Oldest known amphibious beaver fossil discovered

Oldest known amphibious beaver fossil discovered

Oldest known amphibious beaver fossil discovered

North America is rich in rodent fossils that roamed our planet millions of years ago. Researcher Jonathan Calède of the State University of Ohio (United States) was interested in one of them that constitutes a milestone in the evolution of beavers. In a study published August 5, 2022 in Open science of real societydescribes the oldest semi-aquatic rodent in North America, which is also the oldest known amphibious beaver in the world.

A beaver that weighs less than a pound

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

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

A bone that translates a life between water and earth

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

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

In addition, the fossils discovered near the remains of Microtheriomys articulaquaticus confirm its semi-aquatic lifestyle. Excavations have revealed notably fossils of frogs but also of fish.

A change of environment thanks to exaptation

According to Dr. Calède’s findings, this rodent lived about 30 million years ago, which means that the evolution of semi-aquatic beavers may have occurred at least 7 million years earlier than we previously thought. And this study also questions the place of this process: it would therefore have occurred in North America and not in Eurasia. Microtheriomys articulaquaticus steals the show Steneofibra eseri, formerly holder of the oldest semi-aquatic beaver title, 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, ed) for another function, it develops. One example is the existence of feathers in theropod dinosaurs that evolved for thermoregulation and were subsequently co-opted for flight. In the case of this beaver, the ancestor was adapted to dig (a digger) 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, Castoroides ohioensis.  Credit: Jonathan Calede

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

The study also highlights the marked growth of semi-aquatic beavers (diggers have always kept a small size) during their evolution. Their size presumably followed Cope’s law. “It is a postulate, an observation verified in a certain number of groups of organisms: the size of the species tends to increase during the evolution of this group.“, explains Dr. Calède. If Microtheriomys articulaquaticus weighed less than a packet of flour, Beaver Ohioensis it was a giant beaver the size of a black bear. He died 12,000 years ago, while proving that the race for gigantism is no guarantee of success.

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