Potamotherium ('river beast') an extinct genus of Caniformia Carnivora from the Miocene epoch of France and Germany. It has historically been assigned to the family Mustelidae (otters, weasels, etc.), but more recent studies suggest that it represents a primitive relative of (seals, sea lions, etc.)
Classification
The genus was first described in 1833. Carroll (1988) assigned it to the family
Mustelidae as a member of the subfamily
Oligobuninae. However, it was recently suggested that
Potamotherium was not a mustelid at all, but rather a very basal
pinniped.
Berta et al. (2018) placed
Potamotherium along with
Puijila and
Semantor in the family Semantoridae.
[Berta, A., Churchill, M., & Boessenecker, R.W. (2018). "The Origin and Evolutionary Biology of Pinnipeds: Seals, Sea Lions, and Walruses". Annual Review of Earth and Planetary Sciences. 0. doi:10.1146/annurev-earth-082517-010009.
]
Two species have been identified in the genus: P. valletoni, the type species, and P. miocenicum.
Distribution
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.
[Tedford, R. H. et al. (2004): Late Cretaceous and Cenozoic Mammals of North America (ed. Woodburne, M. O.), pp 169–231 (Columbia Univ. Press, 2004)][Mörs, T. & Von Koenigswald, W. (2000): Potamotherium valletoni (Carnivora, Mammalia) aus dem Oberoligozän von Enspel im Westerwald. Senckenberg. Leth. no 80: pp 257–273] It has been interpreted by several researchers as a basal, non-marine ancestor of
Pinniped and
, suggesting a freshwater phase in the evolutionary transition of pinnipeds from land to sea; geochemical analysis supports this hypothesis, finding
P. valletoni to have been a freshwater dweller.
If
Potamotherium was indeed a pinniped instead of a mustelid, its relatives were possibly early
(whose ancestors at the time were small and generally weasel-like).
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Palaeobiology
Physically, Potamotherium resembled a modern otter, and was 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 olfaction, but made up for this with good vision and hearing.
Fossils of Potamotherium are so complete that the shape of the brain can be inferred via a digital endocast of the skull. The coronal gyrus (a fold on the lateral surface of the brain) is broad, slanted backwards and partially split by a small groove. The brain is nearly identical to that of Enaliarctos, an extinct mammal universally agreed to be close to pinnipeds. Modern pinnipeds and the extinct Pinnarctidion have an expanded coronal gyrus with a distinctive vertical orientation. The carnivorans with the largest coronal gyrus are freshwater foragers such as the otter civet ( Cynogale bennetti) and certain otter species (in the genera Lutra and Lontra). They primarily emphasize sensitive whiskers (vibrissae) or the lips while hunting, rather than the hands. It is likely that the same was true for Potamotherium. Modern pinnipeds are unique among marine mammals for their large whiskers, which were probably inherited from an ancestor similar to Potamotherium.
