inland mangroves and coastal flats in West Africa. It is found in waters above 18 °C, and its range spans Senegal to Angola. The dugong (Dugong dugong), the closest living relative of Steller's sea cow, lives in the Indo-West Pacific Ocean in more than 40 different countries. They are coastal animals supported by wide protected sea grass meadows. Steller's sea cow was discovered in 1741 around islands in the Bering Sea and was specialized for cold subarctic temperatures. It ranged from Alaska through the Amchitka and Aleutian Islands, and even to Japan. Steller's sea cow was reported to have congregated in shallow, sandy areas along coastline and mouths of rivers and creeks to feed on kelp. Evolution The evolution of sirenians is characterized by the appearance of several traits that are found in all sirenians. The nostrils are large and retracted, the upper-jaw bone contacts the frontal bone, the sagittal crest is missing, the mastoid fills the supratemporal fenestra (an opening on the top of the skull), there is a drop-like ectotympanic (a bony ring that holds the ear drum), and the bones are pachyosteosclerotic (dense and bulky). Sirenians first appeared in the fossil record in the Early Eocene and diversified throughout the epoch. They inhabited rivers, estuaries, and nearshore marine waters. Sirenians, unlike other marine mammals such as cetaceans, lived in the New World. One of the earliest aquatic sirenians discovered is Prorastomus, which dates back to 40 million years ago, and the first known sirenian, the quadruped Pezosiren, lived 50 million years ago. An ancient sirenian fossil of a petrosal bone was found in Tunisia, dating back to approximately the same time as Prorastomus. This is the oldest sirenian fossil to be found in Africa and supports molecular data suggesting that sirenians may have originated in Africa. Prorastomidae and Protosirenidae, the earliest sirenian families, consisted of pig-like amphibious creatures who died out at the end of the Eocene. With the appearance of the Dugongidae at this time, sirenians had evolved the characteristics of the modern order, including an aquatic, streamlined body with flipper-like fore limbs and no hind limbs, and a powerful tail with horizontal caudal fins which uses an up-and-down motion to move them through the water. The last of the sirenian families to appear, Trichechidae, apparently arose from early dugongids in the late Eocene or early Oligocene. In 1994, the family was expanded to include not