Desmostylia is an extinct order of aquatic mammals native to the North Pacific from the Early Oligocene (Rupelian) to the Late Miocene (Tortonian) from 33.9 to 7.2 or 5.3 Ma. However, phylogenetic models suggests the order first appeared during Middle Eocene between 47 and 39 Ma. Desmostylians are the only known extinct order of marine mammals. Their taxonomic placement within Placentalia is subject to considerable debate.