What is the medium sized marine animal that is part of the order of placental mammals?
Evolution of sirenians Sirenia is the order of placental mammals which comprises modern "sea cows" (manatees and the Dugong) and their extinct relatives. They are the only extant herbivorous marine mammals and the only group of herbivorous mammals to have become completely aquatic. Sirenians are thought to have a 50-million-year-old fossil record (early Eocene-recent). They attained modest diversity during the Oligocene and Miocene, but have since declined as a result of climatic cooling, oceanographic changes, and human interference. Two genera and four species are extant: "Trichechus", which includes the three species of manatee that live along the Atlantic coasts and in rivers and coastlines of the Americas and western Africa, and "Dugong", which is found in the Indian and Pacific oceans.
Verticordiidae Verticordiidae is a taxonomic family of very small saltwater clams, marine bivalve molluscs in the order Pholadomyoida.