The living sirenians consist of three species of manatees and one species of dugong.

The Greek name for this order is derived from the sirens of Greek mythology. Sirens were sea nymphs who lured sailors to their island with their mesmerizing songs.

All three species of manatees belong to the family Trichechidae.

The family Dugongidae includes the dugong found in the Indian and western Pacific Oceans. A second species, the Steller's sea cow was discovered in 1741 and hunted to extinction by 1768. This species lived primarily in the Commander Islands of the western Bering Sea.

Trichechus manatus, the West Indian manatee. There are two subspecies of the West Indian manatee: the Florida manatee (Trichechus manatus latirostris) and the Antillean manatee (Trichechus manatus manatus). These subspecies are distinguished by differences in cranial measurements and by their geographic distribution.

Trichechus senegalensis, the West African manatee. About the same size and shape of the West Indian manatee, the West African manatee differs in some important respects: position of the eyes, snout, and cranial bones.

Trichechus inunguis, the Amazonian manatee. The Amazonian manatee is the smallest of the manatees. Several physical characteristics distinguish it from the other two species. It lacks nails on its pectoral fins, and usually has whitish patches on its underside.

The fossil record of sirenians is incomplete, and the relationship between manatees and their ancestors is poorly known.

Even though the oldest known sirenian fossils were found in Jamaica (Prorastomus sirenoides), it is likely that sirenians originated in Eurasia or Africa. During the middle Eocene period (45 to 50 million years ago), the ancestors of manatees probably reached South America (Savage et al., 1994).

Studies using biochemical analysis of proteins show that the closest modern relatives of sirenians are elephants, aardvarks, and small mammals known as hyraxes. These four orders of mammals are sometimes considered "subungulates", meaning they may be evolutionary offshoots of a primitive ungulate (hoof) stock. The anatomical characteristics they share include the lack of a collar bone and the presence of nails instead of claws. Scientists once mistakenly thought manatees were closely related to walruses based on facial similarities (Reynolds and Odell, 1991).