Fig. 2

A gradual evolution of whale traits. aPakicetus, the terrestrial “Pakistan whale” lived about 50 Mya. Features of the inner ear place this animal at the base of whale evolution. b The amphibious Ambulocetus flourished a few million years after Pakicetus and displayed many more typical cetacean characteristics, but like all fossils found from this period so far, still had fully developed hindlimbs. cRodhocetus likewise had full hind limbs, but a more weakly anchored pelvis, and the sacral vertebrae that are fused in other mammals were loose. d By 38 Mya several species of fully marine whales such as Basilosaurus grew to a very large size, up to 17 m. Its pelvis was no longer connected to the vertebral column, and the tiny remaining hindlimbs were probably used primarily as mating claspers. e The diminutive hind limb parts of later whales such as the North American Squalodon (33–14 Mya) probably showed no external evidence, as in all extant cetaceans. Notice that whales had entered the water long before we find evidence of lost or even reduced limbs. This sequence does not necessarily represent a chain of direct ancestors and descendents. It does, however, exemplify the successive evolutionary changes that some branches of the whale lineage underwent between the Eocene and the Miocene periods, leading to extant whales. (Artist renditions by Nobu Tamura, used with permission. Images not to scale)