These drawings are based on the fossil record and represent animals that lived in shallow seas over 300 million years ago.  These seas were actually drying up into huge swamps.  

The water was so shallow that animals started to walk along the bottom, as well as swimming.   Acanthostega has the large tail for propulsion through the water AND what can be seen as 8 distinct toes that would be useful for walking along the bottom.

The bones on the left depict a fin of a species related to Acanthostega.   Notice that the bones create the framework for "fingers" that enhance the animals capability for walking along the bottom.

Both these species have 8 digits, and were no doubt precursors  to early amphibians which are reported to have had 8 digits. 

How and when 5 digits came into being is a mystery yet to be solved.  One guess would be that one of the digits became slightly "opposable" and made the animal capable of grasping. Once the advantages of being able to grasp were realized, possibly two digits fused to be one opposable digit.  The developed capability of grasping gave rise to the evolution of greater intelligence useful for manipulating objects which could be "grasped" .

This ceramic inlay I bought in the South West and never knew quite what to make of it.

Now I have a theory.  The large tail is for propulsion in water.  The 4 fins have turned into digits useful for locomotion along the ground - no opposable digit would be required at this stage. The horn on the mouth would be used by the animal to announce itself on its transition to land - the artists fantasy.

This theory is of course very speculative, however on first glance I thought this was a strange sea creature that was somehow important to life.. and the match up with the fossil record really is amazing.. I credit the Native American naturalist intuition for the strength of this correlation with the fossil record.