Citation: Sibert, E. C., & Rubin, L. D. (2021). An early Miocene extinction in pelagic sharks. Science, 372(6546), 1105-1107. 10.1126/science.aaz3549
Fossilized remains preserved at the bottom of the ocean have revealed that around 70% of sharks suddenly went extinct about 19 million years ago. The reason why remains unclear, but this enigmatic extinction event provides a warning about the future of sharks in our ocean.
Seas full of sharks
Did you know that sharks have been on our planet for over 400 million years? Although you might find them a little scary, sharks are amazing and diverse creatures that fulfil important roles in the ocean ecosystem.
One way in which scientists study the abundance and diversity of sharks in the past is by examining fossilized shark scales, or denticles. Denticles are found in sediments at the bottom of the ocean and can come in many different shapes and patterns. When scientists find lots of different types of denticles in seafloor sediments, they know that sharks were more diverse at the time when the denticles sank to the bottom of the ocean. Similarly, fewer types of denticles in sediments suggests a lower diversity of sharks. Additionally, the amount of denticles in sediments relates to the number of sharks living in the ocean, with more denticles indicating more sharks.
About 19 million years ago, the diversity and abundance of fossilized denticles dramatically dropped. Prior to 19 million years ago, sediments contained about one shark denticle for every five fish fossils. After that, sediments contained about one shark denticle for every 100 fish fossils. This suggests that shark abundance dropped by over 90%.
What happened to the sharks?
Usually, mass extinctions occur due to rapid environmental change. For example, the dinosaurs went extinct 66 million years ago because of an asteroid impact. However, there is not yet evidence for an event significant enough to cause so many sharks to die. All known climate and fossil records point to relatively stable environmental conditions during this time, so the cause of the sharks’ demise remains a puzzle for scientists to solve.
While we do not know why the mysterious shark extinction event occurred, researchers do know that marine ecosystems have not been the same since. Several million years after so many sharks disappeared, an abundance of fish, seabirds, and whales began to populate open ocean waters. Surviving shark species are still swimming around, but in much smaller numbers than before.
A future extinction?
In the present day, we are seeing alarming declines in shark populations. Unlike the extinction event 19 million years ago, however, the cause it not a mystery. Sharks are suffering due to overfishing and climate change. Regardless of the cause, the past extinction event shows us that loss of sharks can drastically change ocean ecosystems. While we cannot do anything about the shark extinction way back then, we can do our best to help sharks today by advocating for more effective fishing and climate regulations.
I’m a PhD student at URI’s Graduate School of Oceanography, researching how the biological carbon pump influences glacial-interglacial cycles. I use diatom assemblages and diatom-bound nitrogen isotopes to study environmental changes over time. When I’m not thinking about diatoms, I am probably either knitting or hiking!