Oldest known amphibious beaver fossil discovered

North America is rich in fossils of rodents that roamed our planet millions of years ago. The researcher Jonathan Calède of the State University of Ohio (United States) was interested in one of them which constitutes a cornerstone of the evolution of beavers. In a study published on August 5, 2022 in Royal Society Open Sciencehe describes the oldest semi-aquatic rodent in North America which is also the oldest known amphibious beaver in the world.

A beaver weighing less than a kilo

Remains of this now extinct beaver have been unearthed in Montana, a state located in the northwestern United States. “I discovered some of the teeth of this animal during my fieldwork, but the bones important in determining its locomotion were discovered in the 1960s by another scientist, Donald Rasmussen, tell to Science and Future Jonathan Calede. And since then, the bones were kept at the museum of the University of Montana, in Missoula“.

It was a small beaver weighing only 700 grams! A featherweight compared to the European beaver (20 kg) and the Canadian beaver (from 11 kg to 32 kg). Another notable difference: the extinct rodent lacked the typical paddle-shaped tail of modern animals. “He had rather a round tail like a muskrat“, compares the researcher. However, he shared a particularity with today’s beavers: hypsodontia. The growth of their teeth is prolonged for a large part of their life.

A bone that translates a life between water and land

This rodent was baptized Microtheriomys articulaquaticusfrom the Latin words for articulation (joint) and aquatic (aquaticus). And for good reason: the analysis of one of the bones of its ankle, only 10 mm long, shows that it is the oldest semi-aquatic beaver known to date in the world. Because it is by comparing the bone of the newly described species with that of 343 other rodents that the researcher was able to get an idea of ​​its mode of locomotion.

I have studied the ankle bone of several hundred modern rodent specimens to determine the relationship between its form and function in these animals, Explain Jonathan Calede. I then applied this information to the fossil. In practice, this means taking 15 different ankle bone measurements from over 340 animals (a total of over 5100 measurements, therefore) and incorporating them into an analysis capable of detecting differences between animals classified into known locomotion groups“. It is then a question of looking at the bone of the fossil with a a new eye, in the light of the information obtained.

Moreover, the fossils discovered near the remains of Microtheriomys articulaquaticus confirm its semi-aquatic way of life. The excavations have notably revealed fossils of frogs but also of fish.

A change of environment thanks to exaptation

According to Dr. Calède’s results, this rodent would have lived about 30 million years ago, which means that the evolution of semi-aquatic beavers could have occurred at least 7 million years earlier than what we thought before. And this study also questions the place of this process: it would therefore have taken place in North America and not in Eurasia. Microtheriomys articulaquaticus steals the show Steneofiber eseri, previously holder of the title of oldest semi-aquatic beaver, about 23 million years old and discovered in France.

How did beavers become semi-aquatic? The author of this new study offers an explanation: exaptation. “This is’a process by which physical characteristics already existing in an organism are co-opted (similar, editor’s note) for another function, he develops. An example is the existence of feathers in theropod dinosaurs which evolved for thermoregulation and were later co-opted for flight. In the case of this beaver, the ancestor was adapted to dig (a burrower) and this ability to move through the earth was co-opted for movement through water, two fluids, after all.“.

From tiny animal to giant beaver

Ankle bones of Microtheriomys articulaquaticus (in black) and the largest beaver in the world, a Pleistocene fossil, Castoroids ohioensis. Credit: Jonathan Calede

The study also highlights the marked growth of semi-aquatic beavers (burrowers have always maintained a small size) during their evolution. Their size presumably followed Cope’s law. “It is a postulate, an observation verified in a number of groups of organisms: the size of species tends to increase during the evolution of this group.“, explains Dr. Calède. If Microtheriomys articulaquaticus weighed less than a packet of flour, Castoroids ohioensis was a giant beaver the size of a black bear. He died 12,000 years ago, demonstrating at the same time that the race for gigantism is not a guarantee of success.

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