We have long known that the 165-million-year-old success story of the dinosaurs came to an abrupt end 66 million years ago. But now it is also clear in which season the dinosaurs went down.

It was in the spring. Researchers write that in the magazine Scientific Reports.

Become extinct

The dinosaurs had been on the planet for about 165 million years when disaster struck about 66 million years ago. A sizable asteroid entered Earth’s atmosphere and crashed into the Earth’s surface in present-day Yucatan, Mexico. The impact (indirectly) led to one of the largest mass extinctions in the history of our planet (see box).

In recent years, excavations in the huge crater that still reminds of the impact have provided researchers with a fairly good idea of ​​the impact and its direct consequences. In 2019, for example, indications were found that the impact led to tsunamis, earthquakes and forest fires. And evidence was even found that the impact generated sulfate aerosols that blocked out sunlight and, therefore, solar heat, leaving large areas of the Earth significantly colder and darker than usual for long periods of time — on the order of years. And all of that together would have eventually led to the extinction of about 75 percent of life on Earth. The dinosaurs are without a doubt the most famous victims.

Researchers now have a fairly good idea of ​​the consequences of the impact and the way in which it started a mass extinction. However, the timing of the impact was always shrouded in mystery; no one knew in which season disaster struck. And with that, a crucial piece of information was missing, according to researcher Robert DePalma. “The time of year plays an important role in many biological functions. Think, for example, of reproduction, but also of the strategies that animals use to collect food. And, for example, also the interactions between hosts and parasites. “It comes as no surprise, then, that the time of year when a global threat emerges can have a major impact on how much that threat affects life. The question of what season the impact occurred is therefore of crucial importance to the account of the extinction that took place at the end of the Cretaceous.” But researchers couldn’t answer that question until recently. However, DePalma and colleagues are now changing that by convincingly demonstrating that disaster struck in the spring.

Tanis

The researchers rely on research in North Dakota. There we find the so-called Tanis area. Sediments can be found here that date from that crucial period on the border of the Cretaceous and Paleogene, or the period of the impact. Previous research has already shown that the sediments here were deposited by an enormous flood current, caused by the impact. The flood current not only deposited sediments, but also (remnants of) plants, animals and trees. And after DePalma and colleagues showed in 2019 on the basis of all those remains that the sediments date from the period of the impact and we therefore find victims of that impact in those sediments, he now went one step further. He used the fossilized fish bones that were found in these sediments to determine the time of year when they came to an end and to find out in which season the impact took place.

Growth lines

The researchers examined growth lines in the fossilized fish bones. Those growth lines are a bit like the growth rings of a tree; they follow an annually recurring pattern, characterized by periods in which the fish bones alternately expand relatively hard (in summer) and expand relatively slowly (in winter). That pattern can be used to determine how old a fish is. But you can also use it to determine the season in which a fish has died. The researchers have now done the latter. And based on the structure of the growth lines, they can conclude that the fish died during the spring: the period when more food started to become available and the bones expanded just a little faster after a period of slow thickness growth.

Additional research

However, to make sure disaster struck in the spring, the researchers went one step further. They examined the remains of the youngest fish recovered in the Tanis sediments and determined the size of those young fish. Using the growth rate of modern, comparable fish species, they were then able to determine how much time had elapsed between the moment the young fossilized fish were born and the moment they died. And again the researchers came to the conclusion that the fish had died quite shortly after their birth, so still in the spring.

Future

And with that, the question of what time of year the asteroid hit has finally been answered. “It’s great that various pieces of evidence suggest so clearly what time of year it was when an asteroid hit Earth 66 million years ago,” said researcher Anton Oleinik. “One of the things that makes science so great is that we can look at well-known facts and events from a different angle and increase our knowledge and understanding of nature. It proves that we can still make new discoveries in geology and paleontology even in the 21st century.”

And there is certainly more to discover in North Dakota, the researchers think. Research in the area therefore continues unabated and is expected to provide even more insight in the future into the consequences of that one fatal spring day 66 million years ago.