Fossil What?

An animal related to otters, other mustelids, or potentially related to pinnipeds (e.g. seals and sea lions) could have many overall forms. Thankfully, of course, we have quite a few good quality fossils that allow for educated morphological guesswork on the part of illustrators and reconstruction artists. The body plan that has been inferred from these fossils is heavily favoring a body that is much like that of the North American River Otter. River Otters, like Potamotherium, have sleek elongate bodies that are aerodynamic. This allows for swimming in freshwater loaded with current systems, such as rivers, and also allows the animal to be reasonably quick as, in the case of River Otters, they chase fish in the cool river waters. Aside from being mammals, the way River Otters stay warm in those cold waters is with heavy, oiled fur that traps warm air in their coat and waterproofs the fur. The similarity between Potamotherium and River Otters makes a lot of sense; Potamotherium means "River Beast."