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Sirenia is theorder ofplacental mammals which comprises modern "sea cows" (manatees and theDugong) and their extinct relatives. They are the only extantherbivorousmarine mammals and the only group of herbivorous mammals to have become completely aquatic. Sirenians are thought to have a 50-million-year-oldfossil record (earlyEocene-recent). They attained modest diversity during theOligocene andMiocene, but have since declined as a result of climatic cooling, oceanographic changes, and human interference.[1] Two genera and four species are extant:Trichechus, which includes the three species of manatee that live along theAtlantic coasts and in rivers and coastlines ofthe Americas and westernAfrica, andDugong, which is found in theIndian andPacific oceans.[citation needed]
Sirenians, along withProboscidea (elephants), group together with the extinctDesmostylia and likely the extinctEmbrithopoda to form theTethytheria. Tethytheria is thought to have evolved from primitive hoofed mammals ("condylarths") along the shores of the ancientTethys Ocean.
Tethytheria, combined withHyracoidea (hyraxes), forms aclade calledPaenungulata. Paenungulata and Tethytheria (especially the latter) are among the least controversial mammalian clades, with strong support from morphological and molecular interpretations.[citation needed] The ancestry of Sirenia is remote from that ofCetacea andPinnipedia, although they are thought to have evolved an aquatic lifestyle around the same time.
The first appearance of sirenians in the fossil record was during the early Eocene, and by the late Eocene, sirenians had significantly diversified. Inhabitants of rivers, estuaries, and nearshore marine waters, they were able to spread rapidly. The most primitive sirenian known to date,Prorastomus, was found in Jamaica, not theOld World; however more recently the contemporarySobrarbesiren has been recovered fromSpain.[2] The first known quadrupedal sirenian wasPezosiren from the early Eocene.[3]The earliest known sea cows, of the familiesProrastomidae andProtosirenidae, are both confined to the Eocene, and were about the size of a pig, four-legged amphibious creatures. By the time the Eocene drew to a close, came the appearance of theDugongidae; sirenians had acquired their familiar fully aquatic streamlined body with flipper-like front legs with no hind limbs, powerful tail with horizontal caudal fin, with up and down movements which move them through the water, like cetaceans.
InWestern Europe the first and oldest sirenian remains have been found in a newpaleontological site, in Santa Brígida,Amer (La Selva,Catalonia,Spain[4]). The age is dated by the Shallow Bentic Zones in the Eoceno SBZ 15).
The last of the sirenian families to appear,Trichechidae, are thought to have originated from early dugongids in the late Eocene or early Oligocene. The current fossil record documents some major stages in hindlimb and pelvic reduction (well developed hindlimbs attached to a sacrum (Prorastomidae), well developed hindlimbs without proper sacrum (Protosirenidae) and reduced innominate with hindlimbs reduced or absent (Dugongidae and Trichechidae).[5]
Since sirenians first evolved, they have been herbivores, likely depending onseagrasses and aquaticangiosperms (flowering plants) for food. To the present, almost all have remained tropical (with the notable exception ofSteller's Sea Cow), marine and angiosperm consumers. Sea cows are shallow divers with large lungs. They have heavy skeletons to help them stay submerged; the bones arepachyostotic (swollen) andosteosclerotic (dense), especially the ribs which are often found as fossils.
Eocene sirenians, likeMesozoic mammals but in contrast to otherCenozoic ones, have five instead of four premolars, giving them a 3.1.5.3 dental formula. Whether this condition is truly a primitive retention in sirenians is still under debate.
Although cheek teeth are relied on for identifying species in other mammals, they do not vary to a significant degree among sirenians in their morphology, but are almost always low-crowned (brachyodont) with two rows of large, rounded cusps (bunobilophodont). The most easily identifiable parts of sirenian skeletons are the skull and mandible, especially the frontal and other skull bones. With the exception of a pair of tusk-like first upper incisors present in most species, front teeth (incisors and canines) are lacking in all, except the earliest sirenians.
