Biology of Australia
The first fish to convert the swim bladder into lungs did so about 360 million years ago.
Of the 3 living species of lungfish, the Queensland lungfish is the one that most resembles the stage in evolution when the swim bladder of fish was adapted for use as an auxiliary lung. The other two have paired lungs and require air to breathe. The Queensland lungfish cannot breathe entirely with its lung, but must breathe with gills, using its single lung to assist its breathing when oxygen becomes scarce in stagnant water. The main difference between the Queensland lungfish and its 300 million year old ancestors is its size. Its oldest known ancestors grew to about 4 m whereas the present-day descendant only grows to about 1 m.
Sources & Further readingJohn A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995