There’s a familiar story about the dinosaurs, and it goes something like this: About 65 million years ago, a giant asteroid hit the planet. All the dinosaurs died, and mammals, which were smaller and more adaptable, survived. They went on to inherit the Earth, branching out from early mousish forms into all the wonderful mammalian diversity we see today.
When you hold up that story against the available evidence though, it starts to get pretty leaky. A new study estimates that more than 90 percent of mammal species alive at the end of the Cretaceous were wiped out at the same time that most dinosaur species went extinct. That’s right — the dinosaurs didn’t go completely extinct. Some survived and evolved into the multitude of animals that today we call birds. When viewed from this perspective, the story of dinosaurs and the story of mammals start to look strangely similar.
The authors of this latest study, published in the Journal of Evolutionary Biology, improved upon earlier research by incorporating more data from more locations. The fossil record is inherently imperfect, and determining diversity during an extinction event is particularly challenging, since fossils will necessarily be more scarce.
If you limit your research geographically, you’re bound to miss out on species with narrow ranges that lived somewhere else. These species are also more likely to go extinct, since they are more rare and also less adapted to a variety of environments. For this reason, previous studies have tended to underestimate the overall mammalian extinction rate, the authors show. The fossil record is biased towards survivors because species that are numerous and widespread are both more likely to survive and more likely to show up in the fossil record.
For this study, the researchers merged existing datasets from 23 locations between southern Canada and New Mexico, comprising more than 8,000 specimens. They found that of 59 mammal species present in the data, all but four went extinct. That’s an extinction rate of 93 percent, and presumably, if the dataset were wider, that rate would go up.
The authors also found that mammals were surprisingly resilient — within 300,000 years of the asteroid impact, their diversity was twice what it had been at the end of the Cretaceous. In geological time, that’s really quick. “The success of mammals in the Paleocene seems to stem less from high rates of survival than from their ability to adapt to the aftermath,” the researchers write.
So, survival was uncommon, but those survivors were well equipped to go out and inherit the Earth, evolving over millions of years into species as different as blue whales and bats. Compare that to lizards, turtles, and crocodiles — these animals survived the Cretaceous-Paleogene (K-Pg) extinction event in much greater numbers than the mammals or the dinosaurs. But unlike mammals and avian dinosaurs, they did not go on to take on strange new forms and ecological niches in the years following — they look basically the same today as they did a hundred million years ago.
There was a definite power shift that came with the K-Pg extinction. Before, the dinosaurs were the rulers of the planet, the largest and most diverse group. Mammals were there, but they tended to stay small and out of the dinosaurs’ way. In the Paleogene, the opposite became true. Mammals got bigger, and ruled the land, while the remaining dinosaurs took to the skies.
But both mammals and dinosaurs, on balance, did extraordinarily well in the post-asteroid world. There are about 5,000 known mammal species and about 10,000 known dinosaur (bird) species alive today. Mammals might vary more in size, shape, and biology, but birds are pretty awesome, too.