We now know exactly when the dinosaurs went extinct

A study on the extinction event titled "The Mesozoic terminated in boreal spring" has been published in the journal Nature.

An international team of researchers has determined that the meteorite that crashed into Earth and wiped out the dinosaurs struck during the boreal (relating to the northern hemisphere) springtime. Around 66 million years ago, the meteorite crashed into Earth, creating the Chicxulub crater on what is now known as the Yucatan peninsula in Mexico, wiping out all non-avian dinosaurs, pterosaurs, ammonites, and most marine reptiles but allowing mammals, birds, crocodiles, and turtles to survive.

The team was able to study fossilized fish, which were buried by enormous waves resulting from the meteorite impact. The fossils were remarkably well preserved, with almost no geochemical degradation of their bones observed. Using the particle accelerator at the European Synchrotron Radiation Facility (ESRF), the team analyzed the bones of the fish.

"Thanks to the ESRF's data, we found that the bones registered seasonal growth, very much like trees do, growing a new layer every year on the outside of the bone," explained Sophie Sanchez of Uppsala University, and visiting scientist at the ESRF.

"The retrieved growth rings not only captured the life histories of the fishes but also recorded the latest Cretaceous seasonality and thus the season in which the catastrophic extinction occurred," said senior author Jeroen van der Lubbe from the Vrije Universiteit Amsterdam.

"In all studied fishes, bone cell density and volumes can be traced over multiple years and they indicate whether it was spring, summer, autumn, or winter. We saw that both cell density and volumes were on the rise but had not yet peaked during the year of death, which implies that growth abruptly stopped [in] spring," said Dennis Voeten, a researcher at Uppsala University.

While organisms began their reproductive cycles in the Northern Hemisphere in the springtime, organisms in the Southern Hemisphere would've been preparing for winter, seeking shelter in caves, and hibernating.

"Our results will help to uncover why most of the dinosaurs died out while birds and early mammals managed to evade extinction," concluded Melanie During, a researcher from Uppsala University, and the VU Amsterdam and lead author of the publication.

You can read more from the study here.