Scientists believe they know which year the asteroid that killed all the dinosaurs collided with Earth.

- It is a combination of evidence that allows us to now say when it happened, says Melanie During, researcher at Uppsala University in Sweden.

According to the latest estimate, the asteroid that exterminated 76 percent of all species crashed into the earth 66,043 +/- 0.043 million years ago. There is thus an uncertainty of up to 86,000 years in the calculation. Despite this, researchers now believe they are sure that it happened during the boreal spring, that is, when it is spring in the northern hemisphere.

How come? One wonders. Melanie During, who works at the Center for Evolutionary Biology at Uppsala University and is the first author of the study now presented in the journal Nature, tells of a place in southwestern North Dakota in the United States, where there is an almost unlikely accumulation of fossils in what once was time was a lake.

- The place is called Tanis and there, at Hell Creek, we find lots of fossilized dinosaur teeth, shark teeth and fish. Everything is gathered in one place and most of all these animals died at the same time, just minutes after the impact, she says.

How do you know that?

- You can compare the impact with throwing a bowling ball in a sandbox. Enormous amounts of molten rock and soil were thrown into space, says Melanie During.

Once there, they solidified and formed small glass-like hollow bullets. After between 15 and 30 minutes, the bullets fell to the ground over large areas. Many of the animals that died and fossilized in Tanis were fish.

- These bullets are called tektites. Previous research has shown that they can only be formed in space. On Earth, they become completely flat, due to gravity. If you look at the gills in these fossilized fish, you see round tektites everywhere. They are not found in the digestive system, but only in the gills, which means that the tektites came right at the moment of death, says Melanie During.

Melanie During researches at the Evolutionary Biology Center in Uppsala.

Probably, she says, the fish died from these tektites in combination with the big wave that washed over the area.

- There was no tsunami, but the shock wave was so great that the water in the lake shook back and forth, from one side to the other. The same phenomenon can be seen in swimming pools in connection with earthquakes. The scale was probably more than ten meters high and buried all living things in its path.

Some of the fossilized fish are different types of sturgeon that died 'a lot' minutes after the collision. These fish have the property that they grow seasonally, starting in the spring. Growth then peaks during the summer, when the supply of food is at its greatest, and then begins to decline in the autumn. In winter, when there is a shortage of food, they hardly grow at all.

- It can be seen in the skeleton, much like the annual rings on a tree, says Melanie During.

Based on this knowledge, the researchers then analyzed the relationship between two different types of carbon in the skeleton of a number of fossils.

- But we did not look at carbon-14, which gave rise to the carbon-14 method, but at carbon-12 and -13. Carbon-12 uses the fish to make sugar, while the other carbon variant is built into the skeleton, says Melanie During.

The ratio between these two isotopes thus varies depending on how the fish grows during the year. On one and the same fish, the researchers were able to follow the growth up to seven years back in time.

- All fish stopped growing exactly between the lowest value in the winter and the highest value in the summer, that is, in the spring, she continues.

From the fact that she and her colleagues think they know that the geological age Mesozoic (which contains the periods Triassic, Jurassic and Cretaceous) ended in the spring and not at any other time of the year.

What does it matter?

- This bang has employed researchers for decades. One of all the questions is how come all the dinosaurs died, except those who became birds. The mammals that existed at this time, as well as the crocodiles and turtles, also survived. We do not know why, but the time of year may have mattered. Future research will show that.