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[1] The evolution of cetaceans is thought to have begun in the Indian subcontinent from even-toed ungulates (Artiodactyla) 50 million years ago (mya) and to have proceeded over a period of at least 15 million years. [2] Cetaceans are fully aquatic mammals belonging to the order Artiodactyla and branched off from other artiodactyls around 50 mya.
Marine mammal adaptation to an aquatic lifestyle varies considerably between species. Both cetaceans and sirenians are fully aquatic and therefore are obligate water dwellers. Pinnipeds are semiaquatic; they spend the majority of their time in the water but need to return to land for important activities such as mating, breeding and molting.
Other aquatic mammals, such as the Indian rhinoceros, were targets for sport hunting and had a sharp population decline in the 1900s. After it was made illegal, many aquatic mammals became subject to poaching. Other than hunting, aquatic mammals can be killed as bycatch from fisheries, where they become entangled in fixed netting and drown or ...
The traits enabled animals to check area on land for safe spots if being chased by a predator in water, as well as being useful for searching for prey items above the water. The water-based lateral line system was used substantially by these aquatic tetrapods to detect danger from predators. [ 2 ]
Cetacea (/ s ɪ ˈ t eɪ ʃ ə /; from Latin cetus ' whale ', from Ancient Greek κῆτος () ' huge fish, sea monster ') [3] is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins and porpoises.
Cetaceans are descended from land-dwelling hoofed mammals, and the now extinct archaeocetes represent the several transitional phases from terrestrial to completely aquatic. [1] Historically, cetaceans were thought to have descended from the wolf-like mesonychians , but cladistic analyses confirm their placement with even-toed ungulates in the ...
These animals are called "secondarily aquatic" because although their ancestors lived on land for hundreds of millions of years, they all originally descended from aquatic animals (see Evolution of tetrapods). These ancestral tetrapods had never left the water, and were thus primarily aquatic, like modern fishes.
It was a wolf-like mammal, [3] about 1–2 m (3 ft 3 in – 6 ft 7 in) long, [4] and lived in and around water where it ate fish and other animals. The name Pakicetus comes from the fact that the first fossils of this extinct amphibious whale were discovered in Pakistan.