Temporal range: 23.03–7.25Ma Aquitanian - Tortonian
The genus was first described in 1833. In 1988 it was assigned to the family Mustelidae and the subfamily Oligobuninae. However, it has been suggested that Potamotherium was not a mustelid at all, but rather a very basal pinniped. One species has been identified in the genus: P. miocenicum.
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. It has been interpreted by several researchers as a basal, non-marine ancestor of seals and sea lions, suggesting a freshwater phase in the evolutionary transition of pinnipeds from land to sea. 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).
Physically, Potamotherium resembled a modern otter, and was 1.5 metres (5 ft) long, with an elongated, slender body and short legs. With a flexible backbone and a streamlined shape, it was probably a good swimmer. Analysis of fossils suggests that Potamotherium had a poor sense of smell, but made up for this with good vision and hearing.
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