Most Mammals Killed In Dinosaur Extinction

JJ Harrison via wikimedia commons. Were it not for the Chicxulub impact, dinosaurs might still exist, and mammals might look like this

The asteroid that wiped out the dinosaurs also killed off the majority of mammalian species, to such an extent that our entire class may have been lucky to survive the impact and the events that followed.

The popular perception of the Cretaceous-Paleogene extinction event is that while it was bad news for the dinosaurs, it provided the opportunity for oppressed mammals to take over the planet. In Zookeys, however, a team led by Thomas Williamson of the New Mexico Museum of Natural History and Science revealed that many mammals didn't do so well either. 

The authors found that more than 90% of the mammal species in the USA's northern Great Plains disappeared from the fossil record immediately after the asteroid struck. More broadly, two-thirds of North America's mammal species vanished at that time. The authors used recently improved understandings of the relationships between species of the era to enhance our grasp of what happened to the species of the day. The northern Great Plains was chosen as it represents the richest store of late Cretaceous mammalian fossils. 

The mammals that nearly died out were metatherians, a clade that includes modern marsupials. Their descendents lived in the southern hemisphere until geological events caused steep declines outside of Australia.

Research in the area is challenging because, as the authors note, “The dataset consists only of dental characters, because the majority of Cretaceous-Paleocene metatherians are represented solely by teeth.”

“This is a new twist on a classic story,” Williamson said. “It wasn't only that dinosaurs died out, providing an opportunity for mammals to reign, but that many types of mammals, such as most metatherians, died out too - this allowed advanced placental mammals to rise to dominance." Indeed, only one metatherian species has been found in North America in the first part of the Paleocene, and even they were soon surpassed by placental mammals.

Co-author Dr. Steve Brusatte of the University of Edinburgh added, "The classic tale is that dinosaurs died out and mammals, which had been waiting in the wings for over 100 million years, then finally had their chance. But our study shows that many mammals came perilously close to extinction. If a few lucky species didn't make it through, then mammals may have gone the way of the dinosaurs and we wouldn't be here."

It is not yet clear what it was about the placental mammals that made them more suited to the post-apocalyptic environment after the asteroid strike, or if they just got lucky.

Dr. Thomas Williamson. Asiatherium reshetovi is one of the few metatherian species with well-preserved fossils from the Cretaceous era.

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