Other Mammals, Not Dinosaurs, Kept Our Ancestors Down

Dinosaurs so thoroughly dominated the ancient world that they suppressed the evolutionary possibilities open for mammals. This notion has been around for decades now, and it seems to be backed up by the fact that no known mammal became larger than badger-sized during the time dinosaurs were the most prominent animals on land. But, thanks to decades of new fossil discoveries, paleontologists have realized an important twist in the story—it wasn’t dinosaurs that suppressed the evolution of our ancient mammalian ancestors, but other forms of ancient mammal.

Paleontologists who study fossil mammals have suspected the story of “dinosaur dominance” was too simple, but, until now, the critical evidence was missing. Clues that the picture was more complicated now come from a new Current Biology paper by University of Oxford paleontologist Elsa Panciroli and colleagues that followed the evolutionary spread of various mammals during the Age of Dinosaurs—from more ancient types called mammaliaforms to the group of mammals found all around us today, the therians. Because the ancient mammaliaforms diversified and spread into so many forms first, the researchers found, our own mammalian ancestors and relatives didn’t have a chance to truly dig their claws into the world’s ecosystems until after the asteroid strike that sparked a mass extinction 66 million years ago.

“There were tons of different species of mammal in the Mesozoic, so it’s time to move on from this idea that dinosaurs held mammals back,” says Panciroli.

The image of what a Mesozoic mammal is—that is, mammals that lived during the Triassic, Jurassic and Cretaceous Periods—has often been stereotyped as a small, snuffling insectivore that foraged at night. And such shrew-like beasts really did exist during the dinosaurian heyday. Yet they weren’t the only form of mammal around.

Just as dinosaurs thrived, paleontologists have learned, so did mammals—albeit at a smaller scale. The Jurassic Castorocauda had a tail like a beaver’s and ate fish like an otter. The 150-million-year-old Fruitafossor, on the other hand, made a living digging termites and other social insects out of the ground like an aardvark, while the Jurassic Vilevolodon was the mammaliaform equivalent of a sugar glider. In fact, some of these mammals even ate dinosaurs. A fossil of the badger-sized, 125-million-year-old Repenomamus was found with baby dinosaurs in its gut contents.

A little bit of evolutionary orientation helps make sense of the picture, especially because Mesozoic mammals were not a monolith. Around 220 million years ago, a group of weasel-like protomammals called cynodonts spun off a new evolutionary branch. These new creatures were small, covered in fuzz, had good hearing, carried their legs beneath their bodies instead of sprawled out like lizards and had mouths full of different types of teeth that could be used for piercing, nipping and grinding. These new creatures were the mammaliaformes, a broad group that eventually included the ancient equivalents of aardvarks, sugar gliders, squirrels, otters and more.

“I think we overlook the diversity of mammal in the time of dinosaurs in much the same way we overlook their diversity on Earth today,” Panciroli says. The fact that prehistoric mammals don’t have dramatic or common names like some dinosaurs do, Panciroli notes, covers over just how varied and strange they were. “Mesozoic mammals had a large diversity of locomotor modes and dietary preferences,” adds University of Bristol paleontologist Melisa Morales Garcia, who was not involved with the new study.

During the Cretaceous one lineage of mammaliaform spun off the therians. Therian mammals are the most numerous and diverse on Earth today, but their ancestors didn’t just evolve in the Cretaceous and wait for the dinosaurs to go away. The mass extinction that ended the Cretaceous took out numerous mammaliaform species as well as the non-bird dinosaurs, and that shift, paleontologists have found, is what allowed therian mammals to thrive.

In their study, Panciroli and colleagues developed a technique to estimate the capacity for different mammaliaform groups to evolve into new forms. Looking at many of the mammaliaforms from the Age of Dinosaurs, for example, paleontologists have found various species that swam, burrowed, climbed and filled other niches in the ancient habitats. Therian mammals do all these things and more today—but, during the Age of Dinosaurs, they didn’t evolve into the broad diversity of shapes and niches as the mammaliaforms. It was only after the mass extinction of 66 million years ago that therians were able to match what the mammaliaforms had done for millions of years.

It wasn’t the dinosaurs that held the therians back. While the presence of dinosaurs of so many shapes and sizes might have prevented mammaliaforms from attaining large body sizes, Morales Garcia notes, other mammaliaforms had a greater amount of evolutionary influence on therians than dinosaurs.

The rise of rodents is one example. The most successful mammaliaform group of all time was the multituberculates—squirrel-like beasts with chisel-like teeth at the front of their mouths and cheek teeth brimming with cusps. They filled the same niche in the Age of Dinosaurs as many mice and squirrels today. But while some multituberculates survived the end-Cretaceous extinction without missing a beat, the species in Asia virtually disappeared. This left an ecological gap that the earliest rodents started to fill, eventually supplanting the multituberculates. Without the extinction to shake things up, rodents probably wouldn’t have been able to get a toehold in Asia and eventually replace the multituberculates.

Even though dinosaurs had a role to play in keeping prehistoric beasts on the small side, the supposed dominance of the dinosaurs isn’t the whole story. It was other mammals, Panciroli and colleagues suggest, that were more influential in the therian backstory. “Now we have a more complex, but clearer, picture of the diversification of mammals,” Morales Garcia adds.