Origins & taxonomy of marine carnivores:
Polyphyly versus Monophyly

Marine carnivores [true or "earless" seals (Phocidae), sealions or "eared" seals (Otariidae), and walruses (Odobenidae)] are often treated as either an order (Pinnipedia) separate from the terrestrial carnivores (Fissipedia), or as a suborder within the order Carnivora. If, as believed by some mammalogists, sealions and walruses were derived from bears (Ursidae) and true seals from weasels (Mustelidae) as in the above diagram, such a taxon would by polyphyletic.

In fact, recent molecular evidence indicates that all three familes of marine carnivores are descended from weasel-like ancestors. Separation of this holophyletic group as a separate order would make the remaining "fissiped" group paraphyletic. Such an arrangement would not be acceptable to phylogenetic taxonomists, but might be to traditional taxonomists.