Despite their vast differences in their current appearance, the Sirenia, Hyracoidea, and Proboscidea are fairly closely related, in the grand scheme of things. The Paenungulata lived on the tidal shores of the Tethys Ocean. One line began to specialize in swimming and grazing on the shallow continental shelf, one began to specialize in browsing (eating from trees), and one began to specialize in grazing (eating grass and ground shrubs). Over time, these evolved into the three orders in the clade.
American Animals. Witmer Stone and William Everett Clam, 1905.
The Animal Kingdom. Hugh Craig, 1902.