417mya Dipnoi

The Dipnoi are a group of sarcopterygiian fish, are are commonly known as the lungfish. Their "lung" is a modified swim bladder, which in most fish is used for buoyancy in swimming, but in the lungfish also absorbs oxygen and removes wastes. Modern lungfish in Africa and South America are able to survive when their pools dry up by burrowing into the mud and sealing themselves within a mucous-lined burrow. During this time, they breathe air through their swim bladder instead of through their gills, and reduce their metabolic rate dramatically. These fish will even drown if they are kept underwater and not allowed to breathe air!

As lobe finned fish were adapting to live in partial water or on land, 420 million years ago during the Devonian, they seem to have split off into multiple groups. Two such branches are known to survive to the present day, the coelacanths and the lungfish.

The Queensland lungfish (Australian lungfish) and Coelacanth are two of the most famous living fossils; they resemble ancient fossils and unlike most species, seemingly refused to continue to evolve for the past 400 million years. 

Despite their morphological similarities, however, the lungfish and coelacanth are very different genetically, as expected of species which lived separately for more than 400 million years. Because genes do not stop evolving, the molecular DNA of these two species show greater evolutionary distance from each other than to DNA of the rest of the pilgrimage. The Lungfish's Tale reminds us that the rate of morphological changes is not always obviously correlated with that of genetic change.