The evolution of whales
(Amateur Geological Society)
Three major groups pf mammals have returned to the sea – pinnipedia (seals & sea lions), sirenia (dugongs & manatees) and cetacean (whales, dolphins & porpoises). This talk is about the evolution of cetaceans, which have long been recognised as mammals despite their fish-like appearance and habits. Aristotle noted that whales were air –breathing and had a different swimming motion and recognised them as mammals.
This talk is based on the work of Philip Gingerich and his students at the University of Michigan.
Whales evolved from ungulates. For many years they were though to have evolved from Sinonyx, a carnivorous ungulate that looked like a wolf, but since the 1990s, evidence has indicated that they are of archaeodactyl descent because of the unique ankle joint (the astralagus) possessed by archaeodactyls and whales. Morphological and molecular evidence suggests that the closest living relative of whales is the hippopotamus, though these did not appear until 12 million years ago, long after whales and are of African origin rather than the Asian origin of whales.
Fossils from Tethys sediments that have been pushed northwards by the collision of India with Asia into north-west Pakistan have provided some of the key links.
Pakicetus is 50 million years old and looked rather like a large rat-faced dog, with ears that are diagnostic of cetaceans. It inhabited freshwater rivers, estuaries and shallow marine areas on the borders of Tethys.
Indohyus from Kashmir was the size of a smallish dog and is 3 million years younger than the oldest Pakicetus. It seems to have run around under water similar to the present-day African chevrotain and had heavy bones to keep it down.
Ambolocetus natans is clearly a cetacean with functional legs and a skeleton that allowed walking on land. Iy had an astralagus and the beginning of a channel between the jaw and ear.
Rhodocetus, from the Middle Eocene, was 9 feet long and semi-aquatic. It had a large skull, no external ears, a short neck and a smaller pelvis than its predecessors. It is found in Africa and North America as well as Pakistan.
Maiacetus inuus is from the same family but is thinner. The presence of a foetus with teeth and in position for head-first birth (as opposed to the tail-first birth of whales) suggests it probably bred on land.
In 1832, Judge Bry in Louisiana discovered a sea monster (Basilosaurus), which was first thought to be a reptile – hence its name – but Richard Owen found it to be a whale. 45 million years old, it looked like a whale, was fully marine and had tiny back legs but it lacked a melon organ (used by whales for echo-location) and had only a small brain. It had two types of teeth, the front ones were single pegs and the rear ones multiple. Specimens have been found in Egypt, Pakistan and North America and bones from it have been found at Barton-on-sea.
Durodon was frst thought to be a young Basilosaurus when found in the Zeuglodon valley in Egypt. It was possibly an early dolphin.
Aetiocetus, of Upper Miocene age had teeth and baleen.
The development of Archaeodactyls through these Archaeoceta to modern cetaceans required a number of major changes, which can be summarised as:
• Salt regulation is achieved by more efficient kidneys which retain more of the water drunk while excreting salt and whale skin allows water to penetrate but rejects salts (osmosis);
• Breathing has significant changes with nasal drift back to the position of the blowhole, with the exception of sperm whales which retain a frontal position for the blowhole. This drift can also be seen in embryos;
• Reduction of rear legs to vestigial traces. This can also be seen in embryos;
• Changes in the ear, with the development of the involucrum and separation of the jaw from the skull;
• Balance required changes in the inner ear with the development of a much smaller cochlea.
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