The newly-identified species of beaver, Microtheriomys articulaquaticus, represents the oldest semi-aquatic rodent in North America and the oldest amphibious beaver in the world, pushing the advent of semi-aquatic ecology in beavers by 7 million years.
Microtheriomys articulaquaticus lived in what is now Montana, the United States, some 30 million years ago (Oligocene epoch).
The oldest previously identified semi-aquatic beaver, Steneofiber eseri, lived in France 23 million years ago.
Microtheriomys articulaquaticus did not have the flat tail that helps beavers swim today.
The ancient animal likely ate plants instead of wood and was comparably small.
“Beavers and other rodents can tell us a lot about mammalian evolution,” said Dr. Jonathan Calede, an evolutionary biologist at Ohio State University and an author of the study published in the journal Royal Society Open Science.
“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.”
The fossil material of Microtheriomys articulaquaticus — including several associated teeth, two astragali and partial bones — was recovered from the Renova Formation.
Dr. Calede took 15 measurements of the anklebone fossil and compared it to measurements of similar bones from 343 specimens of rodent species living today that burrow, glide, jump and swim as well as ancient beaver relatives.
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,” Dr. Calede 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.”
“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.”
The analysis of beaver body size over the past 34 million years suggests beaver evolution adheres to what is known as Cope’s rule, which posits that organisms in evolving lineages increase in size over time.
A giant beaver the size of a black bear lived in North America as recently as about 12,000 years ago.
Like all but the two beaver species living today, Castor canadensis and Castor fiber, the giant beaver is extinct.
“It looks like when you follow Cope’s rule, it’s not good for you — it sets you on a bad path in terms of species diversity,” Dr. Calede said.
“We used to have dozens of species of beavers in the fossil record. Today we have one North American beaver and one Eurasian beaver. We’ve gone from a group that is super diverse and doing so well to one that is obviously not so diverse anymore.”
Jonathan J.M. Calede. 2022. The oldest semi-aquatic beaver in the world and a new hypothesis for the evolution of locomotion in Castoridae. R. Soc. open sci 9 (8): 220926; doi: 10.1098/rsos.220926
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