National Geographic : 2001 Nov
up-to-60-foot-long snakelike Basilosaurus emerged with shrunken legs. Some evidence suggests that the diminished limbs-still complete down to jointed knees and toes functioned as copulatory guides. Today whale hind parts are tiny and internal, Basilosaurus with reduced but well-developed hind limbs, 37 m.y.a. acting as an anchor for the muscles of the genitalia. Still, the animals retain the genes that code for longer extremities. Occasionally a modern whale is born having sprouted a leg or two-a genetic throwback known as an atavism.