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EARLIEST ANCESTORS Before the recent discoveries in Pakistan, one popular theory of cetacean evolution was that whales were related to the Mesonychid s, an extinct order of Carnivorous Ungulates (hoofed animals), which looked rather like wolves with hooves. These animals possessed unusual triangular teeth that are similar to those of whales. For this reason, scientists had long believed that whales evolved from a form of mesonychid. However, DNA analysis generated an alternative hypothesis. Whale DNA is more similar to that of the Hippopotamid s than to any other living animal. Therefore, a debate arose as to whether hippopotamuses (hippos) or mesonychids were the closest relatives of the whales. The recent discovery of '' Pakicetus '', the earliest proto-whale (see below) has helped to settle the debate. The skeletons of ''Pakicetus'' demonstrate that whales did not derive directly from mesonychids. Instead, they are a form of Artiodactyl (another type of ungulate) that began to take to the water after the artiodactyl family split from the mesonychids. In other words, the proto-whales were early artiodactyls that retained aspects of their mesonychid ancestry (such as the triangular teeth) which modern artiodactyls have since lost. An interesting implication is that the earliest ancestors of all hoofed mammals were probably at least partly carnivorous or scavengers, today's artiodactyls and Perissodactyl s only having switched to a plant diet later in their evolution. Whales, due to the readier availability of animal prey and their need for higher caloric content to live as marine endotherms, naturally retained their carnivory, as did mesonychids, who were however out-competed by better-adapted animals like the Carnivora later on (mesonychids became specialized carnivores when the overall availability of large animal prey was still low; thus their adaptation was likely at a disadvantage when new forms had filled the gaps left by the Dinosaur s). Hippos are artiodactyls too, but the new discovery suggests that the origins of whales and hippos are not directly related. The reason for the physical and genetic similarities between them is that hippos split off from the main Artiodactyl line shortly after the proto-whales did, and thus, like whales, hippos retain some characteristics of early Artiodactyls. Both hippos and whales are Artiodactyls that became adapted to life in the water, but they did so separately and evolved in quite different directions. PAKICETIDS: THE EARLIEST CETACEANS? The is formed from the Ectotympanic Bone only, a feature shared by modern whales. It was initially thought that the ears of ''Pakicetus'' were adapted for underwater hearing, but, as would be expected from the anatomy of the rest of this creature, the ears of ''Pakicetus'' are specialized for hearing on land, and if ''Pakicetus'' is related to the ancestors of whales, underwater hearing must have been a later adaptation2. According to Thewissen, the teeth of ''Pakicetus'' also resemble the teeth of fossil whales, which is another link to more modern whales Whale Origins . AMBULOCETIDS AND REMINGTONOCETIDS The most remarkable of the recent discoveries in Pakistan has been '' Ambulocetus '', which looked like a three-metre long mammalian Crocodile . ''Ambulocetus'' was clearly amphibious, as its back legs are better adapted for swimming than for walking on land, and it probably swam by undulating its back vertically, as otters, seals and whales do. It has been speculated that Ambulocetids hunted like crocodiles, lurking in the shallows to snatch unsuspecting prey. A smaller cousin of ''Ambulocetus'' was the Remingtonocetid family, which had longer snouts than Ambulocetus', and were slightly better adapted for underwater life. It has been speculated that they lived like modern Sea Otter s, hunting for fish in the shallows. PROTOCETIDS The Protocetid s, which include '' Rodhocetus '' and '' Artiocetus '', are another recent discovery. They lived around 45 million years ago. Their principal adaptation was Fluke s (horizontal bars) on their tails, which enable faster swimming; however, they retained substantial hind legs. They lived in shallow seas, and may have had a similar lifestyle to seals, or even dolphins; it is not known whether they ever came onto the land. BASILOSAURIDS AND DORUDONTIDS: FULLY MARINE CETACEANS '' Basilosaurus '' (discovered in 1840 and initially mistaken for a lizard, whence its name) and '' Dorudon '' lived around 38 million years ago, and were fully recognisable whales which lived entirely in the ocean. ''Basilosaurus'' was a monstrous creature, up to 18m long; dorudontids were within the range of modern cetacean size, about 5m long. Although they look very much like modern whales, basilosaurids and dorudontids lacked the ' Melon Organ ' that allows their descendants to sing and use Ultrasound as effectively as modern whales. They had small brains; this suggests they were solitary and did not have the complex social structure of modern whales. ''Basilosaurus'' had two tiny but well-formed hind legs which were probably used as Clasper s when Mating ; they are a small reminder of the lives of their ancestors. REFERENCES SEE ALSO EXTERNAL LINKS |
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