Abstract
Cetaceans (whales, dolphins, and porpoises) are an order of mammals that originated about 50 million years ago in the Eocene epoch. Even though all modern cetaceans are obligate aquatic mammals, early cetaceans were amphibious, and their ancestors were terrestrial artiodactyls, similar to small deer. The transition from land to water is documented by a series of intermediate fossils, many of which are known from India and Pakistan. We review raoellid artiodactyls, as well as the earliest families of cetaceans: pakicetids, ambulocetids, remingtonocetids, protocetids, and basilosaurids. We focus on the evolution of cetacean organ systems, as these document the transition from land to water in detail.
Highlights
Whales, dolphins, and porpoises together constitute the Cetacea (English: cetaceans)
In spite of the variation in body size, all modern Cetacea are relatively similar in shape: they have a horizontal tail fluke used in swimming; their forelimbs are flippers; there are no external hind limbs; their neck is short, and their body is streamlined
Writing before the discovery of Ambulocetus, Fish predicted that the swimming mode of modern cetaceans was preceded by a swimming mode that included dorsoventral sweeps of the feet aided by a long tail, similar to otters
Summary
Dolphins, and porpoises together constitute the Cetacea (English: cetaceans). Skeletons of pakicetids are composites based on bones from a number of different individuals, identified based on their size, their similarity to other primitive whales, the chemical composition of the bones, and the relative abundance at their locality. The skulls show that the orbits (the sockets of the eyes) of these cetaceans were located close together on top of the skull, as is common in aquatic animals that live in water but look at emerged objects.
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