Is it possible to know in which month the asteroid that killed the dinosaurs fell?

Dinosaurs are known to have been wiped out by an asteroid impact on Earth about 66 million years ago, in what became known as the Cretaceous-Paleogene extinction event. However, the time of year when this occurred is a mystery and often generates debate among paleontology enthusiasts.

Artist's 3D illustration shows a scene of the mass destruction of dinosaurs that took place 66 million years ago. A new study pinpoints the month in which this may have happened. Image: Limbitech – Shutterstock

According to a recent study, published in the scientific journal Nature, which builds on previous evidence, it reinforces the idea that dinosaurs probably died in June.

The latest evidence comes from an archaeological site called Tanis, located in the Hell Creek Formation in North Dakota, USA. Tanis is one of several geological sites around the world where scientists have observed succession of sediments bordering the Cretaceous-Paleogene boundary.

As the study highlights, impressive fossils of dinosaurs, primitive mammals, fish and plants have already been found in Tanis. Many of these fossils are exceptionally well preserved, with some showing remains of soft tissue such as skin, and could offer valuable scientific clues.

Fish bones point to time of year when dinosaurs were decimated

First identified in 2008, Tanis has been the focus of paleontologist Robert DePalma's fieldwork ever since. In a 2019 paper, DePalma and his colleagues argued that Tanis captured the moment of the asteroid's impact, due to three factors.

The first was the presence of dinosaur fossils that occurred in the Cretaceous sediments up to the Cretaceous-Paleogene boundary, and exactly at the time of impact. The second factor was a layer of molten spherules: tiny glass balls that had condensed on the rock. When the asteroid hit Earth in the Yucatan Peninsula region of Mexico, it scattered debris and melted these elements for thousands of kilometers.

Researchers compared a live sturgeon with sturgeon fossils from the time of the dinosaurs. Image: Viacheslav Lopatin – Shutterstock

Finally, the third factor was the evidence of “seiche waves” in deep channels. The Tanis site is pretty inland today, but at the end of the Cretaceous period it was located on the western inland coast that divided North America at that time, with sea level about 200 meters higher than it is today. The site was estuarine, which means that fresh and salt waters mixed together.

The seiche waves were generated by the impact that triggered seismic waves that shook the Earth and caused water to flow in and out of the river channels at an accelerated rate at a given moment after the impact.

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In addition to melts within the fossil rocks, the researchers found spherules in abundant amounts in the gills of some of the fish they examined. One can imagine that as they sank into the violently swaying waters of the river channel, they could have swallowed molten spherules from above.

In December 2021, DePalma and his colleagues published an important paper on the timing of the Cretaceous-Paleogene extinction event. In that study, they looked at some of the exceptionally well-preserved fish bones, looking at how the cycle of seasons, from summer to winter, was documented in the structure and chemistry of the bones.

Comparing a live sturgeon with sturgeon fossils from Tanis, they found that in a fin spine, regular layers on a millimeter scale show that the fish died when it was seven years old.

The growth rings confirm that the fish alternated between fresh water in the summer months and saline water in the winter. In this and other specimens analyzed in the same study, the last growth increment corresponds to the transition from spring to summer.

Together, these data suggest that the meteorite hit Earth in May or June, the height of spring and summer in the northern hemisphere.

Water lilies give clues that confirm the thesis

Importantly, these findings confirm earlier evidence based on fossil plants, which suggested that the extinction event took place in early June.

Paleobotanist Jack Wolfe identified a location in Wyoming that showed the meteorite's effect on a freshwater lake. At the point of impact, the lake froze over, preserving fossil plants in astonishing detail.

By comparing fossil plants with modern-day water lilies, he showed that the last Cretaceous water lilies in the lake stopped growing at a point in their production trajectory of summer leaves, flowers and fruit that indicated freezing in early June. .

So, based on this information, it is possible to know that the great mass extinction that ended the dinosaurs on Earth was an event that occurred in the middle of the year.

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