Evolution of sirenians
Sirenia is the order of placental mammals which comprise modern "sea cows" (manatees and the Dugong) and their extinct relatives. They are the only herbivorous marine mammals now in existence and the only group of herbivorous mammals to have became completely aquatic. Sirenians have a 50 million year old fossil record (early Eocene-recent). They attained modest diversity during the Oligocene and Miocene, but have since declined as a result of climatic cooling, oceanographic changes, and human interference. Two genera and four species are extant: Trichechus which includes the three species of manatee that live along the Atlantic coasts and in rivers and coastlines of the Americas and western Africa, and Dugong which is found in the Indian and Pacific oceans.
Early origins
Sirenians, along with their closest living relatives the Proboscidea (elephants), group together with the extinct Desmostylia and likely the extinct Embrithopoda to form the Tethytheria. Tethytheria appears to have evolved from primitive hoofed mammals known as condylarths, along the shores of the ancient Tethys sea.
Tethytheria, combined with Hyracoidea (hyraxes) forms an inclusive group called Paenungulata. Paenungulata and Tethytheria (especially the latter) are among the least controversial mammalian orders, with strong support from morphological and molecular research. The ancestry of Sirenia is remote from that of Cetacea and Pinnipedia, though evolving an aquatic lifestyle around the same time.
Fossil history
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 the Old World.
The earliest known sea cows, of the families Prorastomidae and Protosirenidae, 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 the Dugongidae; sirenians had acquired their familiar fully-aquatic streamliined 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.
The last of the sirenian families who made their appearance, Trichechidae, apparently arose from early dugongids in the late Eocene or early Oligocene. The current fossil record documents all major stages in hindlimb and pelvic reduction from completely terrestrial morphology to the extreme reduction in the modern manatee pelvis, providing an example of dramatic evolutionary change among fossil vertebrates.
Since sirenians first evolved, they were herbivores, likely depending on seagrasses and aquatic angiosperms (flowering plants) for food. To the present, almost all have remained tropical, marine and angiosperm consumers. Sea cows are shallow divers with large lungs. They have heavy skeletons to help them stay submerged; the bones are pachyostotic (swollen) and osteosclerotic (dense), especially the ribs which are often found as fossils.
Eocene sirenians, like Mesozoic mammals but in contrast to other Cenozoic 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.
References
- Sirenian Evolution, Daryl P. Domning; Encyclopedia of Marine Mammals Perrin, Würsig and Thewissen
- Origin of Sirenians