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Hundreds of millions of fossilized shark teeth are buried in the Sharktooth Hill Bone Bed near Bakersfield, Calif., as sharks shed teeth throughout their lives. A few are even as large as this one, shown, from an extinct 40-foot-long shark. A new study from UC Berkeley links rising sea levels more than 15 million years ago to the creation of the renowned bone bed near Bakersfield, the largest accumulation of marine vertebrate in the fossil record worldwide. (Nick Pyenson/University of British Columbia)

Even after visiting Sharktooth Hill Bone Bed for 50 years, Jere Lipps still speaks with a sense of wonder when describing the land. It's a 110-square-mile region northeast of Bakersfield harboring hundreds of millions of fossils left from the time when the Central Valley was covered by an ocean and warmed by a subtropical climate.

Back then, 30 species of sharks ranging from 6 inches to 60 feet long patrolled a bay lapping at foothills that would later become the Sierra. Enormous seals, turtles, dolphins, whales, hippopotamuslike creatures and scores of other now-extinct species inhabited the waters.

Today, the area is a paleontologist's treasure trove. The creatures' bones are preserved and packed together in a layer fewer than 20 inches thick — hence the name "bone bed." Although much of the bone bed is underground, a large region near Bakersfield is exposed and easily accessed.

Lipps, a curator at the UC Berkeley-based UC Museum of Paleontology and an evolutionary biologist, hopes a new study that he co-wrote brings renewed attention to the often-overlooked and unprotected area, which is mostly in the hands of private landowners. The study appears in the June issue of Geology, and it answers the question that's long puzzled researchers studying Sharktooth Hill: What caused such a massive accumulation of fossils in one region, and in such a dense layer?

The researchers' answer: Rising sea levels, possibly due to factors such as a change in Earth's orbit, volcanoes that spewed heat-trapping carbon dioxide, expulsion of the potent greenhouse gas methane from ocean sediments, and undersea volcanic activity that swelled ocean levels from the movement of hot magma.

Any of the first three factors would melt part of the Antarctic ice sheets, Lipps said, adding to the ocean's rise. And the rising waters for a time interrupted sedimentation, preventing the animals' decaying carcasses from being buried by silt and sand. In thousands of years, they piled on top of one another.

Lipps describes Sharktooth Hill Bone Bed as "this amazing place" deserving of protection from development and exploitation by overzealous fossil hunters who could take away an irreplaceable record of marine life more than 15 million years ago.

"This site could be of enormous educational, recreational as well as scientific value," he said. "Since it's unique in the world, it deserves world attention, too."

The study took several years to complete and was meant to settle the reason why the bone bed formed. Throughout the decades, researchers have proposed various ideas, such as cataclysmic volcano eruptions that led to a massive die off in a short period of time, or a series of toxic red algal blooms.

But those proposals didn't sit well with Lipps, who said there was no way such a vast number of marine animals could have squeezed into the ancient bay that covered the Central Valley, which was connected to an ancient ocean to the west.

"That was one of the keys that tipped me off that this could not be a massive catastrophe and die off," he said. "It just seemed like too many animals to be in one place at the same time."

Instead, the layer of bones, containing 200 fossils per square meter, tells the story of normal life during the heights of climate warming 15 million to 16 million years ago in the midst of the Miocene Epoch. The study said Sharktooth Hill contains bones deposited during a 700,000-year time frame during that peak warming period, although Lipps said he thinks it's more like several thousand years.

The deposits speak to a time when sloths, camels, horses, tapirs and a type of large animal resembling a mastodon roamed on nearby land. Some of them were swept into the ancient bay by a river, soon adding their bones to the pile. In the water were 10 types of dolphins or marine animals similar to them, five kinds of whales, as well as a now-extinct species of a walrus, a sea cow and a distant relation of the elephant that walked on the bay's bottom, devouring shellfish with its powerful jaw. The bone bed contains fossils of 20 kinds of birds, a type of marine crocodile, and many species of bony fish and rays. Four large extinct turtle species also left fossilized remains, with one enormous leatherback turtle from the site on display in the Buena Vista Museum of Natural History in Bakersfield. And one of Lipps' achievements at Sharktooth Hill was unearthing a largely intact 12-foot seal fossil.

When conditions changed and the climate cooled, sedimentation began again in the ancient bay, sealing off the dense layer of bones. Subsequent earthquakes thrust up parts of the bone-filled layer, making it accessible today.

The museum is engaged in efforts, along with others, to secure greater protection for the unique region.

"I know a lot of people feel it's just a place to come and hunt fossils," said Koral Hancharick, executive director of the Buena Vista Museum. "But it shows us so much of what was happening with the Earth at the time. It's so important to preserve it, to keep people from looting it and taking a piece of the puzzle away."

Reach Suzanne Bohan at 510-262-2789 or sbohan@bayareanewsgroup.com.