[An answer adapted from Goku's Q&A, less rigorous, but largely correct.] The original title was this: Were cetaceans, pingos, and manatees a variety of independent evolutions into marine mammals? Or is it diverging after entering the ocean? 】
As the subject says, marine mammals can be divided into three tax groups: cetaceans, manatees, and pingopods. The first two are all-aquatic and do not land for life; Pinnipedes are semi-aquatic, living most of their lives in water, and only come to land or ice during mating, calving and molting. They share the common feature of a streamlined body and specialized fins on the forelimbs, but from an evolutionary point of view, there is no close relationship between the three.
Hippopotamus, scientific name Hippotamus amphibius, cetacean hooferaceae
Sperm whale (specimen), scientific name Physeter macrocephalus, whale puppet hoof
More than 90 species of whales are all meat-eating guys who don't blink (is there a mistake to think that baleen whales are vegetarian?). Surprisingly, they originated from primitive ungulates. I didn't expect that the genus of stupid pigs and stupid cows had climbed such a tall relative, which was really fluffy. Among the living ungulates, hippos and cetaceans are most closely related. In the latest taxonomic methods, cetaceans and even-hoofed orders have merged, called cetaceans. From then on, "Blue Sea Long Whale" and "Return to the Back of the Ox" are similar moods...
Blindfolded marten, scientific name Mustela putorius furo, carnivorous ferret family
Spotted seal, scientific name Phoca largha, carnivorous seal family
There are more than 40 species in 3 families (Sea Lion Family, Seal Family and Walrus Family), which evolved from ancient carnivores and may be closer to the Bear Family and the Ferret Family. When I was a child, I read a Japanese popular science book "Animal Ecology", which listed the pinniped order as a pinniped suborder under the carnivorous order, indicating that this division was once recognized; now the pinnipeds have been officially reduced to the pinniped superfamily, belonging to the suborder of carnivorous canines.
African elephant, scientific name Loxodonta africana, proboscis
Dugong, scientific name Dugong dugon, manatee dugong family
Cetaceans and pingos, although not relatives, are under the general order lauya, and the manatee belong to the African order. The manatee family is not prosperous, and there are only 2 families and 4 species left: manatees and dugongs; among the living animals, the closest to the manatees are proboscis, that is, african elephants and Asian elephants. The dugong in the picture above was photographed by a friend in Australia, and there is no such thing as a domestic aquarium. Look at it twice, this is the legendary "mermaid".
Cetaceans, pinnipeds and manatees do not include all marine mammals, and the Ferret family of sea otters (Enhydra lutris) are missing, which most of the time live in the sea and may evolve into pinnipeds over time.