Sharks have been around for the best part of half a billion years. They were around before trees existed; they survived multiple mass extinction events that wiped nearly all other life off Earth. By any metric, they are one of the most successful species on the planet.
And yet a surprise new discovery has revealed that they came perilously close to disappearing forever – and they still haven’t recovered from the impact.
"We actually found this event entirely by accident!" study lead Elizabeth Sibert told IFLS in an email. "[W]e weren't expecting to find any change in the shark community, much less a huge extinction!"
Sibert, along with study co-author Leah Rubin, made the discovery while studying microfossil fish teeth and shark scales. Very little is known about this rather niche area of micropaleontology, Sibert explained, and the project's initial aim was just to get a better understanding of natural ocean biodiversity.
"We decided to generate a long record of fish and shark fossil abundance, going back many millions of years in the same place, just to see what normal background variability was," Sibert said. "[I]t's important to get an idea of 'baseline' before addressing the potential impacts of a global change event."
The study revealed that, for tens of millions of years, the ratio of fish to shark fossils was basically constant, at around one shark fossil for every five fish fossils. But then, 19 million years ago, disaster struck. That ratio suddenly fell to less than one shark fossil for every hundred fish, reflecting a drop in shark numbers of more than 90 percent.
Almost everything about this discovery has been a surprise. The period of time in which it occurred was "previously unremarkable", paleobiologists Catalina Pimiento and Nicholas D. Pyenson, who were not involved in the original study, commented in Science. And what's more, they note, the causes of this "wholesale extinction of shark lineages in ... the largest ecosystem on Earth" is something that the researchers have yet to uncover.
“There is no known climatic and/or environmental driver of this extinction, and its cause remains a mystery," notes Sibert and Rubin's paper, published last week in Science. "Modern shark forms ... represent only a minor sliver of what sharks once were.”
In fact, Sibert explained, we don't even have a clear idea of the extent of the catastrophe. The results showed that sharks were hugely impacted by the event, but whether or not other parts of the ocean ecosystem were affected, and if so, how, remains unknown.
And the fact that the numbers and diversity of shark populations still haven’t recovered from the event is just one more riddle for the scientists to grapple with.
"We aren't sure why shark populations and diversity never recovered after the event," Sibert told IFLS. "One hypothesis is that during the aftermath of the extinction, sharks were less well-adapted to the new environmental or ecological conditions than other large marine predators, such as whales – so in the new conditions, other organisms were more rapidly able to diversify and evolve, effectively out-competing the surviving sharks."
"Like most research endeavors, this first paper offers more questions than it can answer," added Rubin.
But the unexpected findings aren’t only important to our understanding of the past – they also hold a stark message for the future. Marine predators such as sharks and whales are dying off at an alarming rate, and this discovery could potentially provide an insight into the impact that will have on other ocean life.
"The current state of declining shark populations is certainly cause for concern," said Rubin. "This [paper] is a vital first step in understanding what repercussions may follow dramatic declines in these top marine predators in modern times."
Just what those repercussions will be is something we may find out for ourselves depressingly soon – but this new finding may shed some light on what to expect, the authors explain.
"[S]harks ... play such a big and important role in marine ecosystems, and when their numbers are decimated, it can flip the ecosystem into an entirely new state, even if it has been stable for tens of millions of years," Sibert warned. "We humans are currently rapidly reducing the populations of most of the large vertebrate predators in today's oceans - I fear that we are racing towards a tipping point, perhaps like the one we observed 19 million years ago."
But for Sibert and Rubin, the future is an exciting one.
"...[T]here's so much more to learn about the earth," Sibert said. "Sharks have been around for more than 400 million years, and yet this event that we didn't even know about, nearly removed them from the oceans, just 19 million years ago. We didn't know, because we hadn't looked."
"There's always going to be more to discover," she added. "What's next?"