More than 19 million years ago, the world's open oceans were absolutely teeming with sharks, roughly ten times more than today.

Then, suddenly, these large marine predators almost all disappeared.

This devastating and mysterious mass extinction event was only recently discovered through a series of accidental inquiries, and from the evidence we have so far, it's still not clear what caused the abrupt end to so many species.

"I study microfossil fish teeth and shark scales in deep-sea sediments, and we decided to generate an 85-million-year-long record of fish and shark abundance, just to get a sense of what the normal variability of that population looked like in the long term," explains paleoceanographer Elizabeth Sibert from Yale University.

When the team compared the ratio of ancient shark denticles (tiny V-shaped skin coverings that resemble teeth more than scales) to other fish teeth buried up to 5,700 meters deep in the seafloor, they noticed a clear shift in ocean life occurring roughly 19 million years ago.

Before this time, sediment samples held an abundance of both denticles and teeth, which naturally fall off fish bodies and land on the seafloor. After this point, however, only a third of the samples contained any evidence of shark denticles.

In the early Miocene, between 16 and 20 million years ago, open ocean sediments went from holding one shark fossil per five fish fossils to one shark fossil per 100 fish fossils.

This unexpected drop-off in shark abundance is twice as large as what was found for the Cretaceous-Paleogene extinction event, which saw the demise of three-quarters of all plant and animal life roughly 66 million years ago.

The presence of ancient denticles in ocean sediment is only a proxy for the number of sharks that once swam in our oceans, but the same sudden shift in pattern was observed in multiple regions around the world.

In sediment cores from both the North Pacific and the South Pacific, researchers found evidence for a sharp decline in shark abundance, estimated to be over 90 percent. The diversity of sharks swimming in the world's oceans also took a dive during this time, decreasing by over 70 percent.

After this sudden transformation, which probably occurred in less than 100,000 years, the diversity of sharks in Earth's oceans was never the same.

While many species of open ocean sharks disappeared during this extinction event, coastal sharks were a little luckier. Today's lineages are mostly derived from those survivors.

In sediment samples deposited after the extinction event, researchers found no new types of shark denticles, which suggests few shark species have emerged since.

That's a worrisome discovery, as it could mean sharks have a hard time recovering from abrupt extinction events – and unfortunately, we are now entering another.

Since the 1970s, modern ocean sharks have declined by roughly 70 percent, mainly due to overfishing. As a result, coastal sharks are now functionally extinct in 19 percent of Earth's coral reefs.

Once lost, scientists worry this diversity could be gone forever.

"The parallels between this ongoing crisis and the extinction of pelagic sharks more than 19 million years ago thus feels like déjà vu, except that this time we know that the decline of sharks is happening at a faster rate than at any other in the history of the planet," the authors write.

"Pelagic shark communities never recovered from a mysterious extinction event 19 million years ago; the ecological fate of what remains is now in our hands."

Today, climate change poses an extra risk to sharks, which they didn't face before. In the early Miocene, researchers say the climate was relatively stable.

In fact, at this point, there were no known disruptions to ocean ecosystems, which is why it's such a strange time for so many sharks to die off.

"This interval isn't known for any major changes in Earth's history," says Sibert, "yet it completely transformed the nature of what it means to be a predator living in the open ocean."

Shortly after these ancient sharks disappeared, researchers say tunas, seabirds, beaked whales, and baleen whales began to fill in the gaps left behind, which is possibly why it was so hard for shark numbers to recover – their place in the food web had already been taken.

As apex predators, sharks have profound and complex relationships with marine ecosystems, which means their loss can ripple throughout the food chain, causing irreversible changes to the ecosystem.

Today, some scientists warn the loss of sharks has already left a "gaping, growing hole" in ocean life.

If we can figure out what happened roughly 19 million years ago, it could tell us something about where we are headed now.

"This work could tip-off a race to understand this time period and its implications for not only the rise of modern ecosystems, but the causes of major collapses in shark diversity," says earth and planetary scientist Pincelli Hull from Yale University.

The study was published in Science.