Around 66 million years ago, the sky fell on the dinosaurs’ heads. An asteroid smashed into the Yucatan Peninsula, causing cataclysmic climate changes that marked the end of the Cretaceous period, and killed off some three-quarters of animal species. A small proportion of hardy birds survived, but the other dinosaurs went extinct.
They were, however, already in decline.
Manabu Sakamoto from the University of Reading has shown that dinosaur species were going extinct faster than new ones were appearing, for at least 40 million years before the end of the Cretaceous. The dinosaur opera had already been going through a long diminuendo well before the asteroid ushered in its final coda.
Many other researchers had looked at the fates of the dinosaurs before that infamous extinction event and suggested that they were already declining. But most of these studies had simply tabulated raw numbers of species from different blocks of time. This approach has problems: the rocks from certain time periods may simply be better at preserving fossils, or may have been more intensively scrutinized by fossil-hunters.
After a recent attempt to adjust for these biases, using up-to-date information and better statistical techniques, Stephen Brusatte from the University of Edinburgh found “no evidence for a progressive decline in total dinosaur species richness.” Two groups—the horned ceratopsians and the duck-billed hadrosaurs—were fading in both number of species and variation in body shapes, but the others were not. “Recently, the idea that the dinosaurs were reigning strong has dominated the academic debate,” says Sakamoto.