Millions of years ago, there were more than ten times as many sharks as there are now in the world's oceans. However, a mysterious incident nearly caused sharks to share the same fate as dinosaurs 19 million years ago.
The sharks we know of today are only a fraction of the open-ocean sharks that existed before a mystery extinction event nearly wiped them out. For the time being, scientists don't know what triggered the event, but they believe they've identified a previously unknown reset that occurred around 19 million years ago, according to a paper published in Science.
This extinction event shook the world’s oceans, decimating shark populations, and researchers say they are yet to fully recover from the damage.
This mysterious event killed more than 70 percent of the sharks, and the death toll for sharks in the open ocean was even higher than those in coastal waters. To put it into perspective, the shark extinction was twice as severe as what was experienced by sharks during the Cretaceous-Paleogene mass extinction event, which wiped out three-quarters of all the plant and animal species on Earth 66 million years ago.
Apparently, the discovery of the extinction was "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 and lead author of the study.
"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," she said in a press release. "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."
To see whether there was an extinction, the researchers created a framework for identifying various groups of denticles based on 19 denticle features such as shape and ridge orientation. They classified about 1,300 denticles into 88 groups, which don't directly mean shark species, but rather more groups indicating shark population is more diversified.
They found that only nine of the 88 denticle groups that existed before 19 million years ago survived. The decrease in diversity suggests that they experienced an extinction around that period, according to the researchers.
But what was the reason?
Well, there wasn't much to look at since the time this event occurred isn't known for any climate calamities or ecosystem disruptions. There’s no evidence of an asteroid impact around that time either. "This interval isn't known for any major changes in Earth's history," explained Sibert, "yet it completely transformed the nature of what it means to be a predator living in the open ocean."
Scientists don't know much about the cause back then; however, they do know that "the current state of declining shark populations is certainly a cause for concern," says co-author Leah Rubin, an incoming doctoral student at the State University of New York College of Environmental Science and Forestry. Shark populations never fully recovered, and their populations are now declining because of overfishing and other human-caused pressures.
"This paper helps put these decline in the context of shark populations through the last 40 million years. This context is a vital first step in understanding what repercussions may follow dramatic declines in these top marine predators in modern times."