Noun
a family of mammals of order Sirenia including dugongs and Steller's sea cow
Source: WordNetHydrodamalis gigas, Steller's sea cow, could reach lengths of 8 m. citation The three manatee species (family Trichechidae) and the dugong (family Dugongidae) are endangered species. Source: Internet
No fossils exist of other members of the Dugongidae. Source: Internet