Abstract: The Order Sirenia (seacows, herbivorous marine mammals) have a relatively good fossil record in the formerly tropical or subtropical latitudes of the Earth and extending over some 50 million years. Their known diversity (now comprising two living and two extinct families) has significantly increased with recent fossil discoveries; and they now provide one of the best-documented examples of re-adaptation by terrestrial animals to aquatic life. Their phylogenetic relationships have been elucidated by cladistic analysis, and much of their history is now amenable to interpretation in terms of testable functional hypotheses and evolutionary scenarios. This history is now even shedding light on the otherwise poorly-documented history of the aquatic plants on which they have fed.

The Order Desmostylia comprises a few genera of extinct, hippopotamus-like marine mammals that inhabited the North Pacific Ocean in the Oligocene and Miocene. Like the Sirenia, they were closely related to proboscideans; but they probably resembled ground sloths in appearance, gait, and habits, foraging for aquatic vegetation in inter-tidal and sub-tidal coastal habitats.