I'm certainly not a biologist, but modern day amphibians come to mind immediately. A quick google gave me this nice site. http://bio.sunyorange.edu/updated2/pl%20new/20%20amphibians%201.htm For most of earth’s history, there was no life on land. One reason was that, until photosynthesis had contributed enough oxygen to form an ozone layer, too much ultraviolet light from the sun bombarded the land, making life impossible. In the Siluran Period, plants and arthropods made the transition to land 100 million years before vertebrates made a similar transition. Although moving from aquatic to terrestrial life seems like an enormous change for a fish, a variety of modern ray-finned (actinopterygian) fish, such as mudskippers and walking catfish, have adapted to terrestrial life by crawling on their fins, adapting their gills for air breathing, and/or gulping air with a swim bladder.