Evolutionary History of Pinnipeds

There are two main views of the evolution of pinnipeds. The biphyletic view, in which Walruses and eared seals evolved from a bear like ancestor and the true seals evolved from an otter-like carnivore. This view is now generally not accepted as all the evidence is largely morphological which can often be deceiving. The second view is that all of the pinnipeds belong to a monophyletic group and they all share a common ancestor. This view is the most accepted as it has been proved by extensive genetic evidence.

It is thought that after the most common ancestor of pinnipeds, the lineage split into the two superfamilies: the Otaroidea (consisting of eared seals and walruses) and the Phocoidea (the earless seals). Despite this division of the three families, it is thought that the walruses actually share a closer relationship with the phocid seals. Walruses are thought to be a very early divergence from the phocid seals, but not the first.

An animal known as Enaliarctos is thought to be the first known divergence from the last common ancestor of pinnipeds. Unlike modern pinnipeds, skeletal modification in Enaliarctos suggests that is used both its foreflippers and hindflippers in swimming. The two modern superfamilies of pinnipeds use either/or. The presence of elongate hindlimbs with extensive bony processes is an indication that this animal probably spent more of its time on land than in the ocean. The skulls of these animals also show the presence of slicing carnassials. These teeth are found in most terrestrial carnivores, but not in modern pinnipeds, which suggests Enaliarctos may have had to return to land with its prey to ingest it. Phylogenetic analyses of many Enaliarctos fossils indicated that the animal belongs in a sister group in relationship with modern pinnipeds.

Enaliarctos on land. Picture taken from: Mammal evolution an illustrated guide (see references)
Enaliarctos

Skeletons of Enaliarctos have been found from as far back as the late Oligocene which spanned from about 30 to 24 million years ago. This particular epoch marked the transition from the Palaeogene to the Neogene period which was characterized by a period of global cooling culminating in the ice ages in the Pleistocene.

Pinnipedia originally consisted of 5 families. As well as Enaliarctidae, there is another extinct family of called the Desmatophocidae. These primitive seals were closely related to the phocid seals. Whilst fossil evidence is poor, skull fossils found show that they would have relied more on sight to hunt as their inner ear was not as developed for underwater hearing as more modern pinnipeds.