Fossil and modern species illustrate the morphological transition from fishes to tetrapods. Five of the most completely fossils from the time of the transition are known are the osteolepiform Eusthenopteron; the transitional forms Panderichthys and Tiktaalik; and the primitive tetrapods Acanthostega and Ichthyostega. In addition to the clear evidence of the transition from fish fins to vertebrate legs, these fossils show the loss of the gill cover and other morphological shifts associated with the move from the water to the land. Image courtesy of Brian Swartz.