National Geographic : 2001 Nov
LOSING THEIR LEGS Going . . .going . . . but not quite gone. The hind legs that helped propel whales' four footed land-dwelling ancestors have become just nubs of their former selves. Pakicetus walked, but Ambulocetus was more versatile. With its pelvis fused to its backbone, it could support its weight on land, while webbed feet gave it added power underwater as it swam, perhaps by undulating its spine otter-style. Ten million years later, the Four-legged Ambulocetus, 49 m.y.a.