Sharks have held a presence in the ocean for more than 400 million years. The marine predators are older than the oldest fossil forest, and have survived four mass extinctions. However, when researchers looked at the number of shark scales within sediment cores collected from the seafloor, they found that 19 million years ago, open-ocean shark populations dropped by more than 90 percent, reports Michael Greshko for National Geographic.
The decline in shark numbers is twice as significant as that from the Cretaceous-Paleogene extinction event 66 million years ago, where three-quarters of all life was wiped out, including the dinosaurs, Science Alert's Carly Cassella reports. During that event, sharks lost 30 percent of their population, National Geographic reports. The study was published this week in Science.
Deep-sea sediment cores from the ocean floor are used to detail Earth's history. Each sediment layer acts as a marker to a specific period in time and holds chemicals and fossils researchers use to gauge how life responded to changes over time. In the past, sediment cores have been used to detail and reconstruct how Earth's climate has changed over various epochs, per National Geographic.
Researchers uncovered the extinction event using ichthyoliths, or microscopic fossils of shark scales called denticles, along with fish teeth inside the mud cores, reports Karina Shah for New Scientist. The number of fossils buried in each layer of sediment allowed the researchers to track shark population numbers over millions of years, National Geographic reports. Mud cores were taken from two different locations away from land or ocean currents that could disrupt and move around the fossils to track the global changes. The first core was sampled from the middle of the north Pacific Ocean, and the other was from the middle of the south Pacific Ocean, extracted in 1983, National Geographic reports.
When researchers compared both cores, they found that only the samples from the south Pacific site had fossils from 19 million years ago, reports Yasemin Saplakoglu for Live Science. The north Pacific Ocean core had data from 22 million to 35 million years ago and 11 million to 12 million years ago, but in between those timelines, no fossils were found, per Live Science.
When looking at the early Miocene epoch, researchers found the sediments changed from holding one shark fossil per every five fish fossils to one shark fossil per 100 fish fossils, reports Science Alert. The researchers suggest the event occurred abruptly over 100,000 years, and sharks have not recovered since the drop in population numbers.
Paleoceanographer and first author Elizabeth Sibert from Yale University first detailed these findings in a 2016 study published in the Proceedings of the Royal Society B. However, Sibert was unsure if the extinction event affected only one type of shark or all sharks, per National Geographic.
To find out, Sibert and co-author Leah Rubin, a graduate student at SUNY ESF, looked at the species diversity of the shark scales. They classified 798 denticles from the South Pacific core and 465 denticles from the North Pacific core into 80 different shapes and structures, Live Science reports. Sediment samples less than 19 million years old had only 30 percent of diverse fossil types than older samples had, meaning that the extinction event almost wiped out the Pacific Ocean's open-ocean shark species, National Geographic reports. Some shark species were harder hit than others. While most open ocean sharks disappeared after the event, coastal sharks survived, and today's sharks most likely ascended from the survivors, per Science Alert.
While researchers found evidence that the drop in shark populations occurred, there is no known climate disaster or ecosystem disruption during this time.
"We really, truly don't know what caused the extinction," Sibert tells Live Science. "This paper is just the very beginning of what I hope is going to be a really interesting next decade trying to figure out more about what happened at this time."