Illustration of an early mammal ancestor. (Nobu Tamura)

For over 100 million years, mammals skittered and scurried through a dinosaur-dominated world. They were small – the largest was no bigger than a badger – and paleontologists used to think that mammal evolution only took off once a mass extinction wiped all the non-avian dinosaurs off the planet. But over the past 20 years a flood of discoveries has shown that mammals thrived and diversified in the heyday of the dinosaurs, and a new study suggests that our ancestors and relatives had a big evolutionary burst millions of years before dinosaurs flew the coop.

While the moment mammals first scampered onto the evolutionary scene is still a point of contention, paleontologists agree that the wee beasties were around by 175 million years ago at the latest. From there they proliferated into various “archaic” lines that included species similar to aardvarks, beavers, and flying squirrels. And from this tangle, the ancestors of today’s marsupials – think kangaroos and possums – and placentals (like you and me) emerged.

These two groups are bound together in a broader category called therian mammals, and a new analysis by paleontologists David Grossnickle and Elis Newham suggests that this line actually had an evolutionary burst 10 to 20 million years before a wayward asteroid brought the Age of Dinosaurs to a close.

The crucial clues were held in teeth. After drawing from the Paleobiology Database to measure diversity – or the number of species – for fossil mammals, Grossnickle and Newham turned to teeth to see how different they were from each other.

“Teeth tend to fossilize and preserve better than most of the skeletal elements,” Grossnickle says, and are the most numerous type of fossil for early mammals. More than that, Grossnickle adds, tooth shape is tied to diet, so “by tracking molar shape diversity through time we are essentially also tracking ecological or functional diversity through time.”

In the 20 to 10 million years before the mass extinction that toppled all dinosaurs except for birds, the researchers found, therian mammals did not have a large number of species – but there were greater differences between each species. The surprise is what happened to mammals right after the extinction.

“At first I thought I was doing something wrong,” Grossnickle says, but after checking and rechecking the data it was apparent that while the number of mammal species got a boost, they were less different from each other than before the catastrophe. “The traditional story is that mammals began to diversify immediately after the extinction of non-avian dinosaurs,” Grossnickle says, “and that’s not quite what we see.”

“This pretty much agrees with what has been coming out in the last few years about the last part of the Cretaceous mammal record,” says paleontologist Jessica Theodor, who wasn't involved in the latest study. It seems likely that mammals actually thrived during their early years. While the idea that dinosaurs suppressed mammals still survives in the popular imagination, Theodor notes that previous work has pointed out that the reign of the dinosaurs didn’t keep mammals less diverse or abundant – just smaller. With those hulking predators gone, they could grow into the ecological niches left behind by the larger creatures. But they did just fine as tiny critters.

The precise causes of the beastly boom and bust are not totally clear just yet, but Grossnickle has some ideas. Flowering plants were taking off at the time therian mammals were proliferating, he says, so new seeds, fruits, and the insects that evolved to feed on these plants may have underwritten the expansion of mammals. On the other side, the loss of differences between mammal species in the extinction might indicate that animals with restricted diets died out while the generalists survived to uphold the mammalian legacy.

For now, though, it’s safe to say that it’s time to abandon the traditional tale of our long-ago ancestors. Mammals were not meek underdogs just waiting for dinosaurs to disappear. Mammals had their own evolutionary explosion – right under the noses of the terrible lizards.

Brian Switek is a freelance science writer and the author of My Beloved Brontosaurus. For more, read his Scientific American blog and follow him on Twitter or Instagram

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