Beaver Fossil Sheds Light on Mammal Evolution

A beaver fossil found in western Montana is the oldest such specimen ever discovered. Find out how this 30-million-year-old missing link is helping to explain the astonishing diversity in mammal species on our planet.

Like all Canadians, I think of the beaver as an important symbol. It’s our national mascot, and the fur trade involving the beaver is a vital turning point in our country’s history.

Sadly, that fur trade eventually hunted Canadian beavers almost to extinction. Just a generation ago, my parents never saw beavers when they were growing up on their family farms.

During my lifetime, beavers have made a remarkable comeback. Nowadays, at our cottage, the Crane River is chock full of beaver lodges just upstream from Lake Scugog.

Beavers Are Fascinating Animals for Several Reasons

Beavers are fascinating animals for several reasons. They’re amphibious, with specialized, flat tails for swimming. 

They consciously engineer their own habitats by building dams as well as homes for themselves. Averaging fifty pounds in size, they’re enormous for a rodent, second in size only to South America’s capybara.

Until recently, biologists believed that beavers first appeared in Eurasia about 23 million years ago. The oldest known beaver fossil came from France and it lived around that time.

Discovered Fossilized Remains in Western Montana

This week, the journal Royal Society Open Science has published a new study overturning those beliefs. A team of scientists discovered the fossilized remains of a beaver’s teeth and bones in western Montana.

This newly discovered fossil is 7 million years older than the one from France. Its beaver teeth make it unmistakable.

That makes it the oldest beaver fossil ever found. It shows that beavers were already living in North American 30 million years ago.

Compared 15 Precise Measurements with 343 Specimens

The study’s author is Jonathan Calede, an evolutionary biologist at Ohio State University. Professor Calede, one of the scientists who made the discovery, painstakingly compared 15 precise measurements of the fossil’s anklebone with those of over 343 other rodent specimens.

These comparisons helped him determine how this fossilized rodent got around in its habitat. He found that the anklebone the team unearthed had evolved for swimming.

“I’m not claiming this new species is necessarily the oldest aquatic beaver ever,” Professor Calede clarified. “There are other animals that we know, from their teeth, that are related to this species I described.”


New Species Called Microtheriomys Articulaquaticus

Even so, the assistant professor of evolution and ecology considers it a new species. He’s named it Microtheriomys articulaquaticus.

This newly discovered rodent species is much smaller than the beavers I’m used to seeing in the wild. It would only have weighed about two pounds.

It also didn’t have a proper beaver tail. Instead, it had a long, straight tail like other rodents, and it seems to have eaten plants rather than wood, unlike today’s industrious rodent carpenters.

Evolutionary Principle Called Cope’s Rule

The difference in size is consistent with an evolutionary principle called Cope’s rule. It suggests that each generation within a species gradually grows larger over time.

However, Cope’s rule can lead to long-term challenges for species. “It looks like when you follow Cope’s Rule, it’s not good for you.” Professor Calede said. “It sets you on a bad path in terms of species diversity.”

In the case of beavers, there was once a giant beaver species that was as big as a black bear. However, it died out along with dozens of other varieties of large beavers.

“One North American Beaver and One Eurasian Beaver”

“Today we have one North American beaver and one Eurasian beaver.” Professor Calede said. “We’ve gone from a group that is super diverse and doing so well to one that is obviously not so diverse anymore.”

Professor Calede thinks the adaptation of this ancestral beaver’s anklebone results from a process called “exaptation.” That’s when part of an animal’s existing anatomy assimilates into a new way of life.

“In this case, the adaptations to burrowing were co-opted to transition to a semi-aquatic locomotion,” Professor Calede explained. “Beavers went from digging burrows to swimming in water.”

Earliest Beavers Didn’t Swim at All

Scientists believe that the earliest beavers were burrowing animals that didn’t swim at all.

“It’s not necessarily surprising because movement through dirt or water requires similar adaptations in skeletons and muscles.”

Studying rodents helps scientists understand how mammals evolved. “Rodents are the most diverse group of mammals on Earth,” Professor Calede explained. “About 4 in 10 species of mammals are rodents. If we want to understand how we get incredible biodiversity, rodents are a great system to study.”

Unveiled a Deeper Understanding of that Process

Understanding the natural world and humanity’s place in it calls for a deeper grasp of how such a wide range of species has arisen on our planet. These discoveries arising from a single anklebone have unveiled a deeper understanding of that process.

Professor Calder wound up the discussion saying, “Look at the diversity of life around us today, and you see gliding rodents like flying squirrels, rodents that hop like the kangaroo rat, aquatic species like muskrats, and burrowing animals like pocket gophers. There is an incredible diversity of shapes and ecologies. When that diversity arose is an important question.”

We always have more to learn if we dare to know.


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