Pinniped
Pinnipeds, commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammals. They comprise the extant families Odobenidae, Otariidae, and Phocidae, with 34 extant species and more than 50 extinct species described from fossils. While seals were historically thought to have descended from two ancestral lines, molecular evidence supports them as a monophyletic group. Pinnipeds belong to the suborder Caniformia of the order Carnivora; their closest living relatives are musteloids, having diverged about 50 million years ago.
Seals range in size from the and Baikal seal to the and southern elephant seal. Several species exhibit sexual dimorphism. They have streamlined bodies and four limbs that are modified into flippers. Though not as fast in the water as dolphins, seals are more flexible and agile. Otariids primarily use their front limbs to propel themselves through the water, while phocids and walruses primarily use their hind limbs for this purpose. Otariids and walruses have hind limbs that can be pulled under the body and used as legs on land. By comparison, terrestrial locomotion by phocids is more cumbersome. Otariids have visible external ears, while phocids and walruses lack these. Pinnipeds have well-developed senses—their eyesight and hearing are adapted for both air and water, and they have an advanced tactile system in their whiskers or vibrissae. Some species are well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep warm in cold water, and, other than the walrus, all species are covered in fur.
Although pinnipeds are widespread, most species prefer the colder waters of the Northern and Southern Hemispheres. They spend most of their lives in water, but come ashore to mate, give birth, molt or to avoid ocean predators, such as sharks and orcas. Seals mainly live in marine environments but can also be found in fresh water. They feed largely on fish and marine invertebrates; a few, such as the leopard seal, feed on large vertebrates, such as penguins and other seals. Walruses are specialized for feeding on bottom-dwelling mollusks. Male pinnipeds typically mate with more than one female, though the degree of polygyny varies with the species. The males of land-breeding species tend to mate with a greater number of females than those of ice breeding species. Male pinniped strategies for reproductive success vary between defending females, defending territories that attract females and performing ritual displays or lek mating. Pups are typically born in the spring and summer months and females bear almost all the responsibility for raising them. Mothers of some species fast and nurse their young for a relatively short period of time while others take foraging trips at sea between nursing bouts. Walruses are known to nurse their young while at sea. Seals produce a number of vocalizations, notably the barks of California sea lions, the gong-like calls of walruses and the complex songs of Weddell seals.
The meat, blubber and skin of pinnipeds have traditionally been used by indigenous peoples of the Arctic. Seals have been depicted in various cultures worldwide. They are commonly kept in captivity and are even sometimes trained to perform tricks and tasks. Once relentlessly hunted by commercial industries for their products, seals are now protected by international law. The Japanese sea lion and the Caribbean monk seal have become extinct in the past century, while the Mediterranean monk seal and Hawaiian monk seal are ranked as endangered by the International Union for Conservation of Nature. Besides hunting, pinnipeds also face threats from accidental trapping, marine pollution, climate change and conflicts with local people.
Etymology
The name "pinniped" derives from the Latin words pinna and pes, pedis. The common name "seal" originates from the Old English word seolh, which is in turn derived from the Proto-Germanic *selkhaz.Taxonomy
The German naturalist Johann Karl Wilhelm Illiger was the first to recognize the pinnipeds as a distinct taxonomic unit; in 1811 he gave the name Pinnipedia to both a family and an order. American zoologist Joel Asaph Allen reviewed the world's pinnipeds in an 1880 monograph, History of North American pinnipeds, a monograph of the walruses, sea-lions, sea-bears and seals of North America. In this publication, he traced the history of names, gave keys to families and genera, described North American species and provided synopses of species in other parts of the world. In 1989, Annalisa Berta and colleagues proposed the unranked clade Pinnipedimorpha to contain the fossil genus Enaliarctos and modern seals as a sister group. Pinnipeds belong to the order Carnivora and the suborder Caniformia. Of the three extant families, the Otariidae and Odobenidae are grouped in the superfamily Otarioidea, while the Phocidae belong to the superfamily Phocoidea. There are 34 extant species of pinnipeds, and more than 50 fossil species of pinnipedimorphs.Otariids are also known as eared seals due to their pinnae. These animals swim mainly using their well-developed fore-flippers. They can also "walk" on land by shifting their hind-flippers forward under the body. The front end of an otariid's frontal bone protrudes between the nasal bones, with a large and flattened supraorbital foramen. An extra spine splits the supraspinatous fossa and bronchi that are divided in the front. Otariids consist of two types: sea lions and fur seals; the latter typically being smaller, with pointier snouts, longer fore-flippers and heavier fur coats. Five genera and seven species of sea lion are known to exist, while two genera and nine species of fur seal exist. While sea lions and fur seals have historically been considered separate subfamilies, genetic and molecular evidence has refuted this, indicating that the northern fur seal is basal to other otariids and the Australian sea lion and New Zealand sea lion are more closely related to Arctocephalus than to other sea lions.
Odobenidae has only one living member: the walrus. This animal is noticeable from its larger size, nearly hairless skin, flattened snout and long upper canines, known as tusks. Like otariids, walruses can walk on land with their hind limbs. When moving in water, the walrus relies on its hind limbs for locomotion, while its forelimbs are used for steering. Also, it has no outer ears. The epipterygoid of the jaw is well developed and the back of the nasal bones are horizontal. In the feet, the calcaneuses protrude in the middle.
Phocids are known as true or "earless" seals. These animals lack outer ears and cannot position their hind-flippers to move on land, making them more cumbersome. This is because of their massive ankle bones and flatter heels. In water, true seals rely on the side-to-side motion of their hind-flippers and lower body to move forward. The phocid's skull has thickened mastoids, puffed up entotympanic bones, nasal bones with a pointed tip in the back and a non-existent supraorbital foramen. The hip has a more converse ilium. A 2006 molecular study supports the division of phocids into two monophyletic subfamilies: Monachinae, which consists of elephant seals, monk seals and Antarctic seals; and Phocinae, which consists of all the rest.
Evolution
One popular hypothesis suggested that pinnipeds are diphyletic, 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. A 2021 genetic study found that pinnipeds are more closely related to musteloids. Pinnipeds split from other caniforms 50 million years ago during the Eocene. The earliest fossils of pinnipeds date back to the Late Oligocene. Fossil animals representing basal lineages include Puijila, of the Early Miocene in Arctic Canada. It resembled a modern otter, but shows evidence of quadrupedal swimming—retaining a form of aquatic locomotion that led to those employed by modern pinnipeds. Potamotherium, which lived in the same period in Europe, was similar to Puijila but more aquatic. The braincase of Potamotherium shows evidence that it used its whiskers to hunt, like modern seals. Both Puijila and Potamotherium fossils have been found in lake deposits, suggesting that seal ancestors were originally adapted for fresh water.Enaliarctos, a fossil species of late Oligocene/early Miocene California, closely resembled modern pinnipeds; it was adapted to an aquatic life with flippers and a flexible spine. Its teeth were more like land predators in that they were more adapted for shearing. Its hind-flippers may have allowed it to walk on land, and it probably did not leave coastal areas as much as its modern relatives. Enaliarctos was likely more of a fore-flipper swimmer, but could probably swim with either pair. One species, Enaliarctos emlongi, exhibited notable sexual dimorphism, suggesting that this physical characteristic may have been an important driver of pinniped evolution. A closer relative of extant pinnipeds was Pteronarctos, which lived in Oregon 19–15 mya. As in modern seals, the maxilla or upper jaw bone of Pteroarctos intersects with the orbital wall. The extinct family Desmatophocidae lived 23–10 mya in the North Pacific. They had long skulls that with large orbits, interlocked zygomatic bones and rounded molars and premolars. They also were sexually dimorphic and may have been capable of swimming with both or either pair of flippers. They are grouped with modern pinnipeds, but there is debate as to whether they are more closely related to phocids or to otariids and walruses. A 2024 study places them within Phocidae, specifically Phocinae.
Image:Archaeodobenus restoration.PNG|thumb|Reconstruction of Archaeodobenus akamatsui family Odobenidae
The ancestors of the Otarioidea and Phocoidea diverged around 25 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 around this time. The fossil genera Monotherium and Leptophoca of southeastern North America represent the earliest members of Monachinae and Phocinae respectively. Both lineages may have originated in the North Atlantic, and likely reached the Pacific via the Central American Seaway. Phocines mainly stayed in the Northern Hemisphere, while the monachines diversified southward. The lineages of Otariidae and Odobenidae split around 20 mya. The earliest fossil records of otariids are in North Pacific and dated to around 11 mya. Early fossil genera include Pithanotaria and Thalassoleon. The Callorhinus lineage split the earliest, followed by the Eumetopias/''Zalophus lineage and then the rest, which colonized the Southern Hemisphere. The earliest fossils of Odobenidae—Prototaria of Japan and Proneotherium of Oregon—date to 18–16 mya. These primitive walruses had normal sized canines and fed on fish instead of mollusks. Later taxa like Gomphotaria, Pontolis and Dusignathus had longer canines on both the upper and lower jaw. The familiar long upper tusks developed in the genera Valenictus and Odobenus''. The lineage of the modern walrus may have spread from the North Pacific to the North Atlantic through the Caribbean and Central American Seaway 8–5 mya, and then back to the North Pacific via the Arctic 1 mya, or to the Arctic and subsequently the North Atlantic during the Pleistocene.