[4] Finds range from the mid-latitudes of Europe and North America, dated from the Oligocene/Miocene boundary and surviving through to the end of the Miocene.

If Potamotherium was indeed a pinniped instead of a mustelid, its relatives were possibly early bears (whose ancestors at the time were small and generally weasel-like).

[2] Physically, Potamotherium resembled a modern otter, and was 1.5 metres (5 ft) long, with an elongated, slender body and short legs.

The coronal gyrus (a fold on the lateral surface of the brain) is broad, slanted backwards and partially split by a small groove.

Modern pinnipeds are unique among marine mammals for their large whiskers, which were probably inherited from an ancestor similar to Potamotherium.