Oh good, another one-off, ad hoc explanation.
I suppose some things will have to remain a mystery,
Better than another ad hoc explanation.
All that said, there is absolutely NO scientific proof of "evolution" or "transitional" species in the middle of developing legs from fins or vice-versa. NONE. No Fossils, NO remains.
Actually, there are lots. Google tiktaalik. And then remember that they found this transitional species because they figured out where there was a gap in the fossil record, and then looked in geologic formations of the appropriate age. Hypothesis -> prediction -> experiment -> confirmation, just like the scientific method dictates. Let's see creation science do that!
Really. That all ya got??":
Tiktaalik roseae, better known as the "fishapod," is a 375 million year old fossil fish which was discovered in the Canadian Arctic in 2004. Its discovery sheds light on a pivotal point in the history of life on Earth: when the very first fish ventured out onto land.
It "sheds" light on "pivotal point," claimed to be the "very first fish" to try out its new legs? Nice try:
http://creationwiki.org/Tiktaalik
It's yet another extinct creature that's arbitrarily aged at "375 million years old" which was created EXACTLY that way unknown thousands of years ago. You're better off making your desperate case of evolution by citing the current Platypus. It obviously evolved from a duck (or is it the other way around?)