Seals, commonly known as pinnipeds, are a broadly diverse and distributed clade of carnivorous, semiaquatic, fin-footed, primarily marine yet adorable mammals. They contain the extant families
- Otariidae (the eared seals: sea lions and fur seals),
- Odobenidae (whose only living member is the big-old walrus), and
- Phocidae (the true seals, or earless seals).
There are 33 known species of seals, and more than 50 extinct species have been proven from fossils.
One popular hypothesis suggested that pinnipeds (seals) are diphyletic (descended from two ancestral lines), with walruses and otariids sharing a recent common ancestor with bears and phocids sharing one with Musteloidea. However, morphological and molecular evidence support a monophyletic origin.
Nevertheless, there is some dispute as to whether seals are more closely related to bears or musteloids, as some studies support the former theory and others the latter.
Seals split from other caniforms 50 million years ago (mya) during the Eocene. Their evolutionary link to terrestrial mammals was unknown until the 2007 discovery of Puijila in early Miocene deposits in Nunavut, Canada. Puijila had a long tail, short limbs, and webbed feet instead of flippers like a modern otter. However, its limbs and shoulders were more robust, and Puijila likely had been a quadrupedal swimmer—retaining a form of aquatic locomotion that gave rise to the major swimming types employed by modern seals. The researchers found Puijila placed in a clade with Potamotherium (traditionally considered a mustelid) and Enaliarctos. Of the three, Puijila was the least specialized for aquatic life. The discovery of Puijila in a lake deposit suggests that pinniped evolution went through a freshwater transitional phase.
Enaliarctos, a fossil species of late Oligocene/early Miocene (24–22 Mya) California, closely resembled modern seals; it was adapted to an aquatic life with flexible spine limbs modified into flippers. Its teeth were adapted for shearing (like terrestrial carnivorans), and it may have stayed near shore more often than its extant relatives. Enaliarctos was capable of swimming with both the fore-flippers and hind-flippers, but it may have been more specialized as a fore-flipper swimmer.
Enaliarctos long exhibited significant sexual dimorphism, suggesting that this physical characteristic may have been an essential driver of pinniped evolution. A closer relative of extant seals was Pteronarctos, which lived in Oregon 19–15 mya. As in modern seals, Pteroarctos had an orbital wall that was not limited by certain facial bones (like the jugal or lacrimal bone) but was mostly shaped by the maxilla. The extinct family Desmatophocidae lived 23–10 Mya in the North Atlantic and had elongated skulls, fairly large eyes, cheekbones connected by amortized structure, and rounded cheek teeth. They also were sexually dimorphic and may have been capable of propelling themselves with both the fore flippers and hind flippers. Their phylogeny and evolutionary relationship to other seals are poorly understood.
The ancestors of the Otarioidea and Phocoidea diverged 33 mya.
Phocids are known to have existed for at least 15 million years, and molecular evidence supports a divergence of the Monachinae and Phocinae lineages 22 Mya. The fossil monachine Monotherium and phocine Leptophoca were found in southeastern North America. The deep split between the lines of Erignathus and Cystophora 17 Mya suggests that the phocines migrated eastward and northward from the North Atlantic. The genera Phoca and Pusa could have arisen when a phocine lineage traveled from the Paratethys Sea to the Arctic Basin and subsequently went eastward. The ancestor of the Baikal seal migrated into Lake Baikal from the Arctic (via the Siberian ice sheet) and became isolated there.
The Caspian seal’s ancestor became isolated as the Paratethys shrank, leaving the animal in a small remnant sea, the Caspian Sea. The monochines diversified southward. Monachus emerged in the Mediterranean and migrated to the Caribbean and then the central North Pacific. The two extant elephant seal species diverged close to 4 mya after the Panamanian isthmus was formed. The lobodontine lineage emerged around nine mya and colonized the southern ocean in response to glaciation.