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 effects of this extinction were likely felt around the world. The consistent results from the two sediment cores — separated by thousands of miles — suggest that this was truly a “global event,” two paleontologists, Catalina Pimiento of the University of Zurich and Nicholas D. Pyenson of the Smithsonian Institution, wrote in a perspective article that accompanied the study in Science.
So far, the cause of this die-off remains unknown. There were no significant climatic changes in the early Miocene, and there’s no evidence of an asteroid impact around that time. “We have no idea,” said Dr. Sibert.