South Florida Sun-Sentinel (Sunday)
Dinosaur age likely ended in springtime, scientists say
The dinosaur-killing meteor hit in spring.
That is the conclusion of scientists who examined the bones of fish that died on that day when a 6-milewide asteroid collided with Earth.
“These fishes died in spring,” said Melanie During, a graduate student at Uppsala University in Sweden and lead author of a paper published in the journal Nature. “The reign of dinosaurs ended in spring.”
Scientists have known when the meteor hit — just over 66 million years ago, give or take 11,000 years — and where it hit, off the Yucatan Peninsula of Mexico. That ended the Cretaceous period of Earth’s geological history, but even though three-quarters or more of the species of plants and animals disappeared in the mass extinction that followed, it has been hard to pinpoint fossils of anything directly killed by the meteor.
But in 2019, paleontologists published the discovery in southwestern North Dakota of what appeared to be a mass graveyard of creatures that died hours or days after the impact. Although North Dakota was about 2,000 miles from where the meteor hit, the seismic waves of what was the equivalent of an earthquake with a magnitude of 10 or 11 sloshed water out of the lakes and rivers and killed the fish. Tektites — small glass beads propelled into the air by the impact — rained from the skies.
The researchers spent years exploring the site, known as Tanis, which is in the fossil-rich Hell Creek formation that stretches across four states.
With the new results, the fossils now provide insight into the cataclysm that was previously impossible to discern.
Animals in the Northern Hemisphere — some emerging from hibernation or giving birth to young — might have been more vulnerable to extinction.
During first heard about Tanis during a talk in 2017 by Jan Smit, an expert on the dinosaur extinction at Vrije University in Amsterdam, where she was working on a master’s degree. She got in touch with
Robert DePalma, the paleontologist orchestrating the study of Tanis.
In August 2017, During flew to North Dakota and spent 10 days at Tanis excavating fossils of three sturgeon and three paddlefish.
In the lab, the scientists sliced pieces of bone from the lower jaws of the paddlefish and from the pectoral fin spines of the sturgeon. They saw repeating light and dark lines reflecting seasonal changes in the rate of growth. The outermost part of the bones indicated that the fish were becoming more active and growing faster after the end of winter.
Swings in the levels of different types, or isotopes, of carbon in the bones indicated how much plankton was in the water for the fish to eat.
The levels were lower than what they would be during summer’s peak abundance. That added to the “various lines of evidence that we have that these fish perished in spring,” said Jeroen van der Lubbe, a paleo-climatologist at Vrije University and one of the authors of the Nature paper.