Sixty-six million years ago, a massive asteroid crashed into a shallow sea near Mexico. The impact carved out a 90-mile-wide crater and flung mountains of earth into space. Earthbound debris fell to the planet in droplets of molten rock and glass.
Ancient fish caught glass blobs in their gills as they swam, gape-mouthed, beneath the strange rain. Large, sloshing waves threw animals onto dry land, then more waves buried them in silt. Scientists working in North Dakota recently dug up fossils of these fish: They died within the first minutes or hours after the asteroid hit, according to a paper published Friday in Proceedings of the National Academy of Sciences, a discovery that has sparked tremendous excitement among paleontologists.
"You're going back to the day that the dinosaurs died," said Timothy Bralower, a Pennsylvania State University paleoceanographer who is studying the impact crater and was not involved with this work. "That's what this is. This is the day the dinosaurs died."
Roughly three in four species perished in what's called the Cretaceous-Paleogene extinction, also known as the K-Pg event or K-T extinction. The killer asteroid most famously claimed the dinosaurs. But T rex and Triceratops were joined by hordes of other living things. Freshwater and marine creatures were victims, as were plants and microorganisms, including 93 percent of plankton. (A lone branch of dinosaurs, the birds, lives on.)
Four decades of research buttresses the asteroid extinction theory, widely embraced as the most plausible explanation for disappearing dinosaurs. In the late 1970s, Luis and Walter Alvarez, a father-son scientist duo at the University of California, Berkeley, examined an unusual geologic layer between the Cretaceous and Paleogene periods. The boundary was full of the element iridium, which is rare in Earth's crust, but not in asteroids. Walter Alvarez is one of the authors of the new study.
The Hell Creek fossils represent "the first mass death assemblage of large organisms anyone has found" that sits at the K-Pg boundary, study author Robert DePalma said in a statement.
DePalma, a doctoral student at the University of Kansas, began excavating the site at North Dakota's Hell Creek formation in 2013. Since then, DePalma and other paleontologists have found heaps of fossilized sturgeon and paddlefish with glass spheres still in their gills.
They found squid-like animals called ammonites, shark teeth and the remains of predatory aquatic lizards called mosasaurs. They found dead mammals, insects, trees and a Triceratops. They found foot-long fossil feathers, dinosaur tracks and prehistoric mammal burrows. They found fossilized tree gunk called amber that had captured the glass spheres, too.
The site has "all the trademark signals from the Chicxulub impact," Bralower said, including the glass beads and lots of iridium. In the geologic layer just above the fossil deposit, ferns dominate, the signs of a recovering ecosystem. "It's spellbinding," he said.