|
The cetaceans (whales, dolphins and porpoises) are marine mammal descendants of land mammals. Their terrestrial origins are indicated by: * Their need to breathe air from the surface; * The bones of their fins, which resemble the limbs of land mammals * The vertical movement of their spines, characteristic more of a running mammal than of the horizontal movement of fish. The question of how a group of land mammals became adapted to aquatic life was a mystery until discoveries starting in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea. ==Earliest ancestors== The traditional theory of cetacean evolution was that whales were related to the mesonychids, an extinct order of carnivorous ungulates (hoofed animals) that resembled wolves with hooves and were a sister group of the artiodactyls (even-toed ungulates). This theory arose due to similarities between the unusual triangular teeth of the mesonychids and those of whales. However, more recent molecular phylogeny data indicate that whales are more closely related to the artiodactyls, with hippopotamus as the closest living relative.〔 The strong evidence for a clade combining cetaceans and artiodactyls is further discussed in the article Cetartiodactyla. However, the earliest anthracotheres, the ancestors of hippos, do not appear in the fossil record until the Middle Eocene, millions of years after ''Pakicetus'', the first known whale ancestor, appears during the Early Eocene, implying the two groups diverged well before the Eocene. The molecular data is supported by the recent discovery of ''Pakicetus'', the earliest proto-whale (see below). The skeletons of ''Pakicetus'' show that whales did not derive directly from mesonychids. Instead, they are artiodactyls that began to take to the water soon after artiodactyls split from mesonychids. Proto-whales retained aspects of their mesonychid ancestry (such as the triangular teeth) which modern artiodactyls have lost. An interesting implication is that the earliest ancestors of all hoofed mammals were probably at least partly carnivorous or scavengers, and today's artiodactyls and perissodactyls became herbivores later in their evolution. By contrast, whales retained their carnivorous diet, because prey was more available and they needed higher caloric content in order to live as marine endotherms. Mesonychids also became specialized carnivores, but this was likely a disadvantage because large prey was not yet common. This may be why they were out-competed by better-adapted animals like the creodonts and later Carnivora which filled the gaps left by the dinosaurs. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Evolution of cetaceans」の詳細全文を読む スポンサード リンク
|