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> There were transitional changes in the spine/hip area:
> 
> https://www.smithsonianmag.com/science-nature/ancient-whale-fossil-helps-detail-how-mammals-went-walking-land-swimming-seas-180973758/?utm_source=smithsoniandaily&utm_medium=email&utm_campaign=20191211-daily-responsive&spMailingID=41314631&spUserID=ODg3NDcwNDQ2NDA1S0&spJobID=1661079279&spReportId=MTY2MTA3OTI3OQS2
> 
> "Whales don’t swim like fish do. Instead of moving their tales side-to-side like a shark or a sunfish, the marine mammals pump their tails up-and-down to propel themselves forward. But over 50 million years ago, the earliest whales had legs and could walk on land. Adapting to life in the sea required a new way of moving, and a fossil uncovered in Egypt helps estimate the time when whales became primarily tail-powered swimmers.
> 
> ***
> 
> "All told, the paleontologists uncovered almost the entire spine, part of the skull, and pieces of the arms and legs. “It was very clear from the shape and the size of the vertebrae and appendages that this whale is new in this area,” Zalmout says. Further study indicated that the mammal was a species not seen anywhere else in the world.
> 
> "Named Aegicetus gehennae, the ancient swimmer stands out from others found in Wadi Al-Hitan, which fall into one of two groups. Some earlier whales could swim with a combination of paddling limbs and undulating their spines, not unlike otters. Other whales, like Basilosaurus, lived in the sea full time and swam with tails only. Aegicetus fits between the two, representing a moment when whales were just switching to exclusively tail-driven locomotion.
> 
> ***
> 
> "The key feature in this fossil, Zalmout and co-authors point out, is the relationship between the hips and the spine. The earliest whales had hips attached to the spine, just like any terrestrial mammal. This configuration helped the hind limbs support the animal’s weight on land. But in Aegicetus and other whales that came later, the hips are decoupled from the spine and suspended by the flesh of the body. The tight fusion of vertebrae at the hip-spine connection—called the sacrum—also became unfused and more flexible. These whales could no longer paddle with their legs and relied more on undulating their spines to move through the water. The shift indicates two things: that these whales were spending most, if not all, of their time in the water where weight-supporting legs weren’t needed, and that these beasts swam by principally using their tails.
> 
> "Not that Aegicetus was much like a modern orca or sperm whale. The fossil whale, which weighed almost a ton (or about a sixth the weight of the biggest orcas), still had jaws set with different types of teeth instead of the simple cones of today’s dolphins. Nor did Aegicetus swim just like its living relatives."
> 
> Comment: Another partial step as whales adapted more fully to living in water, which required  an enormous number of physical and physiological changes.