It's true. Manatees and their closest relative, the dugongs, are in the Order Sirenia. The term "Sirenia" is derived from the word "siren" which refers to mermaid-like creatures that stories say the sailors of long ago saw at sea (including Christopher Columbus.)
Despite their name, and despite having some similarities to the general body shape and habitat of other marine mammals (like sea lions, walruses or dolphins), manatees are not related to other marine mammals at all. Scientists say there is no "evolutionary relationship" between manatees/dugongs and marine mammals.
Forget the Ungulate
These four species--manatees, elephants, hyraxes and aardvarks--are referred to as "subungulates", meaning they all evolved from a primitive "ungulate" ancestor. An "ungulate" refers to a hoofed mammal, and includes the species in the former order ungulata, such as horses, cattle, deer, swine and elephants. Their common evolutionary background has been studied using biochemical analysis of proteins.
The earliest animal that had a manatee-like appearance is thought to be the Potamosiren, which dates from about 15 million years ago. At that time, that animal did not have the continual tooth replacement adaptation.
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