February 8, 2023

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The Mystery of Nevada’s Ancient Reptile Cemetery

Berlin, Nevada is a treasure chest for paleontologists. Just down the road from now-abandoned gold and silver mines, a stone collection of bones hint at an even richer past. Berlin Ichthyosaur State Park is teeming with dozens of fossils of ancient marine reptiles. This bed of bones is so plentiful and strange that researchers have scratched their heads over it for decades.

“There are places where ichthyosaur skeletons are much denser, including places in Chile and Germany,” said Nick Pyenson, curator of marine mammal fossils at the Smithsonian National Museum of Natural History. “But this place, Berlin ichthyosaurs in eastern Nevada, has eluded explanation for a really long time.” At one particular quarry, at least seven specimens of the genus Shonisaurus — a bloated, bus-sized dolphin with four limb-like fins — lay essentially stacked on top of one another.

Previous hypotheses have largely focused on physical or environmental reasons for fossil aggregation. One suggested that the animals stranded in shallow water and died as a group about 230 million years ago. Or maybe a volcanic eruption destroyed them. Pyenson had a different guess, which his team tested using 3D visualizations of the site, as well as fossils and other clues in the geological record.

In an article in the journal Current Biology today, Pyenson’s team presents evidence that shonisaurs got there to breed. The team concludes that the animals traveled long distances to give birth, as some whales do today. The discovery represents not only an example of “convergent evolution,” where the same traits evolve independently in different species, but also the oldest example of group migration to a given birthplace.

“They make pretty compelling arguments,” says Lene Liebe Delsett, a vertebrate paleontologist at the University of Oslo, Norway, who was not involved in the study. “Ichthyosaurs were the first large marine tetrapods. And throughout the Triassic, they varied quite a bit, so there was a lot of variety. It’s just a very interesting time to learn more about it.”

The origin story of the shonisaurs begins with death – a large part of it.

About 251 million years ago, between the Permian and Triassic periods, Earth’s largest extinction event wiped out about 95 percent of all marine life. This so-called “Great Dying” mowed down the diverse landscape of creatures in the ocean. Some of the animals that grew in their place turned out to be stranger and bigger than ever.

The following Triassic started an evolutionary arms race. Prey evolved harder shells and better mobility, predators crunched through ammonite shells and hunted fish better than ever, and so on. Partially driving this pressure, ichthyosaurs, which evolved from terrestrial reptiles into new species of varying sizes, quickly came to dominate the ocean. The Shonisaurus genus in particular developed into one of the largest marine predators of all. “They reached the size of a whale before anything else,” says Pyenson.

Pyenson is usually more of a whale type; He specializes in mammals that split off from reptiles about 325 million years ago. But ancient marine reptiles like those of the order ichthyosaurs share many similarities with existing marine mammals. Their ancestors came from the land, they were born alive, they had similar fins, and they are tetrapods, meaning they have four limbs. And Pyenson is very familiar with these types of puzzles. About a decade ago, he and his South American collaborators in Atacama, Chile, used 3D mapping and chemical analysis to show that a dense assemblage of at least 40 fossilized whales must have died from a toxic algal bloom 7 to 9 million years ago.