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Saturday, October 23, 2021

The dinosaurs of China

In early September, Ryosuke Motani stood in Anhui Province, China at the foot of a cliff called Turtle Mountain, watching a backhoe scrape part of the mountain’s precious fossil record into dust. The cliff is part of a limestone mine that is the center of the local economy, but it also happens to house one of the world’s richest deposits of ichthyosaur fossils. Motani, a UC Davis professor of paleontology, visited the province last month on a twofold mission: to excavate fossils, and to petition local governments to conserve sites like this one.

“The first time I saw [the mining] I was really shocked, but I got used to it,” said Motani. “It’s really frustrating, but I have to live with it.”

Finding fossils is difficult enough without having them smashed to bits, said Motani. Even with 20 years of experience, Motani says it can be a challenge to tell fossil from stone.

“Usually, you break open a rock and all you can see is a cross-section,” Motani said. “When we find a fossil, it’s not usually so beautiful [as what we see in museums.]”

While mining continues, Motani and a team of paleontologists, including scientists from Milan, Beijing, Chicago and UC Davis, are excavating ichthyosaurs from the same cliffs. An ichthyosaur looks like a cross between a dolphin and a swordfish, with the flippers and tail of a dolphin and a birdlike beak.

This year, Motani discovered the oldest known ichthyosaur fossil at the Turtle Mountain site. About the size of an outstretched hand, this specimen is estimated to be about 246 million years old. Some later ichthyosaurs, which evolved over the following few hundred thousand years, grew up to 60 feet long.

Motani’s discovery provides another clue in the reconstruction of a “recovery scenario,” meaning the events that followed the mass species loss in the early Triassic period. Recovery scenario research is a hot topic in paleontology, as scientists try to infer what environmental changes could have spurred the return of biodiversity.

“Two-hundred and fifty million years ago, there was a very bad mass extinction that destroyed 90 to 95 percent of species on Earth,” said Motani. “After this mass extinction, the planet was virtually empty. There were lots of opportunities for species to grow and diversify. Some reptiles decided to go back into the water, just like the dolphins did. Dolphins’ ancestors were land animals with four legs, but they went back into the water for whatever reason. These ichthyosaurs did the same thing.”

Although no one knows why these lizards might have returned to marine life, Motani speculates that they may have been capitalizing on an easily available food source.

“There were more feeding opportunities in water than on land,” he said.

Ultimately, Motani says, the fossils tell us that the sea level was dropping at a critical time. The waters receded, and the land-dwelling lizards lost their favorite, shallow hunting grounds. The lizards that could live in deeper water were suddenly better adapted to the environment, and had better survival and reproduction rates.

As conditions changed, small flippers gave way to larger flippers, and the lizard-like bodies of the early ichthyosaurs yielded to sleeker, fishlike shapes.

Motani hopes that encouraging tourism in southern China will stimulate the economy while preserving this ancient chapter of China’s history.

EMILY GOYINS can be reached at science@theaggie.org.

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