Did the Dinosaurs Die on a Pleasant North Dakota Spring Day?

The Tanis River was full of activity that day. It traveled through the forest and wetlands on its way to the Western Interior Seaway, a shallow sea several miles away that covered much of what is now North America. The full heat of summer had not yet arrived, but the winter chill was a distant memory. Plants were budding or starting to bloom, and turtles rested along the river banks, taking what warmth they could from the sun. The river had animals similar to today's paddlefish and sturgeon. They foraged for plankton along the riverbed with their long snouts, rostrums, and whisker-like barbels. The fish were plump and healthy, despite the fact that the Plankton numbers wouldn't peak until summer. 66 million years ago, life was good. Researchers behind a new paper say it wasn't until then.

Melanie During said in a recent press call that she and her colleagues believe it happened 2,000 miles to the south on that spring day. A city-sized meteor slammed into Earth, hitting Mexico's Yucatn Peninsula. A cascade of cataclysmic events occurred in seconds and minutes, including the ejection of particles of molten rock so far into space that some may have even circled the Moon. The impact spherules began to fall. They rained back down on the planet, onto the trees around Tanis River and into the water.

The fish were dead less than an hour after the meteorite hit the crater. A wall of water kicked up by the impact, like a wave in a swimming pool during an earthquake, collected the bodies of the fish, turtles, other animals, trees, and more. They were buried by a layer of metal iridium that can be found around the world, like a funeral shroud over evidence of the end-Cretaceous event.

It is like a car crash frozen in place at the site known as Tanis, in North Dakota.

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The sequence of events that took place in the middle of spring in the Northern Hemisphere were noted by her colleagues in a paper published in Nature. The gills of the fish show that they were buried alive, according to Jeroen van der Lubbe. Not everyone in the field agrees with the new paper.

Paleontologists not involved in the research applaud the team's approach and general conclusions about what killed the fish, but also voiced concern that it was built on the assumption that the material from the Tanis bone bed was deposited in the immediate wake of the end. The K-Pg boundary ended with a bang in the Paleogene period, but other researchers are less certain. They don't question the mass death and seiche that piled the bodies, but believe it could have been the result of a smaller catastrophe. Their concern is that they rest on a paper that has not been fully accepted by the field.

The fish died in the spring. The authors of the paper did a good job, says Tyler Lyson, a vertebrate paleontologist at the Denver Museum of Nature and Science. He is familiar with the new paper, but he is not part of it. The Hell Creek Formation is a fossil-rich area that spans the intersection of the Dakotas, Montana, and Wyoming and includes layers from the K-Pg boundary.

The Tanis site is part of Hell Creek, but the new paper assumes the Tanis bone bed is from the K-Pg kill layer.

The fish and other animals may have been killed in a regional event not related to the Chicxulub impact. Either the fish died in the spring or when the asteroid hit. There are two very different conclusions.

The new research is not the first to suggest doom came in the Northern Hemisphere springtime.

Van der Lubbe said the two teams arrived at their conclusions independently, and that his team's work is unique because it includes analysis of osteocytes, tiny pores or cells within the fish bones, that reveal additional information about the animals. The fish from the Tanis site were in good health and did not suffer from famine or a dry spell before they died.

A paper published in the Proceedings of the National Academy of Sciences claimed that Tanis was the site of the immediate effects of the Chicxulub. The young researcher's apparent reluctance to open Tanis to more scrutiny by other teams heightened the skepticism of the claims.

They failed to find a single impact spherule, the tiny, telltale tektite upon which much of the mass extinction was due, when they examined other blocks of fish taken from the Tanis site before DePalma began work there a decade ago. The initial paper is a really good one, but everything about this locality is just weird. I would like to see more evidence that this is the K-Pg.

He understands that the core Tanis team wants to protect their own research interests, but that it would be nice for other scientists to go to this locality and have a look and collect their own samples. We all have different eyes because we have seen different things from different K-Pg boundaries around the world.

He says that it is independent verification and the way science works.

There is a suggestion in the new paper that the event in the Northern Hemisphere would be out and about.

The pattern of the species around the world that survived the mass extinction has nothing to do with seasonality. Smaller animals with slower metabolisms, which lived underground or underwater, seemed to have the best chance of surviving. The animals were protected from the initial impact. They were able to survive a period of harsh cold and limited resources because of their lower energy needs.

More researchers will be able to access the Tanis site and conduct their own analysis to verify the work done there by DePalma and others. I think it is really cool if it is a snapshot of the K-Pg boundary. It's amazing to think that we could know what season the extinction was in.

It is a critical interval of time, and if it hadn't been for that day, we wouldn't be here.