Oregon was once home to rodents of unusual size.
Paleontologists have unearthed fossilized remains from 21 ancient rodent species in eastern Oregon, including the skull and teeth of a previously unknown miniature beaver called Microtheriomys brevirhinus. At about the size of a squirrel, this particular beaver would have been ten times smaller than modern relatives, as Tara Kulash reports for The Oregonian. The finds appeared in the May issue of Annals of Carnegie Museum.
In 2012, the beaver fossils were unearthed less than a mile from the visitor's center for the John Day Fossil Beds National Monument, reports Jeff Barnard for the Associated Press. Nine other new species of rodents have been found in the area — some in the same fossil beds, most on nearby government lands. Some of the fossils are more than 20 million yeas old. The sediment surrounding the beaver fossils contains layers of volcanic ash, and by dating radioactive elements within the ash suggests the fossils are from the Oligocene period between 28 and 30 million years ago.
Aside from its size, what makes this ancient beaver unique is that it's probably not related to other early beavers unearthed in Oregon. Burrowing beavers, from the same period, used their claws and teeth to dig holes in the ground, but the remains M. brevirhinus suggest it probably led more of an aquatic lifestyle. It's skull and teeth bear similarity to larger aquatic beaver species found in Asia and Europe, but paleontologists will need to find its fossilized limbs to be sure.
The find could also lend insight into rodent evolution more generally, as University of Oregon paleontologist Samantha Hopkins explained to the AP in an email:
“While there is relatively little castorid (beaver species) diversity today, there are hundreds of species (many of which are really important members of their faunal communities) in the fossil record of the Northern Hemisphere, and a better understanding of their diversity and evolutionary relationships has a lot to tell us about processes driving mammalian evolution over the last 40 million years."
While the burrowing beaver's line went extinct, it's also possible that this ancient aquatic mini beaver has modern relatives.