June 15 , 2016

by Shaena Montanari

It’s a story that is essentially paleontological folklore by now—dinosaurs dominated the Cretaceous landscape to such a degree that mammals could never get a foothold. That is, until an asteroid struck and evened out the playing field for everyone, dethroning the dinosaurs and allowing the mammals to begin their rise. Although this is a neat and tidy tale, new research out today in Proceedings of the Royal Society B by University of Chicago paleontologist David Grossnickle and University of Southampton paleobiologist Elis Newham may prove there is more than meets the eye when considering the rise of early mammals.

The group of mammals that contains placental mammals (like us) and marsupials is called Theria. The rise of therians is often assumed to have been suppressed by competition with larger dinosaurs before 66 million years ago. Mammals were seemingly small before the Cretaceous-Paleogene extinction event, and not so diverse taxonomically. But when paleontologists began to use more quantitative approaches to examine the size and shape of early mammal fossils, especially their teeth, the story began to shift.

“The traditional view is that mammals were suppressed by the dinosaurs’ success, and that they didn’t really take off until after the dinosaurs went extinct. This study shows that therian mammals, the ancestors of most modern mammals, were already diversifying before the dinosaurs died out,” said lead author David Grossnickle. He found that when closely analyzing diversity patterns and tooth shape of early mammals from the early Cretaceous to the middle Paleocene, the idea that dinosaurs “suppressed” mammals did not seem to be true.

Looking at morphological differences between teeth from a wide variety of extinct mammals, Grossnickle and Newham were able to show that there was increasing morphological diversity 10-20 million years before the big extinction event. Disperate tooth shapes means these mammals also had a wide variety of diets. Instead of a huge explosion in diversity of dietary habits after the extinction, they found the opposite—mammals were on the rise, then the extinction event appeared to selectively eliminate some mammals from the fossil record, perhaps due to the fact they were dietary specialists that lost their food source.

Dietary generalists seemed to have the edge for making it through the extinction event. Although this is essentially what many paleontologists believed all along, now it has been shown quantitatively—but the increase in morphological differences between the molars of mammal groups before the boundary was unexpected. It is still unclear what might explain this trend: “We can’t know for sure, but flowering plants might have offered new seeds and fruits for the mammals. And, if the plants co-evolved with new insects to pollinate them, the insects could have also been a food source for early mammals,” he says.

Mammals clearly went on to recover from this extinction event and were able to explode in taxonomic diversity shortly after the asteroid rammed the planet. Understanding how diets and ecology impact mammal survival in the face of extinction is extremely relevant, especially today. “The types of survivors that made it across the mass extinction 66 million years ago, mostly generalists, might be indicative of what will survive in the next hundred years, the next thousand.”