Sirenia (redirected from Sea cows)

Sirenia

An order of herbivorous aquatic placental mammals, commonly known as sea cows, that includes the living manatees and dugongs and the recently exterminated Steller's sea cow. The order has an extensive fossil record dating from the early Eocene Epoch, some 50 million years ago.

The earliest known sirenians were quadrupedal and capable of locomotion on land. Fossils clearly document the evolutionary transition from these amphibious forms to the modern, fully aquatic species, which have lost the hindlimbs and transformed the forelimbs into paddlelike flippers. The living species have streamlined, fusiform bodies with short necks and horizontal tail fins like those of cetaceans, but no dorsal fins. The skin is thick and nearly hairless. The nostrils are separate, and the ears lack external pinnae.

Sirenians typically feed on aquatic angiosperms, especially seagrasses, but in ecologically marginal situations they also eat algae and even some animal material. They are normally found in tropical or subtropical marine waters, but some have become adapted to fresh water or colder latitudes. Body sizes have ranged from less than 3 m up to 9–10 m (30–33 ft). Sirenians mate and give birth in the water, bearing a single calf (occasionally twins) after about 13–14 months of gestation and then nursing it from one pair of axillary mammae. The closest relatives of sirenians among living mammals are the Proboscidea (elephants).