SAN FRANCISCO – In a coastal bay fronting an ancient sea where Bakersfield now stands, the waters once swarmed with 40-foot sharks, ancestral seals larger than any known today, and the ancestors of countless other marine animals long extinct.
That was 15 million years ago, but evidence of their existence lies in a priceless layer of fossils where a team of scientists working from the University of California Berkeley has discovered clues telling how a fantastic fossil-rich site known as Sharktooth Hill formed in the water and was exposed to the surface much later.
The area was shallow and rich with sea life. The marine animals lived and died there by the millions for as long as 700,000 years, the scientists say. There is no evidence, they say, that the animals all died at once from some lethal red tide or in a violent earthquake. The loss of life there was gradual. Eventually, sediments buried the bones and created a fossil-rich underwater shelf that is arguably the richest “bone bed” in the world, the scientists say.
Much later, earthquakes heaved the undersea burial ground upward as the restless San Andreas fault lurched and lurched again. Even more fossil layers were uncovered as sea-level surges during a long period of global warming finally subsided, the scientists said.
Amateur fossil collectors and paleontologists alike have been digging up treasured bones at Sharktooth Hill for many decades. Yesterday, in the journal Geology, the scientists reported on their latest findings and conclusions about the area's origins.
Nicholas Pyenson, a former Berkeley graduate student now at the University of British Columbia; Randall Irmis, another Berkeley colleague now at the University of Utah; and their professor, Jere Lipps, an evolutionary biologist and curator of the University of California Museum of Paleontology, are among the principal authors of the report.
Sharktooth Hill gets its name from the many huge teeth of ancestral sharks known as megalodon found there and that provide clear evidence it was once a rich watery feasting ground for the giant predators, ancestors of today's great white sharks. The waters also teemed with sea turtles whose shells were three times larger than any of today's leatherbacks, plus whales, sea cows, porpoises and fish large and small, the team reports.
The fact that the fossil bones of land animals like tapirs and primitive horses were also dug from the fossil bed shows how close to an early shore lay what is now a monument to early marine life, said Pyenson, the lead author of the report.
“It revealed long cycles of life and death that lasted many thousands of years during the Miocene epoch, when the Earth was hot and the ocean was warm,” Pyenson, a specialist in fossil whales, said in an interview.
During that epoch, the Sierra Nevada range was still rising, and sand from the growing mountains must have run down to the shore and into the shallow waters of the broad Miocene embayment that geologists now call the Temblor Sea. Eventually over many years, the sand buried the bodies of all the marine and land animals to create a layer of bones 6 to 20 inches thick above a deeper layer of dense mud riddled with the burrows of long-extinct shrimp, Lipps said.
Scientists call these dense layers of fossils “bone beds,” and they are packed together as if they were cemented. Sharktooth Hill is the largest in the world, Lipps said.
To tease out, describe and identify many of the bones, Lipps and his colleagues carved a single block of the bone bed about 3 feet square and hauled it to Berkeley's Museum of Paleontology for analysis.
Sharktooth Hill has been known for its trove of fossils on the surface since 1850, and Lipps has been researching there for 50 years. A small segment of the hill has been designated for protection as a National Natural Landmark, but Lipps said a much larger fossil-rich area covering several square miles needs protection, too.
“It's as important to science and the public as the Dinosaur National Monument in Utah and Colorado, and the La Brea Tar Pits in Los Angeles,” he said.