Seals, sea lions, and walruses are all part of the same lineage called pinnipeds.  According to molecular DNA evidence, their nearest kin are skunks and weasels.[1] [2]  However, their earliest fossils look nothing like skunks and weasels.  Instead, they look much like they do today.  The earliest among them is Enaliarctos.  Some of the features it has in common with modern pinnipeds include flippers instead of feet, equal length elongated   According to Berta et al,

 

Enaliarctos documents an early, yet complete, stage in the acquisition of features associated with aquatic locomotion in pinnipeds.

 

Yet Berta et al also note the olecranon process and the number of lumbar vertebrae are different from later pinnipeds, and state that Enaliarctos is therefore intermediate between terrestrial animals and later pinnipeds.[3]  Enaliarctos is the best the fossil record can provide for an intermediary between terrestrial land mammals and pinnipeds.

     Enaliarctos shows some affinity to bears,[4] and it was widely believed for a long time that pinnipeds' closest relatives were bears.  The molecular DNA evidence indicates that indeed bears are closely related to the mustelids and to the pinnipeds, probably diverging from them just before the mustelids and pinnipeds diverged from each other.[5]  This fact would mean that a relatively swift divergence among the three groups occurred, to explain why the reconciliation of DNA data with fossil data is problematic.  As Arnason and Widegren said,

 

It is likely that the evolution of otariids and phocids (subdivisions of pinnipeds) was fast in the early stages of marine adaptation; hence evolutionary linkages with mustelids may be difficult to [6]

 

It would not be the only rapid diversification to occur among the carnivores.  There was also a swift three-way split of the feline lineage into cats, vivets, and hyaenas.[7] 

     Swift divergence among pinniped ancestors was followed by a much longer period of about 23 million years during which natural selection made comparatively small adjustments, causing adaptive radiations at the genus and species level, which resulted in speciation within the pinniped group.  Moreover, the transformation is not complete, for the pinnipeds today fill an ecological niche similar to that of the ancestors to whales Pakicetus and Ambulocetus, which were partly terrestrial and partly aquatic.  Given several millions of years, the pinnipeds might evolve to become fully aquatic like the whales.  Yet this is still occurring in slow motion.    

          The pinnipeds exemplify the operation of two distinct mechanisms – one mechanism being natural selection, which brought about gradual evolution with regard to the diversification among seals, sea lions, and walruses; and the other mechanism being some poorly understood mutational force which catalyzed an evolutionary event so rapid and so intense that it brought about a radically new and different form while leaving only a trace in the fossil record.