Are sharks going extinct? Today, they are considered an endangered species. But scientists studying deep sediment layers from the ocean floor found evidence that sharks may have nearly faced extinction millions of years ago, too.
Analysis of microfossils in core samples revealed a previously unrecognized extinction that wiped out around 90% of sharks 19 million years ago. Pacific ocean sharks — and possibly worldwide open ocean populations of the species — disappeared thanks to a mysterious event.
The study, published in Science, explains that not only did populations crash, but they also became a lot less diverse. With the knowledge of this long-ago event, and with current populations under threat, scientists and conservationists looking into the endangerment of sharks today may be able to glean insight from the past.
Counting Fish Teeth and Shark Scales
Evidence for what happened in our planet’s past usually comes from the fossil record buried in rock or sediment. By looking closely at what’s buried in each layer, scientists can document events chronologically. And that’s what the Yale University researchers were doing when they discovered this mysterious, unexplained extinction event affecting the sharks.
The study involved analyzing deep sediment cores from sites in the North and South Pacific, as Australian ABC reported. These were chosen as they were least likely to have experienced much change caused by ocean floor drift or currents. Cataloging microfossils, called ichthyoliths, in deeper and deeper layers of sediment eventually took them to a point where there was a marked difference in the abundance of shark fossils compared with fish.
The team was looking at the ratio of fish teeth to shark scales in each sediment layer. Since shark skeleton is cartilaginous, it does not calcify to form fossils. However, the scales that form shark skin do form fossils because these denticles contain the mineral bioapatite. They survive extremely well in the red clay sediment cores and allow researchers to not only count them but also examine their morphology.
Deeper, older core samples contained around one dermal denticle from Pacific ocean sharks for every five fish teeth. However, at the 19-million-years mark, the denticles became very scarce, dropping to around one for every 100 fish teeth.
Loss of Individual Sharks and Diversity
Current shark populations are only a fraction of what was teeming in the oceans 19 million years ago — not only in numbers but also in species diversity. In addition to counting denticles and teeth, the scientists also studied the size and shape of the shark scales. They saw that around the time of the dramatic decline in shark numbers, there was also a change in the morphology of the denticles.
The Smithsonian Institute notes that denticles help sharks swim faster. The shape of the scales and their surface features vary according to species. Grouping the scales found during the sediment surveys, the researchers saw that geometric-shaped scales almost vanished while those with surface striations or grooves could still be found. The extinction event seemed very selective for certain Pacific ocean shark species.
Why Are Sharks Going Extinct?
Sharks have been around for 400 million years. Science News notes that sharks survived the asteroid strike that wiped out the dinosaurs 66 million years ago, although between 30% and 40% were lost at the time.
Sharks also survived the Paleocene-Eocene Thermal Maximum that saw a dramatic rise in global carbon dioxide and temperature around 56 million years ago.
However, something happened around 19 million years ago that seriously impacted populations and took place over a time frame of only 100,000 years. LiveScience notes that the study doesn’t show if this happened over a day, a week, a year or 1000s of years. It was a surprising discovery to the research team and the reason for the extinction event still isn’t known.
Today’s sharks are mostly under siege from human activity. A study published in Nature calls out overfishing as the major pressure on Pacific ocean sharks and other populations today. Harvesting them for their fins is impacting many populations around the world.
Although the scientists still don’t know what caused the massive die-off 19 million years ago, the work is an important part of today’s conservation efforts.
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