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Study Solves Century-old Enigma Of Prehistoric Marine Mass Grave

June 8, 2009

Good old-fashioned detective work has turned up the first conclusive explanation for the origin of a massive bonebed in southern California, according to a new study led by a UBC paleontologist.

The Sharktooth Hill bonebed is exposed over approximately 100 sq. kilometres of land, located at the southern end of the Central Valley of California. It is one of the largest concentrations of marine vertebrate fossils in the world, containing an average of 200 fossils per square-metre, including the skeletal remains of whales, seals, sea turtles, sharks and land mammals.

What caused the collection of millions of fossils in a layer of only 10 to 50 centimetres of sediment has puzzled scientists since the bonebed’s discovery in the 1850s.

“Scientists have proposed two kinds of explanations based on the accumulation of fossils,” says Nick Pyenson, a post-doctoral fellow in the UBC Dept. of Zoology. “One group of ideas suggests a catastrophic incident such as a volcano eruption, a toxic algal bloom or even 12 metre-long relatives of the great white shark. Another kind of explanation is that the bonebed simply formed over a long period of time.”

Conducting the paleontological equivalent of crime scene investigations on the bonebed, its fossil specimens and the surrounding geological data, Pyenson and his colleagues Randall Irmis and Jere Lipps (Pyenson and Irmis were graduate students at the University of California at Berkeley at the time) were able to disprove the one-time catastrophe theory.

“Our evidence suggests that the bonebed formed over a 700,000 year time-span approximately 15 million years ago,” says Pyenson. Details of the investigation are published in the June issue of the journal Geology.

The team, which included paleontologists from the Natural History Museum of Los Angeles County, examined the size, wear and abrasion of more than 3,000 specimens of bones and teeth at local museums and found only five indicating shark bites. A lack of volcanic sediments and presence of land mammal remains further support the deposit-over-time theory.

“The bonebed formed during the Middle Miocene, which coincides with a prolonged period of exceptionally warm global temperatures,” says Pyenson. “The associated changes in sea levels played an important role in forming the Sharktooth Hill bonebed, which explain its marvelous richness and expanse.

“More importantly, we now have a better handle on the kinds of factors – both geologic and biologic – that bias our interpretation of this snapshot of the ocean life from the Middle Miocene,” says Pyenson.

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