Marine Mammal Protection Act Indian manatee, sea otter, polar bear, and Pacific walrus. Species not present in U.S. waters include the West African and Amazonian manatee, dugong, Atlantic walrus, and marine otter. In efforts to conserve and manage marine mammal species, the Service has appointed field staff dedicated to working with partners to conduct population censuses, assess population health, develop and implement conservation plans, promulgate regulations, and create cooperative relationships internationally. Various Marine Mammal Management offices are located on either coast. The Service's Marine Mammal Management office in Anchorage, Alaska is responsible for the management and conservation of polar bears, Pacific walruses, and northern
Mammal classification through the new concept of cladistics. Though field work gradually made Simpson's classification outdated, it remained the closest thing to an official classification of mammals. See list of placental mammals and list of monotremes and marsupials for more detailed information on mammal genera and species. Molecular studies by molecular systematists, based on DNA analysis, in the early 21st century have revealed new relationships among mammal families. Classification systems based on molecular studies reveal three major groups or lineages of placental mammals- Afrotheria, Xenarthra, and Boreotheria- which diverged from early common ancestors in the Cretaceous. The relationships between these three lineages