Earliest amphibious beaver lived 30 million years ago

It was previously thought that beavers living an amphibious life first evolved in Eurasia. A new discovery in Montana refutes this idea, suggesting a North American origin. The research also shows that amphibious beavers date back much further than what was previously thought. 

In a recent study, Ohio State evolutionary biologist Jonathan Calede described the ankle bone of a 30 million-year-old beaver he named Microtheriomys articulaquaticus. This makes this aquatic beaver seven million years older than the previous oldest from France. 

Calede explained that a lot can be learned about mammal evolution from rodents. “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.”

“Rodents are the most diverse group of mammals on Earth, and 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,” said Calede.

It was immediately apparent when the scientists uncovered the bones that they were from a beaver. The ankle bone was key to determining the animal’s lifestyle, and 15 measurements were taken of the bone. 

The measurements were compared to those of 343 modern specimens from burrowing, gliding, swimming, and jumping mammals as well as other ancient beavers.

After running computer analyses, Calede determined that beavers evolved their aquatic lifestyle through exaptation, which means co-opting one structure for a new purpose.        

“In this case, the adaptations to burrowing were co-opted to transition to a semi-aquatic locomotion,” said Calede. “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.”

Analysis of the beaver lineage shows that the animals in the lineage increased in size, to the point that a beaver the size of a black bear lived 12,000 years ago. Now what was once a diverse group of rodents is reduced to two species – one in North America and one in Eurasia.   

The study is published in the journal Royal Society Open Science.

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By Erin Moody , Earth.com Staff Writer