Shark extinction 19 million years ago remains a mystery

In a new study led by Earth scientists at  Yale University and the College of the Atlantic, researchers have discovered a massive shark extinction that occurred roughly 19 million years ago. 

The experts report that at this time in Earth’s history, there were more than 10 times more sharks patrolling the world’s oceans than there are today. The cause of the massive die-off remains a mystery.

“We happened upon this extinction almost by accident,” said study lead author Elizabeth Sibert, a Hutchinson postdoctoral associate in Yale’s Department of Earth and Planetary Sciences and the Yale Institute for Biospheric Studies. 

“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.” “What we found, though, was this sudden drop-off in shark abundance around 19 million years ago, and we knew we had to investigate further.”

According to Sibert, more than 70 percent of the world’s sharks died off. The death rate was even higher for sharks in the open ocean compared to those in coastal waters. The death toll was twice that of an extinction event that sharks experienced during the Cretaceous-Paleogene mass extinction event 66 million years ago, when three-quarters of all living species were wiped out. 

The circumstances are mysterious, considering that there is no known climate or ecosystem disruption that occurred at the time of the shark die-off.

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

“The current state of declining shark populations is certainly cause for concern and this paper helps put these declines in the context of shark populations through the last 40 million years,” said study co-author Leah Rubin. 

“This context is a vital first step in understanding what repercussions may follow dramatic declines in these top marine predators in modern times.”

The researchers noted that past discoveries of extinction events have led to waves of new research to learn the origins of the die-off and whether it signaled a larger, previously unknown, disturbance in global ecosystems.

For example, further research may confirm whether the shark extinction event caused remaining shark populations to change their habitat preferences to avoid the open ocean, explained the researchers.

Professor Pincelli Hull is an assistant professor of Earth and Planetary Science at Yale, who was not part of the study.

“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 Professor Hull. “It represents a major change in ocean ecosystems at a time that was previously thought to be unremarkable.”

The study is published in the journal Science.

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By Chrissy Sexton, Earth.com Staff Writer