EVOLUTIONARY HISTORY

 

Pictorial Representation Of Evolution


          The Major group of marine mammals have separate evolutionary origins, from different groups of terrestrial mammals. The cetaceans arose >50 million years ago, and they are now universally thought to be monophyletic (all arising from the same ancestor). 

           There were 3 major phase of cetacean radiation.

The first occurred about 45-53 MYA, (Eocene) in the shallow, warm, tropical waters of the ancient Tethys sea. It involved the initial radiation of the most primitive cetaceans, the archaeocetes (now extinct).It included the appearance of Ambulocetus, a 4-meter walking protocetacean, which has been seen as “Missing link” in cetacean evolution.

The Second major phase resulted in initial radiations of the odontocetes (toothed whales) and mysticetes (baleen whales), a bout 25-35 MYA, (Oligocene).

The final radiation, in the Miocene about 12-15 MYA, involved the appearance of modern cetacean, especially the delphinoids and balaenopterids.

Traditionally, a diphyletic Origin of the pinnipeds was proposed with walruses and eared seals evolving from ursid (bear like) ancestors, and true seals originating from mustelid otter-like ancestors.

Monophyletic origin of pinnipeds from an aquatic carnivore most probably ursid ancestor in north pacific about 30-35 MYA. The fossil record goes back at least 25-27 MYA.

The sirenians have a long evolutionary history with fossil record of > 50 MYA.

Picture From Research Gate

Some types of marine mammals are very long lived and among the mammals reaching the greatest ages. It includes the baleen whales, the larger toothed whales, some dolphins, Killer whale a few pinniped and sirenians. May routinely live to be as old as humans nearly 80-90 years. On the other hand, some species such the porpoises have a short life span rarely reaching the age of 20 years.

Picture From Nature Genetics



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