-day blue whales, and other cetaceans, many of which are now classified as endangered.

“We should want a world,” he said, “with these large ocean giants in it.”