What is the medium sized marine animal that is part of the order of placental mammals?
Dugong The dugong ( , ; "Dugong dugon") is a medium-sized marine mammal. It is one of four living species of the order Sirenia, which also includes three species of manatees. It is the only living representative of the once-diverse family Dugongidae; its closest modern relative, Steller's sea cow ("Hydrodamalis gigas"), was hunted to extinction in the 18th century. The dugong is the only strictly marine herbivorous mammal.
Desmostylia The Desmostylia (from Greek δεσμά "desma", "bundle", and στῦλος "stylos", "pillar") are an extinct order of aquatic mammals that existed from the late Oligocene (Arikareean) to the late Miocene (Tortonian) ().