PhysOrg.com has an interesting, but ultimately uninformative, write up of some research that will be appearing in the Journal of Vertebrate Paleontology. Here is the context:
The different body forms seen in the lineage of whales point to different methods of swimming underwater. Previous studies have proposed a possible process to evolve from the ancestral form, paddling with all four legs, to the modern-day whale in which the tail oscillates up and down.
Living vertebrates that are capable swimmers employ a whole range of different techniques, including five particularly well-defined methods: quadrupedal paddling, paddling only using the back legs, undulation of the hips, tail undulation, and tail oscillation.
Interestingly, it had been suggested that during whale evolution that each of these steps occurred in turn, but that the hip undulation stage might have been by-passed. The discoveries indicate that the complete opposite was true, and as Uhen says, “wiggling hips were a significant step in the evolution of underwater swimming in whales.”
The discoveries refer to new finds of material attributed to Georgiacetus vogtlensis. Again, from PhysOrg:
In particular, previously unknown bones from the tail show that it lacked tail flukes. On the other hand, it did have large back feet, and Uhen suggests that it used them like modern whales use their tail flukes. Undulating, or moving the body in a wave-like fashion, was a key factor in the evolution of swimming.
You can see what the skeleton of G. vogtlensis looks like here.