I do not find it possible to explain the transition of fish to terrestrial life in an evolutionary, purposeless, materialistic framework which I believe Darwinian evolution is. To explain the transition from aquatic to terrestrial life in non-Darwinian terms, I would use principles of development, expanded to include the development of large groups of animals as well as individuals. I find developmental principles, more compatible than Darwinian mechanisms for dealing with the transition you described, and other changes in the fossil record. For me principles of development apply to changes in the great animal phyla as well as to individual organisms. Development has several critical characteristics that make it a better candidate than evolution for explaining transitions and other phenomena in the fossil record.
Individual development provides a window of phyletic developmment.
As much if not more change in the biological world comes about by evolution than by Darwinian mechanisms. I hold then that the transition from aquatic to terrestrial life of fishes was driven by developmental processes. Evolution may have fine-tuned the process toward the end of the transition, but did not initiate it. Underlying the entire transition is the fundamental driving purpose of the universe, the appearance of human beings created to be in joyful relationship to their Creator. I would appreciate any comments anyone may have on this developmental perspective.