With their streamlined bodies, intelligence, graceful swimming, tusks (in the case of the walrus) and adorable faces, most people could easily identify a seal nowadays. However, all lineage of animal must have evolved from somewhere, right? Where did Pinnipeds (eared seals, true seals and walrus) come from? Did they all evolve from the same common ancestor? Or did they evolve from separate lineages and develop similar adaptations due to convergent evolution?
First of all, we have explain what different types of Pinnipeds there are. Pinnipeds are typically split into 3 groups. The Odobenidae (containing a single extant species, the walrus), Otariidae (also typically known as eared seals, contain 15 extant species) & Phocidae (also known as earless seals, contain 19 extant species). All three families are considered Pinnipeds but they all differ from each other enough to be separate groups. The differences can include, locomotion on land, swimming methods, body morphology, etc. However, one thing I would like to outline in this breakdown is the swimming methods, as they're the most distinct difference between each family. Odobenidae are known to use both their forelimbs and hindlimbs for locomotion in the water, rather clumsily I may add. Otariidae use just their forelimbs, with their hindlimb possibly being used as a rudder when swimming slow, but seemingly not used at all otherwise. Finally, Phocidae use just their hindlimbs for movement in the water, their forelimbs are either not used at all or are used as paddles when the animal is moving slow.
With the three different families of Pinnipeds explained, we can start exploring their origins. Originally, scientists believed that Pinnipeds were diphyletic (meaning evolved from two ancestral lines) but since more effort has been put into the study of these mammals, it is widely considered that they were monolyphetic (evolved from a single common ancestor) instead. A few discoveries of proposed basal Pinnipeds have came to light in this timeframe. First it was the discovery of an extinct Pinniped called Enaliarctos (Meaning; Sea Bear) from coastal areas of Oregon. This animal was heralded the ancestor of all extant Pinniped species due to it's extremely similar morphology. Study of Enaliarctos showed that it probably would've used both it's forelimbs and hindlimbs for swimming, just like modern walrus. It is proposed that eared seals lost the use of their hindlimbs for swimming, earless seals lost the use of their forelimbs and walrus lost neither and retained the use of both sets of limbs. The reason why a species of animal would've stopped using either set of limbs has been debated for a while, but no reliable answer or suggestion has been put forward.
That's not the only fossil discovery that rocked the structure of Pinniped lineage, however. The discovery of a semi-aquatic mammal from Canada called Puijila (Meaning; Young Seal) created a very interesting morphological link of mammals returning to the sea from a terrestrial lifestyle multiple times and one that precedes the familiar structure of Enaliarctos. This would mean that Puijila is fossil evidence that mammals did indeed transition from land, to sea and possibly back to land and back into the sea again.
Whichever fossil find is truly the ancestor of Pinnipeds, we are still working out. However, looking into either fossil's morphology offers an insightful look of the transitions and similarities mammals shared, even across completely separate genus'.
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