Good morning! A new beaver fossil find in Montana might be important to the way we think about beavers and how they evolved. Plus, it’s a fun story to start a wednesday with.

30-million-year-old amphibious beaver fossil is oldest ever found

A new analysis of a beaver anklebone fossil found in Montana suggests the evolution of semi-aquatic beavers may have occurred at least 7 million years earlier than previously thought, and happened in North America rather than Eurasia.

In the study, Ohio State University evolutionary biologist Jonathan Calede describes the find as the oldest known amphibious beaver in the world and the oldest amphibious rodent in North America. He named the newly discovered species Microtheriomys articulaquaticus.

Calede’s findings resulted from comparing measurements of the new species’ anklebone to about 340 other rodent specimens to categorize how it moved around in its environment—which indicated this animal was a swimmer. The Montana-based bone was determined to be 30 million years old—the oldest previously identified semi-aquatic beaver lived in France 23 million years ago.

Beavers and other rodents can tell us a lot about mammalian evolution, said Calede, an assistant professor of evolution, ecology and organismal biology at Ohio State’s Marion campus.

7 million years older than we thought. Of course beavers have existed longer and done more than we ever thought possible. It looks like these beavers ADAPTED to aquatic life, even though they weren’t designed for it.

Running computational analyses of the data in multiple ways, he arrived at a new hypothesis for the evolution of amphibious beavers, proposing that they started to swim as a result of exaptation—the co-opting of an existing anatomy—leading, in this case, to a new lifestyle.

“In this case, the adaptations to burrowing were co-opted to transition to a semi-aquatic locomotion,” he said. “The ancestor of all beavers that have ever existed was most likely a burrower, and the semi-aquatic behavior of modern beavers evolved from a burrowing ecology. Beavers went from digging burrows to swimming in water.

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

So one day 35 million years ago some burrowing beavers thought, “Dam I’m in a rut. I feel like I’m going around in circles and not getting anywhere in life” And he crawled across to where the sparkling water met the sky and thought “I think I can do this!”

Fossils of fish and frogs and the nature of the rocks where Microtheriomys articulaquaticus fossils were found suggested it had been an aquatic environment, providing additional evidence to support the hypothesis, Calede said.

Fossils are usually dated based on their location between layers of rocks whose age is determined by the detection of the radioactive decay of elements left behind by volcanic activity. But in this case, Calede was able to age the specimen at a precise 29.92 million years old because of its location within, rather than above or below, a layer of ashes.

“The oldest semi-aquatic beaver we knew of in North America before this was 17 or 18 million years old,” he said. “And the oldest aquatic beaver in the world, before this one, was from France and is about 23 million years old.

Beavers have been adapting to massive changes for 30 million years. No wonder they’re so dam good at it.