New study suggests sharks almost went extinct 19 million years ago

Introduction

When people think of extinctions that reshape Earth’s history, they typically refer to the extinction of the Dinosaurs 65 million years ago. Massive environmental change usually triggers extinctions, such as an asteroid impact or shift in climate. Recently, researchers believe in discovering a previously unknown extinction event from 19 million years ago.

Researchers tell the New York Times that the extinction event occurred in the ocean. They believe the event decimated shark populations. In a paper published Thursday in Science, the team suggests that sharks have yet to recover from the damage.

Scales cover sharks’ bodies, known as “dermal denticles.” The scales act as protective armor that also reduces drag, helping the sharks swim smoothly. Oceanographer at Yale University, Elizabeth C. Sibert, explains that these scales are microscopic. Each scale is roughly the width of a human hair. Yet, the scales are standard in the fossil record, as sharks lose around 100 denticles for every tooth lost.

Therefore, dermal denticles are valuable to scientists who seek to understand sharks’ past.

Study

Dr. Sibert received a box of mud in 2015 with around 40 million years of history. First, researchers drilled the clay from deep into the seafloor of the Pacific Ocean. The box contained fish teeth, shark denticles, and other marine microfossils. Then, using a microscope and paintbrush, Dr. Sibert picked through the sediment, counting the number of fossils and separated them in time by several hundred thousand years.

Halfway through her sample, Sibert noticed an abrupt change in the fossil record. Nineteen million years ago, there was a drastic change in the ratio of shark denticles to fish teeth. Older samples contained about one denticle for every five fish teeth (about 20%). More recent models, however, had ratios closer to 1 percent. Therefore, sharks suddenly became much less common, relative to fish, during the early Miocene era, concludes Dr. Sibert.

An earlier study using the same data set finds that sharks declined in abundance by roughly 90 percent about 19 million years ago. “The sharks almost completely disappear,” claims Dr. Sibert.

While the results suggest that something happened to shark populations about 19 million years ago, the researchers still wonder if an actual extinction event occurred. “We wanted to know if the sharks went extinct or if they just became less prominent,” claims Sibert.

Further Investigation

To test the theory of extinction, Dr. Sibert and Leah D. Rubin, a marine scientist from Maine, developed a framework to identify distinct groups of denticles.

Together, the scientists settle on 19 denticle traits, including their shape and ridge orientation. Next, the team sorted roughly 1,300 denticles into 88 groups. Unfortunately, each group did not correspond to the exact shark species. However, seeing more groups indicates that a shark population is more diverse, the team concludes.

Of the 88 denticles, only nine persisted after the predicted event 19 million years ago. The reduction in denticles suggests that sharks experienced an extinction around that time, concludes Sibert and Rubin. They claim that this event was more detrimental to shark populations than the dinosaur-killing asteroid impact 66 million years ago.

Currently, the cause of the extinction event is unknown. No significant climatic changes are known in the early Miocene. There is also no evidence of another asteroid impact. “We have no idea,” claims Dr. Sibert. “It’s a fascinating Mystery,” adds Rubin.

Sharks have yet to recover from this incident thoroughly. Also, their populations are declining in abundance over recent decades due to overfishing and other human pressures, according to Nicholas Dulvy, a marine conservation biologist at Simon Fraser University. As a result, according to Dulvy, future scientists that pull up sediment cores from the 20th and 21st centuries may see an even larger extinction signal than this one 19 million years ago.

According to Dulvy, “there is a genuine risk that it’s going to look worse than any of the mass extinctions that have ever happened on Earth.”