Sharks are incredibly resilient predators that have been prowling our oceans for 450 million years. Their evolution has come a long way and they have survived some incredibly stressful periods in Earth’s history. Outliving the dinosaurs and populating our modern oceans with over 3000 different and unique species, sharks are one of the most evolutionarily successful creatures on earth. 

But a recent study of shark fossils and teeth samples has shown that they used to be far more abundant, both in terms of numbers and diversity in species. Researchers from Yale University and the College of the Atlantic have attributed this to a massive extinction event 19 million years ago. It came at a period in history when there were more than 10 times more sharks patrolling the world’s oceans than there are today.

“We happened upon this extinction almost by accident,” said Elizabeth Sibert, a Hutchinson postdoctoral associate in Yale’s Department of Earth and Planetary Sciences and the Yale Institute for Biospheric Studies in a Yale News press release. She is the lead author of the new study, which appears in the journal Science.

This study of microfossil fish teeth and shark scales in deep-sea sediments revealed to the team that current shark diversity is a small remnant of a much larger array of forms that were decimated by a previously unidentified major ocean extinction event. What is more surprising is that there are no known records of a major climatic/environmental event that could have triggered this massive die-off. Previous extinction events, like the extinction, that sharks experienced during the Cretaceous-Paleogene mass extinction event 66 million years ago (that wiped out three-quarters of the plant and animal species on Earth) have already been studied and their causes have been determined.

The extinction led to a reduction in shark diversity by more than 70 percent and an almost 90  percent loss in the total number of sharks in the sea. The team tracked the evolution of modern day sharks and came to the conclusion that modern sharks began to diversify within 2 to 5 million years after this massive extinction. The shark species that inhabit our oceans now represent just a minor sliver of what sharks once were.

In the last century, shark populations have been decimated because of overfishing and human greed. Even though most large shark species are on the no fishing list, smaller sharks are a part of the human diet across most countries. But this event, in the early Miocene, was catastrophic for sharks from which they never recovered, according to the team. from which they never recovered. This abrupt extinction occurred independently from any known global climate event and ~2 million to 5 million years before diversifications in the highly migratory, large-bodied predators that dominate pelagic ecosystems today, indicating that the early Miocene was a period of rapid, transformative change for open-ocean ecosystems.

For now, researchers don’t know the cause of the shark die-off. The team happened upon this extinction almost by accident according to Elizabeth Sibert, a Hutchinson postdoctoral associate in Yale’s Department of Earth and Planetary Sciences and the Yale Institute for Biospheric Studies. She is lead author of the new study, which appears in the journal Science.

Impact of findings

“The current state of declining shark populations is certainly a cause for concern and this paper helps put these declines in the context of shark populations through the last 40 million years,” Rubin said. “This context is a vital first step in understanding what repercussions may follow dramatic declines in these top marine predators in modern times.”

According to the researchers, the study of this mass extinction event can help us understand the changes sharks had to undergo to adapt to new conditions, repopulate favorable areas and how they adapted to life in the open ocean and reef systems close to the coasts. Additional research might also help to explain why shark populations did not rebound after the die-off 19 million years ago.

Adding to the mystery is the fact that there is no known climate calamity or ecosystem disruption that occurred at the time of the steep drop in shark populations. “This interval isn’t known for any major changes in Earth’s history,” said Sibert, “yet it completely transformed the nature of what it means to be a predator living in the open ocean.”

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,” said Pincelli Hull, an assistant professor of Earth and planetary science at Yale, who was not part of the study. “It represents a major change in ocean ecosystems at a time that was previously thought to be unremarkable.

Modern-day sharks might have to adapt to drastic changes in the near future because of changes in global temperatures. There is a lot of evidence that shows a range of aquatic and terrestrial creatures suffering from climate change and extinction because of drastic changes in the ecosystem they inhabit. Other mass extinction events, like the one discovered here, could provide clues on how we could establish a better, more favorable planet for all animals that call it home.