New research by UW scientists says a group of mammals called "multis" flourished long before dinosaurs went extinct, contradicting previous theories about the rise of mammals.

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The standard story on the origin of mammals holds that our ancient forebears didn’t creep out of the evolutionary shadows until after the demise of dinosaurs. But researchers at the University of Washington have discovered that one class of rodentlike creatures was booming long before an asteroid slammed into the Earth and put an end to the age of giant reptiles.

“If you pull out a textbook, it will basically say that with the extinction of the dinosaurs, the opportunity became available so that mammals could evolve into diverse forms,” said UW paleontologist Greg Wilson, lead author of a study published Wednesday in the journal Nature. “What we’re seeing is that at least in this important group of mammals, extinction of the dinosaurs wasn’t the driver.”

Instead, the flowering of those early mammals closely tracked the rise of modern flowering plants. By analyzing fossil teeth, the researchers concluded that the early mammals quickly adapted and diversified to take advantage of the new sources of food.

The animals Wilson and his colleagues studied have such a difficult name — multituberculates — that even experts call them “multis” for short. They’re also referred to as the lost tribe of mammals, because they died out 35 million years ago and have no living relatives. But during a heyday that lasted nearly 100 million years, multis were the most abundant mammals on Earth.

“Long before there were rodents or rabbits, there were these great multituberculates,” said University of Chicago paleontologist Zhe-Xi Luo, who was not involved in the UW study. “That’s why they are very important for understanding the history of life.”

Scientists know a lot about multis because they flourished on nearly every continent and left behind a rich fossil record that’s mostly teeth, but includes a few specimens so well-preserved they “look like road kill,” Wilson said.

The earliest multis were about the size of mice, and probably lived on a similar diet of bugs and fruit. But a full 20 million years before dinosaurs vanished, multis exploded in diversity, spinning off new species that ranged up to the size of marmots. To understand why, Wilson and colleagues from Finland and Australia peered into the mouths of the long-extinct beasts.

Multis get their name from their back teeth, which are ridged with multiple tubercles, or cusps. The researchers developed a new way to quantify that bumpiness, using laser and CT scans to create the dental equivalent of high-resolution topo maps. They analyzed jaw sections from 41 types of multis. What they found is that as the mammals diversified, their back teeth became bumpier and more complex.

“If you look at the complexity of the teeth, it will tell you about diet,” Wilson explained.

Where carnivores have simple teeth suited to ripping flesh, the teeth of plant eaters are broader and bumpier because they have to chew harder to extract nutrients from vegetation.

So the evolution of bumpier tooth surfaces would have allowed multis to consume a wide variety of modern plants — which were undergoing an evolutionary explosion of their own, Wilson said. And that could explain why the early mammals were able to thrive despite living in a world dominated by dinosaurs.

Luo finds the argument compelling. “It’s really shining a light on this mystery group,” he said.

Oklahoma State University paleontologist Anne Weil, another member of the tiny fraternity of multituberculate experts, is more cautious, noting that the link between the rise of the early mammals and modern plants is still circumstantial. But she’s delighted to see multis featured in one of the most influential science journals, which is sure to inspire interest and follow-up studies.

“They’re really underappreciated,” Weil said.

Early mammals will never have the cachet of dinosaurs, Wilson admitted. “It’s not the sexiest avenue of research, looking under the microscope at these small teeth,” he said. “Most people would rather deal with big bones and skeletons.”

But research on the origins of mammals is a hot topic now and is yielding a more nuanced picture. Early mammals lived side by side with dinosaurs for millions of years, Wilson pointed out. “The age of dinosaurs was not the dark ages of mammals.”

Multis also survived the asteroid impact and resulting ecological mayhem that scientists are convinced triggered the extinction of dinosaurs and many early mammals 65 million years ago. That could be because many multis lived in burrows, which might have provided some protection, Luo said.

What finally appears to have done the multis in was the rise of true rodents, which diversified even more quickly and muscled into the same ecological niches.

“You could say multituberculates were a good match against the dinosaurs,” Luo said, “but they were no match for the rodents.”

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com