Fossil lutrines

A couple of weeks ago, I got an email from someone who asked me some very interesting questions about the evolution of otters. This was a specialty of mine in graduate school, and this recent inquiry has resulted in me digging up some images of fossil "lutromorphs" – or otterlike animals – from an old research paper of mine. So I thought I'd share some of them with the readers of this blog.

This is Potamotherium, an "otter" from 30 million years ago. I put that in quotes because otters as we know them now didn't exist yet. Otters are members of the weasel ("mustelid") family of carnivores. Potamotherium, however, was an "arctoid" carnivore – a taxonomic group comprising what would become the bear family, the weasel family and the raccoon family millions of years later.




Despite being such a "primitive" carnivore, though, Potamotherium's skeleton (below) looks almost exactly like that of modern otters.




The otterlike body-shape ("lutromorphy") is a classic mammalian form. It's so well-suited to its environment that it's evoloved over and over again in many different groups of animals.

For example, this is the fossil "seal-otter" known as Semantor, from approximately 5-6 million years ago. Semantor is an example of a "morphological intermediate" between otterlike animals that lived on land and the fully-aquatic pinnipeds we know today.




And this fossil animal, Enaliarctos, was an otter-like bear which lived in the inland sea of California 22-24 million years ago.




Enaliarctos was a "hemicyonine ursid," which roughly translated means "half-doglike bear." Like I said, way back then, the carnivores were not as well-differentiated as they are now. Anyway, doing some reading on the web, I guess today Enaliarctos is considered to be the common ancestor of seals and sea lions. Back when I was in grad school, Enaliarctos was thought to be ancestral to sea lions only, but now there seems to be more evidence that it was the granddaddy to modern-day phocid seals, as well...