Fossils from the day an asteroid killed dinosaurs

A tangled mass of fish from the recently dug-up deposit in North Dakota's Hell Creek formation.
A tangled mass of fish from the recently dug-up deposit in North Dakota's Hell Creek formation.PHOTO: UNIVERSITY OF KANSAS

WASHINGTON • Sixty-six million years ago, a massive asteroid crashed into a shallow sea near Mexico. The impact carved out a 140km-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 last Friday in Proceedings Of The National Academy Of Sciences, a discovery that has sparked tremendous excitement among palaeontologists.

"You're going back to the day that the dinosaurs died," said Professor Timothy Bralower, a Pennsylvania State University palaeoceanographer who is studying the impact crater and was not involved with this work.

Roughly three in four species perished in what is called the Cretaceous-Paleogene extinction, also known as the K-Pg event or K-T extinction.

The killer asteroid most famously claimed the dinosaurs and hordes of other living things. A lone branch of dinosaurs, the birds, lives on.

The asteroid extinction theory is buttressed by four decades of research, and is widely embraced as the most plausible explanation for the disappearance of dinosaurs.

 
 
 
 

In the late 1970s, Dr Luis Alvarez and Dr 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.

Dr 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, said study author Robert DePalma.

Mr DePalma, a doctoral student at the University of Kansas, began excavating the site at North Dakota's Hell Creek formation in 2013. Since then, he and other palaeontologists have found, among many things, squid-like animals called ammonites, shark teeth and the remains of predatory aquatic lizards called mosasaurs.

At the time of the dinosaurs, the Hell Creek site was a river valley. The river fed into an inland sea that connected the Arctic Ocean to a prehistoric Gulf of Mexico.

After the asteroid struck, seismic waves from a magnitude 10 to 11 earthquake rippled through this sea, according to the study authors. This caused not a tsunami but what is known as seiche waves, the back-and-forth sloshes sometimes seen in miniature in a bathtub.

Seiche waves from the inland sea reached 9m, drowning the river valley in a pulse of water, gravel and sand. The rain of rocks and glass followed. The tektites dug "small funnels in the sediment laid down by the seiche", said study author Jan Smit, a palaeontologist at Vrije University in Amsterdam, "so you know for sure they are coming down when the waves are still running upriver".

This is preservation, in other words, of a fresh hell.

WASHINGTON POST

A version of this article appeared in the print edition of The Sunday Times on March 31, 2019, with the headline 'Fossils from the day an asteroid killed dinosaurs'. Print Edition | Subscribe