Leave It To 'Jurassic Beaver'... To Redefine The Evolutionary History Of Mammals

But it was a fake beaver. Happy #FossilFriday!

This creature lived millions of years ago and had the gall to look and act like a beaver.

Nobu Tamura / Via spinops.blogspot.com

Around 164 million years ago, in the warm, sub-tropical lakes of the Jurassic, a curious creature could have been found swimming and scurrying as it effortlessly transitioned from land to water.

Until 2006, most scientists would have assumed that such a creature was unequivocally some kind of reptile or amphibian. Prevailing logic held that the earliest mammal ancestors of that time were small, shrew-like creatures confined only to land.

That was before Castorocauda lutrasimilis was discovered in the hills of Inner Mongolia. The find was remarkable in both preservation and significance. Not only was it a complete skeleton, it also recorded the earliest known evidence of fur. And it shattered the notion that early mammals had to exist only on land as tiny, dinosaur-fearing, insect-eating afterthoughts.

Was it really a mammal?

By most standards, it was not technically a mammal, but a mammaliaform. True mammals include all living mammal species and any species (even extinct ones) ever included in the genera of those living mammals. Mammaliaforms include all living mammals and their closest extinct relatives, including genera that are completely extinct today.

They have most of the same attributes as true mammals, but are more distantly related from an evolutionary perspective.

April I. Neander / Via luo-lab.uchicago.edu

Castorocauda belongs to a group of extinct critters called Docodonts. In the figure above 'extant mammals' (the opposite of extinct) are the true mammals.

Was it really a beaver?

No, it was decidedly not a beaver. It just appears to have had very similar features and lived in a similar kind of environment to the modern beaver. If it were still around today, it would be less related to beavers than humans are.

Castorocauda is a beautiful example of convergent evolution, the process by which unrelated organisms evolve similar traits as a result of adapting to similar environments or niches. It had a broad tail and webbed toes, similar to that of a modern beaver. These adaptations were presumably for moving around in aquatic environments.

Federicofoto / Getty Images

For the record, we interviewed* a modern beaver and they resent being compared to this primitive, beaver-like creature. (*this did not happen, but it's probably true, right?)