Cetacean evolution

The evolutionary history of cetaceans is thought to have occurred in the Indian subcontinent from even-toed ungulates 50 million years ago, over a period of at least 15 million years. Cetaceans are fully aquatic marine mammals belonging to the order Artiodactyla, and branched off from other artiodactyls around 50 mya (million years ago). Cetaceans are thought to have evolved during the Eocene or earlier, sharing a closest common ancestor with hippopotamuses. Being mammals, they surface to breathe air; they have 5 finger bones (even-toed) in their fins; they nurse their young; and, despite their fully aquatic life style, they retained many skeletal features from their terrestrial ancestors. Discoveries starting in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea.

The two modern parvorders of cetaceans – Mysticeti (baleen whales) and (toothed whales) – are thought to have separated from each other, from the archaeocetes, around 28-33 million years ago in a second cetacean radiation, the first occurring with the archaeocetes. The adaptation of echolocation in toothed whales distinguishes them from fully aquatic archaeocetes and early baleen whales. The presence of baleen in baleen whales occurred gradually, with earlier varieties having very little baleen, and their size is linked to baleen dependence (and subsequent increase in filter feeding).

The aquatic lifestyle of cetaceans first began in the Indian subcontinent from even-toed ungulates 50 million years ago, over a period of at least 15 million years, however a jawbone discovered in Antarctica may reduce this to 5 million years. Archaeoceti is an extinct parvorder of Cetacea containing ancient whales. The traditional theory of cetacean evolution, first proposed by Van Valen in 1966, 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 early whales. However, molecular phylogeny data indicates that whales are very closely related to the artiodactyls, with hippopotamuses as their closest living relative. Because of this, cetaceans and hippopotamuses are placed in the same suborder, Whippomorpha.Cetartiodactyla (formed from the words Cetacea and Artiodactyla) is a proposed name for an order containing both cetaceans and artiodactyls. 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, appeared during the Early Eocene, implying the two groups diverged well before the Eocene. Since molecular analysis identifies artiodactyls as being very closely related to cetaceans, mesonychids are probably an offshoot from Artiodactyla, and cetaceans did not derive directly from them, but possibly sharing a common ancestor.

...
Wikipedia