Manatees and dugongs are marine mammals of the order Sirenia. They originated in the middle Eocene period around 45 to 50 million years ago. They are gentle, plant-eating herbivores sometimes nicknamed "sea cows". The name manatee comes from the Haitian word manati. Today, their closest living relatives are elephants [Proboscideans], hyraxes [Hyracoidea] and aardvarks [Tubulidentata]. Manatees are classified as [sub-]ungulates, even though they live in the water. Their thick-skinned, streamlined, almost hairless bodies have forelimbs modified into flippers. The tail is enlarged and horizontally-flattened.