“Sea monster” fossils point to an ichthyosaur migration 230 million years ago


Fossil experts think they’ve solved a decades-old mystery: How did at least 37 school-bus-sized marine reptiles die and become embedded in rock about 230 million years ago in what is now central Nevada? If scientists at the Smithsonian National Museum of Natural History and other institutions are right, the fossil cemetery near an old silver mine represents an early example of migration, one of the most basic and ingrained animal behaviors.

The bones found at the Nevada site are from the giant ichthyosaur Shonisaurusresembling a giant deformed dolphin. Shonisaurus slid like barges thousands of miles across an ocean known as the Panthalassa, the ancient version of what is now the Pacific Ocean, to breed and give birth to their offspring, according to a new study in Current Biology.

The find offers a rare glimpse into the behavior of prehistoric animals, something not always captured by individual fossils. It raises the possibility that further clues embedded in sediment and soil will allow for a deeper understanding of marine reptiles that inhabited the planet long before humans.

The earliest known Evidence of migration goes back more than 300 million years, to ancient Bandringa sharks with long, spoon-billed 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 wildebeest.

Climate change could play a role in reports of larger-than-average fish in unexpected areas. (Video: John Farrell, Brian Monroe/Washington Post)

Evidence from similar fossils found in other regions suggests so Shonisaurus migrated to central Nevada from parts of present-day California, Alaska, and New Mexico.

If yes, this behavior could connect the history Shonisaurus, the largest creature to travel the oceans in the Triassic period, with modern giants – the blue whales now seen with their calves in the Gulf of California. Whales typically migrate to warmer waters to give birth and then to cooler waters that are rich in nutrients.

“You have to wonder if the same ecological rules apply, even though there are more than 200 million years between them [whales and Shonisauruses]said Nicholas D. Pyenson, one of the authors of the new paper, who works in the Department of Paleobiology at the National Museum of Natural History.

Not all experts in the field believe that Pyenson and his colleagues have solved the mystery of the abundance of Shonisaurus Bones on site and the absolute absence of other ichthyosaurs.

“This study is probably not the final word, but it’s a good step forward,” warned Martin Sander, a professor of paleontology at the University of Bonn in Germany and a research fellow 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 hard to prove.”

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

Charles L. Camp, a UC Berkeley paleontologist, was the first to excavate the alternating layers of limestone and mudstone at this site in the 1950s. He immediately wondered what could be responsible for the large accumulation of Shonisaurus skeletons.

“He thought it might be a mass stranding,” like whales, said Neil P. Kelley, another author of the paper and an assistant professor in the Department of Earth and Environmental Sciences at Vanderbilt University.

But the fossil evidence refutes this hypothesis and shows that the skeletons did settled far from the shore under water.

Trying to explain why Shonisaurus Bones were the only ichthyosaur fossils uncovered at the Nevada site, becoming 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 considered migration to be the most likely scenario after ruling out other possibilities. Testing of the sediment revealed the absence of the mercury levels that would have indicated volcanic activity believed to have caused the largest mass extinction event 252 million years ago.

The researchers were also able to rule out the possibility that a deadly algal bloom poisoned the marine reptiles.

In the end, only the migration scenario made sense.

“Shonisaur definitely occurs elsewhere, so the genus had a wide geographic range, and it’s very reasonable that these large individuals traveled long distances, as most large marine vertebrates do today,” Kelley said. “It should be possible to collect additional data in the future that could test the hypotheses we present in the paper, including migration.”

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

Sander of the University of Bonn said that like sea turtles, ichthyosaurs were originally land animals, “but they appear in the fossil record as adult marine animals. We don’t have the right stones to show how the ichthyosaurs went into the sea.”

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

Pyenson can’t help but think that the ultimate fate of Shonisaurus contains a lesson for modern blue whales and other cetaceans, many of which are now classified as endangered.

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

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