Did dinosaurs need to die for mammals to thrive?

New research bolsters a shift in thought among scientists about the rise of the mammals.

Courtesy of © Nobu Tamura
Artist's rendering of an early therian mammal, Purgatorius unio.

Textbooks tell a simple story about the rise of the mammals: Tiny, rodent-like animals lived boring lives as insect-eaters, scurrying around the feet of the reigning dinosaurs for millions of years. Then, when the non-avian dinosaurs died some 66 million years ago, mammals had the space to thrive. And they took over.

But that's not the whole story, says David Grossnickle.

"Their evolution is a little more independent from dinosaurs' than is commonly thought," Mr. Grossnickle tells The Christian Science Monitor.

Grossnickle, a PhD student at the University of Chicago and a Field Museum Fellow, is the lead author of the latest research paper suggesting that mammals were actually thriving before the end of the Age of the Dinosaurs. 

"This paper, along with some other work in the past decade, has really been throwing a lot of cold water" on the idea that mammals were boring little creatures until the demise of the dinosaurs left an opening for them to take over the landscape, says Stephen Brusatte, a paleontologist and evolutionary biologist at the University of Edinburgh in Scotland, who was not part of the study but is also finding evidence for an early rise of mammals in his own work

"They weren't really big – no mammals living with the dinosaurs as far as we know were bigger than a badger – but these small mammals were doing all sorts of things with their teeth, eating all sorts of different foods," Dr. Brusatte tells the Monitor. "The rise of mammals in a sense started a long time before the dinosaurs died, a long time before the extinction."

In fact, the group of mammals that later yielded our own lineage likely began growing and filling new ecological niches 10 to 20 million years before the dinosaur-killing mass extinction, according to Grossnickle's new paper, published Wednesday in the journal Proceedings of the Royal Society B.

Snacking to success

Grossnickle examined teeth from therians, a group that includes placental and marsupial mammals, that lived leading up to and through that extinction event. 

"If we see a greater and greater diversity of molar shapes, then we're most likely seeing a greater and greater diversity of diets," he explains.

"And that's basically what we see leading up to the boundary," he says, referring to the transition between the Cretaceous period and the subsequent Paleogene period, also known to scientists as the Cretaceous-Tertiary (K-T) boundary or the K-Pg boundary.

Instead of just having teeth that were best at crunching on insects' exoskeletons, mammals began to sprout teeth that could help them munch on meat and plants, too. This meant that the animals were able to take advantage of a broader variety of habitats and grow larger bodies. 

"They're still probably not much bigger than a dog or badger [for scale] right up until the boundary," Grossnickle says, "But some of the largest fossils from the Mesozoic are found right there."

Mammals had been living their small, simple lives for tens of millions of years, so what changed?

Around the same time that mammals began to diversify at the end of the Cretaceous period, flowering plants also exploded in new diversity, says Grossnickle. This likely provided new food sources for the little animals, who would have snacked on the plants and the insects that inhabited them.

Some research suggests some dinosaurs were declining around that time as well. That pattern is very much up for debate, but if it is part of the story, mammals may have begun taking over the ecological niches left by the dwindling dinosaurs, Grossnickle says.

And that leads to the burning question. 

Could mammals have risen to dominance even without the end-Cretaceous mass extinction event?

"That's pretty speculative," Grossnickle says. 

"I hate to say strongly that yes, we would be here today if there wasn't the mass extinction, but I think [mammals] would have been doing pretty well," he says. 

Brusatte is even more skeptical. "I still think that extinction was the critical moment," he says, "that allowed mammals to wrestle control from the dinosaurs."

"If there were no asteroid," he says, referring to the humongous space rock that slammed into the Earth 66 million years ago and is thought to have triggered the dinosaur-destroying mass extinction, we'd probably have "dinosaurs everywhere" today, but mammals, too. "Mammals would still be quite small, but doing a lot in that small-bodied world." 

Or, as Ross MacPhee, curator in the department of mammology at the American Museum of Natural History, who was also not part of this study, tells the Monitor, "Diversification was jet-propelled after the K-Pg extinctions, but there was a long, subtle lead-up."

Debunking dinosaur domination?

As for the question of whether or not the dinosaurs' reign was suppressing the mammals, Roger Close, a quantitative palaeobiologist at the University of Birmingham in Britain who was not part of this study, suggests a closer look at what happened after the Cretaceous-Paleogene boundary, 66 million years ago. 

"In order to draw concrete conclusions about whether mammals were suppressed ecologically by dinosaurs, you would have to expand the time periods you were looking at to quite a few million years after the last period that they look at in the study," Dr. Close tells the Monitor. It might take 10 or 15 million years for mammals to expand their morphological diversity after the mass extinction event, he says: that extinction likely wiped out a substantial number of mammal species, too, and they would have needed time to recover.

Close also advises caution when extrapolating trends like this from the fossil record. Some sections of time leading up to the K-Pg boundary are rich with fossils, while others are spotty at best.

Dr. MacPhee agrees. "The terrestrial record is extremely patchy," before the K-Pg boundary, he says. "It is essentially limited to western North America and various point sources in Eurasia, with little to no evidence of what was going on in more southerly parts of the world. The authors acknowledge all this, but the data gap is there." 

Still, says Close, this study "underscores the diversity that we didn't fully appreciate in this group."

J. David Archibald, a professor emeritus of biology and curator of mammals at San Diego State University who also wasn't part of this study, says this research adds to other work suggesting mammals weren't doing so poorly before the K-Pg boundary. 

"It fits basically what I and others had argued, but we didn't look at ecomorphological differences," Dr. Archibald tells the Monitor. "I think their data is much more rigorous."

How to survive an extinction

"The K-Pg boundary is the last major mass extinction before the one that we're probably in now," Grossnickle says. "Looking at before and after the boundary and seeing what survived and what didn't may give us some clues to the future as to what mammals on Earth are going to be best adapted for survival in the next few hundred years or few thousand years."

Grossnickle's analysis finds that leading up to the K-Pg boundary, mammals had increasingly different body sizes and shapes, and diets. But right after the mass extinction, the animals that survived and diversified were much more similar, although they were different species. 

His interpretation: Animals that ate more general diets tended to survive, and those with more specialized ones struggled to survive. 

"It kind of tells us what happens after a mass extinction," Archibald says. "Things, in terms of ecology, become rather humdrum," at least for a little while.

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