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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.
They greatly affected cetacean evolution , because they spread across Earth's oceans. [7] They had long snouts, large eyes, and a nasal opening located farther up the head than in earlier archaeocetes — suggesting they could breathe with the head held horizontally, similar to modern cetaceans — a first step towards a blowhole.
In cetaceans, evolution in the water has caused changes to the head that have modified brain shape such that the brain folds around the insula and expands more laterally than in terrestrial mammals. As a result, the cetacean prefrontal cortex (compared to that in humans) rather than frontal is laterally positioned.
The following is a list of currently existing (or, in the jargon of taxonomy) 'extant' species of the infraorder cetacea (for extinct cetacean species, see the list of extinct cetaceans). The list is organized taxonomically into parvorders, superfamilies when applicable, families, subfamilies when applicable, genus, and then species.
The limbs of more aquatic Eocene cetaceans did not preserve very well. Ambulocetus demonstrated that cetaceans swam by flexing the spine up and down (undulation) before they had evolved the tail fluke, forelimb propulsion evolved relatively late, and that cetaceans went through an otter-like phase with spinal undulation and hindlimb propulsion ...
It is unclear at present whether protocetids had flukes (the horizontal tail fin of modern cetaceans). However, what is clear is that they are 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 ...
The most basal of marine cetaceans, ... Cetaceans portal; Evolution of cetaceans; Notes References. This page was last edited on 16 November 2024, at 18:55 ...
However, they did not recognize hippopotamuses and classified the ruminants as the sister group of cetaceans. Subsequent studies established the close relationship between hippopotamuses and cetaceans; these studies were based on casein genes , [ 7 ] SINEs , [ 10 ] fibrinogen sequences, [ 11 ] cytochrome and rRNA sequences, [ 5 ] [ 12 ] IRBP ...