Whale evolution. Illustration of six prehistoric whale ancestors and two modern whales (bottom). From top to bottom, the prehistoric genera are: Pakicetus (52 to 48 million years ago), Ambulocetus (47 to 41 million years ago), Remingtonocetus (45 to 43 million years ago), Protocetus (45 to 43 million years ago), Basilosaurus (40 to 33 million years ago) and Dorudon (40 to 33 million years ago). All lived during the Eocene, and form part of the evolutionary tree that resulted in modern cetaceans (whales). Starting from a semi-aquatic habitat, successive stages of evolution resulted in fully aquatic marine mammals, the loss of rear limbs, and the evolution of differing feeding mechanisms. The two modern groupings of whales at lower right are (top to bottom): baleen whales (Mysticeti) and toothed whales (Odontoceti). This version of this image has a colour-coded geologic timescale as its background. For an unlabelled version of this image, see C046/8250.