The last theory I heard was that fish developed calcium deposits in order to move up river deltas into fresh water. Calcium is important for muscle function and you would have to bring a stable supply with you to survive.
The fact that mammals still cycle calcium in and out of their bones to manage blood chemistry was as I recall used to support that theory.
I was trying to read the article, but then I refreshed and it looks like the site is now suffering from the internet kiss of death. Resource limits reached.