An anonymous reader quotes a report from The New York Times: Researchers believe they’ve now pinpointed a previously unknown planetary-scale reset that occurred about 19 million years ago. This extinction event transpired in the world’s oceans, and decimated shark populations. The boneless fishes still have not recovered from the damage, the team suggests in a paper published Thursday in Science. Scales cover the bodies — and even the eyeballs — of sharks. Known as “dermal denticles,” these scales function like protective armor and their ridges also reduce drag as the animals swim, said Elizabeth C. Sibert, an oceanographer and paleontologist at Yale University. These scales are microscopic — each one is only about the width of a human hair — but sharks slough off about 100 denticles for each tooth they lose, making them common in the fossil record. This abundance makes them valuable to scientists seeking to understand the past, said Paul Harnik, a paleobiologist at Colgate University, not involved in the research. “It’s a sheer numbers game.”
In 2015, Dr. Sibert received a box of mud spanning about 40 million years of history. The reddish clay, extracted from two sediment cores that had been drilled deep into the Pacific Ocean seafloor, contained fish teeth, shark denticles and other marine microfossils. Using a microscope and a very fine paintbrush, Dr. Sibert picked through the two sediments and counted the number of fossils in samples separated in time by several hundred thousand years. About halfway through her data set, Dr. Sibert spotted an abrupt change in the fossil record. Nineteen million years ago, the ratio of shark denticles to fish teeth changed drastically: Samples older than that tended to contain roughly one denticle for every five fish teeth (a ratio of about 20 percent), but more recent samples had ratios closer to 1 percent. That meant that sharks suddenly became much less common, relative to fish, during an era known as the early Miocene, Dr. Sibert concluded. Dr. Sibert and her collaborators, in an earlier study using the same data set, had also found that sharks declined in abundance by roughly 90 percent about 19 million years ago. These declines in relative and absolute shark abundance suggest that something happened to shark populations about 19 million years ago, Dr. Sibert concluded.
But there was still the question of whether a true extinction occurred, she said. “We wanted to know if the sharks went extinct, or if they just became less prominent.” To test the idea of an extinction, Dr. Sibert recruited Leah D. Rubin, a marine scientist then at the College of the Atlantic in Maine. Together, they developed a framework to identify distinct groups of denticles. The researchers settled on 19 denticle traits — such as their shape and the orientation of their ridges. Dr. Sibert and Ms. Rubin sorted roughly 1,300 denticles into 88 groups. These groups don’t correspond exactly to shark species, but seeing more groups is an indicator that a shark population is more diverse, the researchers proposed. Of the 88 denticle groups initially present before 19 million years ago, only nine persisted afterward. The reduction in shark diversity suggests that they experienced an extinction around that time, Dr. Sibert and Ms. Rubin concluded. In fact, this event was probably even more cataclysmic to sharks than the dinosaur-killing asteroid impact that occurred 66 million years ago, they said. “There were just a small fraction that survived into this post-extinction world,” Dr. Sibert said. The researchers have no idea what caused this massive die-off. “There were no significant climatic changes in the early Miocene, and there’s no evidence of an asteroid impact around that time,” the report says.