Origins & taxonomy of marine carnivores:
Polyphyly versus Monophyly

    Marine carnivores comprises three families, true or "earless" seals (Phocidae), sea lions or "eared" seals (Otariidae), and walruses (Odobenidae), were formerly treated as an order (Pinnipedia) evolutionarily separate from the terrestrial carnivores (Fissipedia), but are now recognized as a suborder evolved within the order Carnivora. The diagram here shows the hypothesis that sea lions and walruses are derived from bears (Ursidae), and true seals from weasels (Mustelidae). If so, the taxon Pinnipedia would by polyphyletic.

    In fact, molecular evidence indicates that all three families 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, who emphasize relationship, but might be to traditional taxonomists, who emphasize similarity.