The Sirenia (commonly referred to as sea cows) are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. Four species are living, in two families and genera. These are the dugong (one species) and manatees (three species). Sirenia also include Steller's sea cow, extinct since the 18th century, and a number of taxa known only from fossils. The order evolved during the Eocene, more than 50 million years ago. Their forelimbs are modified to form flippers, their hindlimbs are reduced to nothing more than a vestigial pelvis, and their tail is enlarged and flattended horizontally to form a fluke or paddle. Sirenians are massive, sometimes weighing over 1150 kg. Their body is streamlined and mostly nearly hairless.