Some 66 million years ago, on the Yucatán peninsula in Mexico, a 12 kilometre-wide (7.5 mile) asteroid crashes to Earth. The impact causes an explosion whose magnitude is hard to imagine today – several billion times more powerful than the atomic bomb dropped on Hiroshima.
Most of the animals on the American continent are killed immediately. The impact also triggers worldwide tsunamis. Tonnes and tonnes of dust are ejected into the atmosphere, plunging the planet into darkness. This "nuclear winter" causes the extinction of many plant and animal species.
Among the latter, the most emblematic: the dinosaurs. But how were the dinosaurs faring before this cataclysm? This is the question we try to answer in our new study, the results of which have just been published in the scientific journal Nature Communications.
We were interested in six families of dinosaurs, the most representative and the most diversified of the 40 million years that preceded the arrival of the asteroid.
Three of these families were carnivorous: the Tyrannosauridae, the Dromaeosauridae (including the Velociraptor, made famous by the Jurassic Park movies) and the Troodontidae (small dinosaurs similar to birds).
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The other three were herbivores: the Ceratopsidae (represented in particular by the Triceratops), the Hadrosauridae (the richest of all the families in terms of diversity) and the Ankylosauridae (represented in particular by the Ankylosaur, a dinosaur covered in bony armour with a club-like tail).
We knew that all these families had survived until the end of the Cretaceous, marked by the fall of the asteroid. Our goal was to determine at what rate these families diversified – formed new species – or became extinct.
For five years, we compiled all the known information on these families in order to try to work out how many of them there were on Earth at a particular time, and which species were in each group. In palaeontology, each fossil is given a unique number for the sake of traceability, which allows us to follow it through the scientific literature over time.