When Earth’s temperatures spiked and oxygen levels in the ocean plummeted some 252 million years ago, most life on the planet was doomed. Some …
When Earth’s temperatures spiked and oxygen levels in the ocean plummeted some 252 million years ago, most life on the planet was doomed. Some estimates suggest the extinction event, a mass extinction known as the Great Dying, killed off up to 70% of all land species but marine animals felt the impact even greater. As much as 96% of all species in the world’s oceans perished. But not sharks. The slender prowlers of the deep persevered. “They’re the ultimate survivors,” says Elizabeth Sibert, a paleobiologist and oceanographer at Yale University. The next extinction event, at the end of the Triassic period, couldn’t kill them off and neither could the asteroid that ended the reign of the dinosaurs some 66 million years ago. The longevity of sharks is legendary but they may have come much closer to extinction than we once believed, according to a new study, published in the journal Science on Thursday. The research, led by Sibert, shows that a previously unknown extinction event pushed sharks to the brink 19 million years ago, leaving only about one in 10 sharks in the open ocean alive. “Something happened that knocked out 90% of them overnight,” Sibert says. What that something was remains uncertain, but it was definitely devastating. The asteroid that wiped out the dinosaurs resulted in the extinction of just 30 to 35% of all shark species — this event was two to three times worse. How did Sibert and her co-author Leah Rubin, a doctoral student at the State University of New York College of Environmental Science and Forestry, stumble upon this enigmatic marine die-off? It was a bit of a surprise, involving some deep-sea detective work and a mountain of microfossils. When a marine animal dies, its remains descend into the abyss, decomposing from whole creature to constituent parts. Tiny pieces of the fallen, like the scales present on a shark (denticles) and teeth from fish rain down upon the ocean floor and, over time, pile up in the sediment. “The ocean floor is basically a graveyard for all of the life that’s living anywhere in the water column,” says Sibert. Each denticle or tooth is like a tiny headstone. As they’re embedded into the sediment, they leave a fossilized record of life. These fossils are known as “icthyoliths.” And denticles don’t just get subsumed by the sediment when a shark dies either.
