Mass extinction events are essentially a form of zoological regime change. At the end of the Great Dying 252 million years ago – when over 90 percent of all life on Earth died – the predecessors to the dinosaurs began to diversify into many species, thereby setting in motion the events that led to the rule of the famous reptilian beasts. Similarly, the reign of mammals only began because an asteroid slammed into the Earth 66 million years ago, effectively killing off the non-avian dinosaurs and allowing our ancestors to spread across the world.

A new piece of research, however, is about to rewrite this story. As it turns out, mammals were rapidly diversifying as much as 20 million years before the space-borne apocalypse arrived. Although they survived the impact event and eventually took over much of the world, they were initially severely hampered by it, and many went extinct along with their dinosaurian counterparts.

“While mammals undeniably did far better over the end-Cretaceous Mass Extinction than non-avian dinosaurs, the accepted story that they hit the ground running is not the case,” Elis Newham, a Ph.D. student at the University of Southampton and co-author of the study, told IFLScience. “Mammals… took several million years to recover their ecological diversity to previous levels.”

 

^{One of the early mammals, Purgatorius unio, as depicted by an artist. This little guy lived around 66 million years ago. Nobu Tamura}

In order to determine the diversification rate of these ancient tiny mammals, they looked at fossilized molars taken from hundreds of extinct specimens. Many previous mammalian fossils dating back to the Late Cretaceous period – from 101 to 66 million years ago – had small teeth that were presumably designed for eating insects, and showed little diversity in form.

However, as this team of paleontologists found, the variety of tooth shapes increased dramatically in the period of time leading up to the demise of the non-avian dinosaurs. This means that they were beginning to evolve into many distinct groups at the same time beasts like the Tyrannosaurus rex were appearing.

When the asteroid hit, the more specialized mammalian species with restricted diets died out, but the more generalist critters survived. It is from this lineage that all living mammals on Earth evolved from.

A recent study revealed that the non-avian dinosaurs were on the decline up to 50 million years before they went extinct, with speciation rates far lower than the extinction rates. Several culprits have been suggested, but the authors in this case suspected the rise of opportunistic mammals may have pushed key dinosaurian species out of their ecological niches. This new study, published in the Proceedings of the Royal Society B, only adds credence to this theory.

“There does seem to be a fascinating correlation between our findings of an early therian (the ancestors of placental and marsupial mammals) radiation, and the start of the fall of non-avian dinosaurs,” Newham pointed out. However, this does raise an additional question that hadn’t been previously considered: What caused such an early diversification of mammals in the first place?

“We can’t know for sure, but flowering plants might have offered new seeds and fruits for the mammals,” study co-author David Grossnickle, a Field Museum Fellow and Ph.D. candidate at the University of Chicago, said in a statement. “And, if the plants co-evolved with new insects to pollinate them, the insects could have also been a food source for early mammals.”

 

^{Triceratops, one of the latest dinosaurs to evolve and one that would have been witness to the diversification of mammals before the mass extinction event. Los Angeles Museum of Natural History/Allie_Caulfield/Wikimedia Commons; CC BY-SA 3.0}

Earth is going through another biological regime change right now, of course. Thanks to humanity’s detrimental impact on ecosystems across the world, and our rapid alteration of the climate, species are going extinct at rates comparable to a mass extinction.

As highlighted by this new study, perhaps the more generalist species will be able to make it through this evolutionary bottleneck, and the specialists that dominate large swaths of the planet will, once again, perish.