Hundred of millions of years ago, sea creatures crawled up on land and started to become mammals. Then much later, a few went back into the sea, but left few fossils to show us how they did it -or at least that's what we used to think.
These fossils raise almost as many questions as they answer. Read more at Smithsonian magazine. Link
For more than a century, our knowledge of the whale fossil record was so sparse that no one could be certain what the ancestors of whales looked like. Now the tide has turned. In the space of just three decades, a flood of new fossils has filled in the gaps in our knowledge to turn the origin of whales into one of the best-documented examples of large-scale evolutionary change in the fossil record. These ancestral creatures were stranger than anyone ever expected. There was no straight-line march of terrestrial mammals leading up to fully aquatic whales, but an evolutionary riot of amphibious cetaceans that walked and swam along rivers, estuaries and the coasts of prehistoric Asia. As strange as modern whales are, their fossil predecessors were even stranger.
These fossils raise almost as many questions as they answer. Read more at Smithsonian magazine. Link
well seeing as how mammals currently live in both land and sea id say they are both fit. no mistakes in variation...just variation