#AceNewsReport – June.06: When palaeobiologist Elizabeth Sibert set out to build a record of fish and shark populations over millions of years, she didn’t expect to be solving a mysterious disappearance case:
Sharks were nearly completely wiped out 19 million years ago: But according to a recent study in Nature, today’s shark populations have dropped by an alarming 71 per cent over the past five decades, with overfishing being one of the main drivers of the decline: Sharks ‘read magnetic fields like a map’
Sharks are the ocean’s world travellers, and now scientists have found they use magnetism to navigate.
Tiny shark and fish fossils collected from the bottom of the ocean showed that sharks ruled the oceans for some 40 million years, their numbers 10 times higher than they are today.
And then they vanished.
Global shark populations were wiped out by up to 90 per cent around 19 million years ago, even though there were no signs of sudden climatic or environmental changes.
The findings were published today in Science.
“We discovered this almost entirely by accident,” said Dr Sibert, who is based at Yale University’s Institute for Biospheric Sciences.
“This is the biggest extinction that sharks have ever seen.”
A gap in the record
It all started when Dr Sibert, along with her co-author Leah Rubin, then a student at the College of the Atlantic, decided to explore whether fish and shark populations had experienced any major changes over the past 85 million years.
They looked at microfossils in deep-sea sediment cores — collected in the North and South Pacific oceans — and compared the frequency of fish microfossils to shark microfossils.
The shark microfossils were in the form of dermal denticles — the tiny, plate-like scales that cover a shark’s skin.
Before 19 million years, the researchers found one shark fossil per five fish fossils, indicating that the ocean was once teeming with sharks.
But after that point, they counted just one shark fossil per 100 fish fossils.
After thriving for around 40 million years, shark numbers fell by a whopping 90 per cent.
Since then, global shark populations have not recovered from the die-off, said Dr Sibert, who was at Harvard University while conducting this research.
“The sharks basically just disappeared overnight.”
The researchers also analysed the shape of the denticles, which can vary significantly across shark groups.
This revealed that the number of different shark groups had dropped by around 70 per cent.
After the extinction event, the most common denticle types resembled those seen in most sharks today — smooth and linear — which may help modern sharks swim efficiently over long distances.
Geometric denticles with interlacing ridges were much rarer after the mass die-off.
Today, they’re mostly found on small, deep-sea shark species, such as the Cookiecutter shark and Lantern shark.
Cause of decline a mystery
The sudden wipe-out 19 million years ago wasn’t the first extinction for sharks, but it was more devastating than previous events.
The Cretaceous-Paleogene mass extinction that occurred 66 million years ago — which killed off the dinosaurs — decimated three-quarters of the world’s plant and animal species.
But the mysterious extinction event discovered by Dr Sibert and Ms Rubin resulted in a decline that was twice as severe for sharks compared to what they experienced during the Cretaceous-Paleogene event.
While scientists suspect the Cretaceous-Paleogene event was caused by a massive asteroid or comet impact, there was nothing of the sort 19 million years ago.
“We weren’t expecting this, because this period is not known for rapid extinctions or major global change,” Dr Sibert said.
It’s also unlikely that the sharks were out-competed by other marine predators — such as whales, tuna, and seabirds — as these groups didn’t appear until around 5 million years after the event.
But their sudden disappearance suggests that something major was going on during this relatively unknown period of time.
“It’s an example of the biology telling us that this is a really important interval in Earth’s history that we’ve been overlooking,” Dr Sibert said.
Lessons for the future
Sharks have been cruising the oceans for over 400 million years, making them older than the earliest trees.
They’re one of the long-term survivors in Earth’s story.
Today, more than 400 species of sharks inhabit the oceans, with 180 of these living in Australian waters.
“What we see today is a tiny fraction of the diversity that they once knew,” Dr Sibert said.
As top predators, sharks keep marine ecosystems in balance by keeping prey populations under control.
Looking at mass extinction events of the past can give us a window into the future, and how big changes in predator populations can reshape ecosystems.
“Losing 90 per cent of the abundance of a really important predator group is a huge deal to an ecosystem, and can really derail how the ocean works,” Dr Sibert said.
Catherine Boisvert, an evolutionary developmental biologist specialising in sharks at Curtin University, said that despite their looming extinction, sharks are still not a high enough priority in conservation efforts.
“If we can save the whales, we can surely save the sharks,” said Dr Boisvert, who was not involved in the study.
“And we have to, because it has enormous consequences on the ecosystem, and that diversity simply doesn’t come back.”
The next step for the researchers is to figure out what led to the massive shark die-off 19 million years ago, and why they never bounced back.
“The million-dollar question is why?” Ms Rubin said.
“We still haven’t figured that out, but the great thing about this research is that it just keeps offering us more and more questions to dive into.”
#AceNewsDesk report ……Published: Jun.06: 2021:
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