Sharks living in the open ocean seem to have experienced a previously unknown mass extinction about 19 million years ago. The event may have wiped out nearly 90 per cent of sharks at the time.
Many sharks are currently threatened with extinction as a result of human activities, including overfishing, plastic pollution and illegal shark finning. What makes this situation more striking is that sharks have existed for at least 420 million years and have been considered resilient to large mass extinctions, several of which have happened during that time.
Elizabeth Sibert at Yale University – who conducted the study while at Harvard University – and Leah Rubin at State University of New York College of Environmental Science and Forestry say they have now found the first evidence of a mass extinction of the “pelagic” sharks that live in the open oceans.
They isolated microfossils of shark scales, called ichthyolith denticles, from samples of mud taken from the sea floor in both the North and South Pacific Ocean. The mud samples come from the upper 15 metres of the seafloor, and were deposited over the past 40 million years.
Sibert and Rubin counted and characterised a total of 1263 fossilised denticles. They say the sediment samples reveal a sudden drop in the abundance and diversity of shark scales around 19 million years ago, during an epoch known as the Miocene.
“There seems to have been a major extinction event in the early Miocene, which knocked out about 90 per cent of sharks in the open ocean,” says Sibert. This is more than twice the level of extinction that sharks experienced during the Cretaceous-Paleogene extinction 66 million years ago, which wiped out the dinosaurs.
Sibert says the extinction occurred relatively abruptly, geologically speaking, over a span of 100,000 years.
The sharks don’t seem to have recovered following this drop, says Rubin. The abundance and diversity of shark scales in the mud have remained at the same level from 19 million years ago to the present day.
“By studying these bits of [shark] skin, shed over the course of their lives and buried in the seafloor, they have discovered an unknown event,” says Nicholas Pyenson at the Smithsonian National Museum of Natural History in Washington DC.
The researchers are unsure why this mass extinction occurred. “There are no significant climate events during the early Miocene,” says Rubin. As sharks are top predators, this mass extinction must have cascaded down the food chain and affected other oceanic wildlife, she says.
“It seems that the extinction here is highly selective, as only sharks appear to be impacted, rather than pelagic groups more generally,” says Matt Friedman at the University of Michigan in Ann Arbor. Selective extinctions are known throughout the geological record, and although it is early to speculate, the mass extinction may have only affected the biology of sharks, says Friedman.
However, Charles Underwood at Birkbeck, University of London remains sceptical. “Shark denticles, unlike shark teeth, have rarely been studied in detail,” he says. The change in shark denticle abundance and diversity could be related to a shift in denticle type. This means the fossil evidence may reflect a change in preservation potential of shark remains rather than an extinction event, he says.
Today, there are more than 400 species of shark left in the world’s oceans. However, oceanic sharks and rays have declined by more than 71 per cent over half a century.
Journal reference: Science, DOI: 10.1126/science.abj2088
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