An Ancient Mammal Paves the Way for Modern Rodents

Paleontologists identify an ancient mammal species and ancestor to modern animals like rats and mice

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Illustration by April Isch / University of Chicago

Like most early nocturnal mammals, the Rugosodon was active at night. This reconstruction shows the Rugosodon searching for food among ferns and cycads on the lakeshores in the darkness

On the geological timescale, humans’ stay on the planet earth amounts to little more than the blink of an eye. Before we emerged, earlier creatures enjoyed golden ages for more than twice as long as primates have existed, and hundreds of times longer than the paltry few hundred thousand years Homo sapiens have walked the earth. One example is the order of small, furry mammals known as multituberculates, which were as common as rodents are now for roughly 130 million years — including much of the age of dinosaurs. They disappeared 35 million years ago, leaving no living descendants — the longest run of any mammalian lineage ever. Their species ranged from mouse- to beaver-size, and could be found digging burrows, scampering along the ground or scaling trees like squirrels.

Though bits and pieces of their fossilized remains have been discovered around the world, intact skeletons of early multituberculates are rare. So four years ago, when scientists at the Chinese Academy of Geological Sciences received a multituberculate fossil from a local who had unearthed it at the Tiaojishan fossil bed, it was cause for excitement — especially when, after examining the sediment layers the fossil came from, they realized it was a very old one. The scientists dated it to 160 million years ago, making it one of the earliest multituberculates ever, and in a report published this week in Science, they reveal key details of the life of a long-extinct creature and its descendants.

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The most notable discoveries relate to the teeth and ankles of the mouse-size animal, a previously unknown species of multituberculate the team has named Rugosodon. They managed to remove a loose tooth from the rock; after taking detailed images using a scanning electron microscope, the researchers found that the tooth has a ridged surface similar to that of omnivorous modern mammals like the African dormouse. That evidence suggests that this creature ate both plants and insects. “This helps us to bridge a gap — the later herbivorous multituberculates really went through a stage of omnivory,” says Zhe-Xi Luo, a professor of vertebrate paleontology at University of Chicago and an author of the Science paper. “The vast majority of early mammals are mostly insectivores … and the latest multituberculates are perfectly good herbivores.”

Examining Rugosodon’s limbs, though, they made an even more surprising discovery. Although its bone structure made it clear that the animal ran along the ground — a behavior that usually requires stiff ankles like our own — Rugosodon was incredibly flexible, capable of rotating 180 degrees. They were ankles you’d sooner expect to see in a tree-climbing species, where suppleness is a boon — see modern squirrels, which have similar flexible ankles. Luo says seeing flexible ankles in this very early specimen suggests that the feature may have been common to all multituberculates.

The discovery of such a well-preserved, early multituberculate skeleton is thrilling, says Richard Cifelli, a professor of vertebrate paleontology at University of Oklahoma. “Most of the fossils of multis, and for that matter most mammals from the age of dinosaurs, are known from jaw fragments and teeth,” he says. “In fact, my own specialty, by default, has ended up being working mostly with isolated teeth. I kid you not, I have established new species on not only single teeth, but teeth that are incomplete. So this is really something.”

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These particular fossil beds are likely home to many more specimens dating to the early history of mammals. Last week, two papers in Nature — one of them co-authored by Luo — described two mammal fossils found in the Tiaojishan beds, which are in the same geographic area as those that yielded China’s famous feathered dinosaurs. “To many paleontologists, the so-called middle Jurassic, about the time period of this formation, is the sweet spot on the fossil record,” says Cifelli, who compares Tiaojishan to the Burgess Shale, a famous fossil-hunting spot in western Canada. “This is when you’re first getting explosive diversification of mammals, and that is demonstrated in spades by the stuff that’s coming out of this formation.”

But for all their bounty, these beds in China provide but the merest peephole into what was living on the planet more than 100 million years ago. There are many unanswered questions: Did this furry, mouselike creature run along the shores of the Sundance Sea in what is now North America? Which of the conifers and ferns that covered the planet before flowering plants evolved did it eat? And why, after surviving for so long, did they finally die out? Until paleontologists find more fossils, our imaginations will have to suffice.