The age of dinosaurs may have ended in our youth

The meteor that killed the dinosaurs hit in the spring.

That’s the conclusion of scientists who examined the bones of fish that died that day when a six-mile wide asteroid collided with Earth.

“These fish die in the spring,” says Melanie While, a PhD student at Uppsala University in Sweden and lead author of a new study. The paper was published Wednesday in the journal Nature. “The period of dinosaur domination ends in the spring.”

Scientists already know when the meteor hit — just over 66 million years ago, or take 11,000 years — and where it hit, off Mexico’s Yucatán Peninsula. That ended the Cretaceous period of Earth’s geological history, but although three-quarters or more of plant and animal species disappeared in the ensuing mass extinction, it’s hard to pinpoint. exactly fossils of anything that directly killed the meteorite.

But in 2019, paleontologists announced 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 is about 2,000 miles from where the meteorite fell, earthquakes the equivalent of a 10 or 11 magnitude earthquake have splashed water out of lakes and rivers and killed fish. Tektites – tiny glass beads pushed into the air by impact – rain from the sky.

Researchers have spent years exploring the site, known as Tanis, in the fossil-rich Hell Creek Formation that spans four states. An article in The New Yorker describes Tanis as a wonderland of fossil finds; The original scientific papers describing the site were sparser in detail, focusing on the geological context.

With new scientific results, the fossils now provide insight into the cataclysm that was previously indistinguishable.

“It’s amazing that we can capture an event, a moment that happened 66 million years ago – literally a rock falling and immediately hitting the Earth – and it’s amazing,” said Stephen. we can pinpoint that event at a particular time of the year. L. Brusatte, a paleontologist at the University of Edinburgh, who was not involved in the study. “I think it’s a detective story of the highest caliber.”

Animals in the Northern Hemisphere – some emerging from hibernation or giving birth – may be more vulnerable to extinction. “If it were spring, then many organisms would not be able to go into hibernation,” Ms. Trong said at a phone news conference arranged by Nature.

The animals of the Southern Hemisphere, hiding in the autumn, may have been more sheltered from the sudden, drastic change in climate. “If you can hibernate, that increases your chances,” Ms. Trong said. “If you can lock yourself in a cave or if you can shelter in the water, that could help.”

Dr. Brusatte agrees. “I think there’s some potential here to help understand patterns and processes of extinction,” he said.

She first heard about Tanis during a 2017 talk by Jan Smit, an expert on dinosaur extinction at Vrije University in Amsterdam, where she is doing her master’s degree.

She was intrigued by his description of fossil finds in North Dakota. “I actually started typing him an email from my phone from the back of the room, saying, ‘Hey, if you have these fish, we can do isotopic analysis on their bones, please. are not? “” said Mrs. Trong.

She contacted Robert DePalma, the paleontologist coordinating research on Tanis. In August 2017, Ms. Trong flew to North Dakota and spent 10 days in Tanis unearthing fossils of six species of fish: three sturgeon and three paddlefish.

In the lab, the scientists cut thin pieces of bone from the lower jaws of paddlefish and from the spines of the pectoral fins of sturgeon. They see repeated light and dark lines that reflect seasonal variations in growth rates, similar to tree belts. The outermost part of the bone shows that the fish is becoming more active and growing faster after the end of winter.

“In April, I guess,” Ms. Trong said. “It’s definitely not summer.”

Fluctuating levels of different types, or isotopes, of carbon in bones indicate how much plankton there is in the water for fish to eat. Lower levels than would be expected during peak summer abundance. Jeroen van der Lubbe, a paleoclimatologist at Vrije University and one of the authors of Nature, said:

Tektites were found trapped in fish gills but not in the digestive tract. Ms. Trong said: “They couldn’t continue to swim. “They instantly died.”

Another team of scientists, led by Mr. DePalma, independently performed the same analysis on fish and fish fossils. reported almost the same conclusions last December in the journal Scientific Reports.