Dolphins Evolving... To Get Out of Ocean?

A dolphin with four fins -- back legs, essentially -- has been caught in Japan.

My question is: is this more evidence that dolphins evolved from land mammals (which they obviously did)...

Or is it evidence that they are trying to evolve in order to to get the hell out of the horribly polluted oceans?

With "fishermen" throwing bombs and dumping cyanide onto coral reefs to "fish," I sure would.