The ancestors of cetaceans began to move into the water and develop aquatic characteristics about 50 million years ago. They have been evolving ever since, gradually becoming more and more aquatic. Today's cetaceans live entirely in water. The ancestors of pinnipeds began moving into water much more recently, about 23 million years ago. Because of this, modern pinnipeds are not as fully aquatic as cetaceans. The ancestors of sirenians began moving into the water about 40 million years ago, and have been evolving since then.
Think of it as the same thing as land mammals versus water mammals. Marine mammals first evolved to life on land and then adapted to return to the water, but not all mammals did that. Similar thing with birds. All birds evolved from common flying ancestors, whereas flying mammals evolved from land mammals at a much later point in evolutionary history. Hope that helps.
Marine mammals do not live on land, and no marine mammals lay eggs. Marine mammals include whales, dolphins and porpoises.
Marine mammals such as whales, dolphins and porpoises, are collectively labelled cetaceans (of the order Cetacea, comprising 87 modern species). Cetaceans appear to have evolved from a land mammal that began adapting to marine environments 50,000,000 years ago; the earliest known 'proto whale' (ancestor of modern Cetaceans) is named Pakicetus- a land-based quadruped that eventually evolved into Ambulocetus, which lived in shallow waters but were still able to walk on land.
Marine mammals live and water and they need to surface to breathe. While land mammals can breathe just as humans do.
With skin and egg shells able to retain water, reptiles could remain out of water indefinitely, unlike their amphibian ancestors. Mammals and dinosaurs (from which birds evolved) later evolved from reptiles.
well over time the land animals change a little and they need to be underwater to live unlike their land animal ancestors
Not as whales, no. Marine (sea) mammals (whales, dolphins, etc.) are descended from terrestrial (land) mammals, but they're not identical with those land mammals... among the more obvious changes, they've lost their legs. The related links section has a link to a Wikipedia page showing what some of the distant ancestors of whales might have looked like.
Land dwelling mammals called archaeocetes evolved to live in the ocean around 50 million years ago. Their descendants include dolphins and whales. Sea turtles, also, evolved from land dwelling turtles more than 110 million years ago.
Evolved from lobe-finned fish, tetrapods are a group of vertebrates that includes amphibians, reptiles, birds, and mammals. One of the key characteristics that distinguish them is having limbs with digits, which evolved from the lobed fins of their fish ancestors. This transition from water to land was a crucial step in the evolutionary history of vertebrates.
Theory of evolution
There are no land mammals in or on Antarctica. There are only marine mammals.
The habitats of mammals are quite diverse depending on the particular species. Most of them are land mammals but there are some which are marine mammals like whales and others which are amphibious like hippos.