June 3 (UPI) -- A new survey of tiny fossils trapped in ancient deep sea sediment layers suggests the world's sharks suffered a massive die-off around 19 million years ago.
Until now, this tremendous loss of shark diversity and abundance was unknown. Scientists have yet to determine a cause for the mysterious die-off -- described Thursday in the journal Science.
"We happened upon this extinction almost by accident," lead author Elizabeth Sibert said in a press release.
"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," said Sibert, a research fellow at the Yale Institute for Biospheric Studies.
Researchers were surprised to find a sudden drop off in the abundance of shark-derived microfossils, including scales and teeth, beginning around 19 million years ago.
Scientists determined shark numbers declined by approximately 70 percent. The losses were most pronounced in the open ocean.
During the course of the die-off, scientists estimate shark losses were twice what they were during the Cretaceous-Paleogene mass extinction event, the catastrophe that wiped out the dinosaurs and killed more than 75 percent of the planet's plant and animal species some 66 million years ago.
Most mysteriously, there is currently no paleoclimate data to suggest the planet or its climate experienced a significant disturbance. And yet, for some reason, sharks perished in great numbers, reshaping marine ecosystems.
"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," Sibert said.
As always, a more complete understanding of how animal populations and ecosystems evolved in the past can provide insights and context for today's dramatic ecological changes.
Though some shark species, like great whites, are doing better than they were a few decades ago, overall, shark populations have been steadily declining over the last century.
The authors of the new paper said they hope the discovery will spark investigations of this particular time period in Earth's history -- investigations that might offer clues to the cause of the massive die-off event.
"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," Pincelli Hull, an assistant professor of Earth and planetary science at Yale, who was not part of the study, said in the release.
"It represents a major change in ocean ecosystems at a time that was previously thought to be unremarkable," Hull said.