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It is divided into toothed whales (Odontoceti) and baleen whales (Mysticeti), which diverged from each other in the Eocene some 50 million years ago (mya). Cetaceans are descended from land-dwelling hoofed mammals, and the now extinct archaeocetes represent the several transitional phases from terrestrial to completely aquatic. [1]
It is possible that some protocetids had flukes. However, it is clear that they were adapted even further to an aquatic life-style. In Rodhocetus, for example, the sacrum (a bone that, in land-mammals, is a fusion of five vertebrae that connects the pelvis with the rest of the vertebral column) was divided into loose vertebrae. However, the ...
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.
Whales evolved from land-living mammals, and must regularly surface to breathe air, although they can remain underwater for long periods of time. Some species, such as the sperm whale, can stay underwater for up to 90 minutes. [2] They have blowholes (modified nostrils) located on top of their heads, through which air is taken in and expelled.
A few toothed whales, such as some orcas, feed on mammals, such as pinnipeds and other whales. Toothed whales have well-developed senses – their eyesight and hearing are adapted for both air and water, and they have advanced sonar capabilities using their melon. Their hearing is so well-adapted for both air and water that some blind specimens ...
Though it was known that cetaceans descended from land mammals before the discovery of Ambulocetus, the only evidence of this in the fossil record was the 52-million-year-old (fully terrestrial) Pakicetus and the Paleocene mesonychians (as there was a hypothesised link between cetaceans and mesonychians). The limbs of more aquatic Eocene ...
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Though the whale may have been primarily aquatic, the discovery of ankle bones lends to the idea that this fossil may have been a transition between sea-based and land-based mammals. While whales eventually returned to the sea, the anthracotheres , ancestors of the hippopotamus , are thought to have descended from an ancestor shared with the whale.