Lifestyle transitions are major evolutionary events often reflected by extreme morphological modifications. One particularly obvious example is the secondary adaptation of whales to an aquatic lifestyle. Toothed and baleen whales (Cetacea) are included in the Cetartiodactyla clade with even-toed ungulates (“Artiodactyla”). Their early fossil record documents species within the paraphyletic “Archaeoceti” that are crucial to understand this major evolutionary transition from the terrestrial to the aquatic environment. Members of the earliest known family, the Pakicetidae, were mostly terrestrial with some adaptations to water. They lived during the early Eocene (ca. 50 million years ago). And it is mostly during the middle to late Eocene when the quadrupedal whales “Protocetidae”

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