The family has one surviving species, the dugong (Dugong dugon), one recently extinct species, Steller's sea cow (Hydrodamalis gigas), and a number of extinct genera known from fossil records.

Heart rate readings are from 40 to 96 bpm and vary between individual dugongs.

[2] The genera Eosiren, Eotheroides, and Prototherium have been assigned to Halitheriinae in the past, but recent cladistic analysis recovers these genera as basal to the clade formed by Trichechidae and Dugongidae.

Moreover, Halitheriinae is paraphyletic with respect to Dugonginae and Hydrodamalinae, and further use of the name should be discontinued because the type genus is based on a non-diagnostic tooth.

[8] Orycteropodidae Macroscelididae Chrysochloridae Tenrecidae Procaviidae Elephantidae Dugongidae Trichechidae