Quirks and Quarks

Sharks were almost wiped out in a mysterious extinction 19 million years ago, new study finds

A new study found a 90-per-cent decline in shark abundance and diversity, but no clear cause.

Evidence found in deep sea sediments documents the loss of species, but not why it happened

Microfossil shark scales, known as denticles, arranged in the image of a shark. Denticles are found in deep ocean sediment cores. (Leah D. Rubin)

Scientists studying ocean floor sediments have discovered a previously unknown mass shark extinction that may point to an unknown period of climate change or other significant environmental catastrophe.

"About 19 million years ago, there was a major extinction in open ocean sharks where sharks were about 10 times, maybe even 100 times more abundant in the ocean than they are today," said Elizabeth Sibert, a Hutchinson Postdoctoral Fellow at the Yale Institute for Biospheric Sciences at Yale University.

Sibert, the lead author of a new study published this month in the journal Science, found the evidence for the extinction in deep ocean sediments that contain layers of shark teeth and scales — a record that goes back tens of millions of years. 

When a shark dies in the ocean, its remains fall into the depths where nearly all of the carcass is eaten or decays. However teeth and scales don't decay and build up in sedimentary layers over millennia.

Sibert told Quirks & Quarks host Bob McDonald that she was studying these microfossils in ocean sediment cores to get an idea of the changes in which shark species were present, and what their abundance was throughout history. 

I think the sharks are just like the canary in the coal mine. They're telling us something's up here — let's look closer.- Elizabeth Sibert, Yale University

She was expecting to find minimal changes during periods of Earth's history when the climate became warmer or cooler and, of course, at the mass extinction event 66 million years ago that wiped out the dinosaurs.

"We certainly weren't expecting to find this dramatic 90-per cent decline in shark abundance and diversity 19 million years ago at a time that is really not known in the earth science community as being a period of major global change or extinction," she said.

An aerial view of sharks swimming in the shallow Mediterranean Sea after dozens of sandbar and dusky sharks gathered off the coast of Israel in April 2021. (Jack Guez / AFP via Getty Images)

Co-author Leah Rubin, who is beginning her PhD studies at the State University of New York College of Environmental Science and Forestry, said these findings give context for the state of declining shark populations today.

One-quarter of the current global diversity of sharks is currently threatened with extinction.

"This context is a vital first step in understanding what repercussions may follow dramatic declines in these top marine predators in modern times," said Rubin. 

Looking for clues in shark scales

Insight into just what shark species suffered most in this extinction event came from the abundance of different kinds of shark scales, also known as denticles. There are two broad groups of denticles, those with parallel ridges and those with a more geometric pattern, reflecting different shark lifestyles.

Examples of some distinctive linear denticles on the left, and geometric denticles on the right observed in the study led by Elizabeth Sibert. (Elizabeth Sibert / Leah Rubin / Science)

"We know that a lot of sharks that undergo long-distance migrations, things like great white sharks, have these very clearly linear denticles," said Sibert.

The second type features complex curved shapes on its surface, she said, pointing to a different and rarer group of shark species, similar to modern cookiecutter sharks. 

Cookiecutter sharks sit in the water and wait for larger predators to go by when they burst forth to take a chunk out of them. (Don Kobayashi / NOAA / PIFSC)

Cookiecutter sharks are about a metre in length, and instead of preying on smaller organisms, like most sharks in the open ocean, behave more like mosquitoes.

"It sits in the deep sea and waits for large predators like whales, great whites and other big organisms to come by and it darts into them, takes a single bite and then darts away," described Sibert.

Cookiecutter sharks can attack and take chunks out of other animals in the ocean. (Wayne Hoggard / NOAA / NMFS / SEFSC)

At the time of the extinction event, she found steep declines in denticle abundance for both types of sharks — about 60 per cent for the linear denticles and 90 to 95 per cent for sharks with geometric denticles.

"It was the long distance swimmers who survived, but I really want to emphasize that it was only a small subset of them that actually did survive," she said.

Source of mysterious extinction

Sibert said her findings now lead to an obvious question: what caused this mass extinction of one of the world's most important ocean predators?

"That's the million dollar question. And I think the thing that I'm honestly most excited about this is that we don't really know what happened."

Great white sharks descended from the long-distance swimming sharks that survived the mass extinction 19 million years ago. (©Ron and Valerie Taylor)

Sibert said there are few paleontological records from around 19 million years ago that could shed light on what might have caused this mass extinction.

Whether that's due to a lack of research into this specific period, or a lack of preservation of the key sediments, is not yet clear, so Sibert thinks this should be a target of considerable research interest.

"I think the sharks are just like the canary in the coal mine. They're telling us something's up here — let's look closer."

Produced by Sonya Buyting and Jim Lebans. Written by Sonya Buyting. You can hear the interview with Elizabeth Sibert by clicking the link above.