A modern whale on the beach faces fairly grim prospects. There was a time, however, when whales moved freely between land and sea. Indeed, the earliest known whales, which date back as far as 50 million years ago, had well-developed hindlimbs and are believed by most paleontologists to have evolved from four-legged terrestrial mammals. Yet details of the transition from whales with large functional legs, such as Ambulocetus (right), to their streamlined descendants with only internal vestigial legs at most, have remained elusive, owing to a paucity of intermediate forms in the fossil record. New fossils described yesterday at the annual meeting of the Society of Vertebrate Paleontology in Mexico City, however, are providing insight into the timing of this extraordinary transformation.

Lawrence Barnes of the Natural History Museum of Los Angeles and his colleagues found in Washington State the bones of an as yet unnamed ancient baleen whale from the so-called Late Oligocene epoch. Included among the remains is part of a pelvis. The 27-million-year-old bone displays a deep, cuplike socket that once held the head of a thigh bone, or femur. This ancient whale, he says, appears to have had small, external legs. Barnes estimates that the legs were about one and a half feet long and might have enabled the 20-foot-long animal to shuffle around on the beach. Its close ancestors, he surmises, had larger legs. Only later, in the Middle Miocen epoch, did whales reach the modern condition of having no external vestigial hindlimbs. Barnes additionally points out that because such whales were long thought not to have legs, fossil whale limbs may well have been overlooked by collectors in the past.