Sirenians, along with Proboscidea (elephants), group together with the extinct Desmostylia and likely the extinct Embrithopoda to form the Tethytheria. Tethytheria is thought to have evolved from primitive hoofed mammals ("condylarths") along the shores of the ancient Tethys Ocean.
Tethytheria, combined with Hyracoidea (hyraxes), forms a clade called Paenungulata. Paenungulata and Tethytheria (especially the latter) are among the least controversial mammalian clades, with strong support from morphological and molecular interpretations. The ancestry of Sirenia is remote from that of Cetacea and Pinnipedia, although they are thought to have evolved an aquatic lifestyle around the same time.