Beavers have been shaping North American ecosystems for 7 million years, finds research by ETSU scientists

Beavers are iconic members of North American ecosystems. Besides being large, charming rodents, they are also famous for their habits of cutting down trees and damming rivers, behaviors that leave a major impact on their environments. In fact, beavers are said to change their environments more than any other animals (besides humans). According to new research on beaver fossils, it seems these rodents have been modifying forests and rivers for many millions of years.

These insights come from a study published in the journal Palaeontologia Electronica by Kelly Lubbers, bonebed paleontologist at The Mammoth Site and alumnus of ETSU’s paleontology Master’s program, and Dr. Joshua Samuels, associate professor in the ETSU Department of Geosciences and curator at the Gray Fossil Site & Museum.

Modern North American beavers belong to the species Castor canadensis, and they are often compared with the extinct species Castor californicus, which are found in the fossil record between about 2 and 7 million years ago. The two species are so similar that some researchers have questioned whether they are in fact distinct species. The main factor that distinguishes them is their size, with the extinct C. californicus being generally larger.

This image shows the skull and jaws of the modern North American beaver (Castor canadensis) and a fossil skull of Castor californicus. Image from Josh Samuels.

“Size alone is not a good metric for distinguishing species,” says Lubbers, “so we wanted to take a closer look and see if we could find any other similarities or differences that might tell us if they really should be considered different species.”

To do so, the team took extensive measurements on more than 120 beaver specimens, including skulls and skeletons of modern and fossil beavers from all over North America. Comparing the results, they found only minor differences between the modern and extinct species, including significant overlap in their body size. This suggests the two might not represent wholly separate species, but instead a single lineage showing subtle changes over time.

These strong similarities also indicate that the extinct beavers had a very similar lifestyle to their modern relatives, which suggests that North American environments have been shaped by beaver activity for the last 7 million years.

Left: Kelly Lubbers examines fossils in the collections of the Mammoth Site in South Dakota. Image by Claire Scarborough. Right: Dr. Josh Samuels examines a fossil under the microscope in the lab at the Gray Fossil Site & Museum. Image by David Moscato.

“Based on our findings, I am quite confident that Castor californicus would have had a lifestyle and ecology indistinguishable from living beavers,” says Samuels.

“This means that like beavers today, we can infer that C. californicus was semiaquatic, consuming fibrous plant matter, and modifying ecosystems by felling trees and dam building,” Lubbers adds.

The researchers say that more work will be needed to determine confidently whether these beavers represent the same species, including further study on particular parts of the beavers’ bodies. “Castor californicus was initially described as a separate species based on its teeth,” says Lubbers. “I think by comparing the teeth of fossil and extant beavers, this will give us a better indication as to whether they really are separate species or not.”


Study: Lubbers, Kelly E. and Samuels, Joshua X. 2023. Comparison of Miocene to early Pleistocene-aged Castor californicus (Rodentia: Castoridae) to extant beavers and implications for the evolution of Castor in North America. Palaeontologia Electronica, 26(3):a35

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