Mr. Brian Switek (student in ecology and evolution at Rutgers) says in “Maiacetus, the good mother whale” on Scienceblogs.com:
When the English anatomist William H. Flower proposed that whales had evolved from terrestrial ungulates in 1883 he cast doubt upon the notion that the direct ancestors of early whales chiefly used their limbs for swimming. If they did, Flower reasoned, whales would not have evolved their distinctive method of aquatic locomotion, typified by vertical oscillations of their fluked tails. Instead Flower suggested that the stock that gave rise to whales would have had broad, flat tails that paved the way for cetacean locomotion as we know it today, and he closed his lecture with a vision of such creatures shuffling about the water’s edge;
We may conclude by picturing to ourselves some primitive generalized, marsh-haunting animals with scanty covering of hair like the modern hippopotamus, but with broad, swimming tails and short limbs, omnivorous in their mode of feeding, probably combining water plants with mussels, worms, and freshwater crustaceans, gradually becoming more and more adapted to fill the void place ready for them on the aquatic side of the borderland on which they dwelt, and so by degree being modified into dolphin-like creatures inhabiting lakes and rivers, and ultimately finding their way into the ocean.
Dr. Flower, Mr. Switek reports, was wrong in some of his ideas on whale evolution, but right in others. Mr. Switek goes on to say:
Just announced in the journal PLoS One, Maiacetus was a member of the Protocetidae, an extinct group of early whales that beautifully illustrate an important phase of whale evolution.
These are generalities, of course, because not all members of the Protocetidae were the same. In particular they were marked by differences in limb proportions, and Maiacetus fits snugly into the continuum of anatomical types. Rodhocetus, for example, has very large hind limbs ended with flat, paddle-like feet. Protocetus, by contrast, had much smaller hind limbs that were probably not very important to locomotion; its tail was more important to its swimming style. Maiacetus is closer to Rodhocetus in form but its hind limbs are smaller and do not show the same adaptations for hind-foot propulsion. What does this mean?