Most fish in the sea evolved on land

Ray-finned fish evolved from fresh water fish

source: Colin Barras
New Scientist vol 213 no 2851, February 11th 2012 p17

Greta Vega and John Viens from Stony Brook University, New York, have discovered that ray-finned fish, which account for 96% of marine and freshwater fish on earth, descended from freshwater fish. Ray-fins only appeared in the sea some 170 million years ago. Most dolphins and whales also evolved in fresh water. Wiens believes that lakes, rivers and land may be less prone to mass extinctions than seas, so may be able to re-seed oceans after the loss of sea species. However, lakes and rivers are now threatened by pollution and dams, which could  affect their ability to resupply oceans, at a time when increasing acidification is threatening ocean life. 
FI,BC