The ancestors of a pentadactyl tetrapod should be pentadactyl themselves. This is not what we find in the fossil record. My favorite evolutionist writer is Harvard's Stephen Jay Gould. Gould is a recognized authority, has an excellent way with words, and often shows a refreshing openness about the difference between actual evidences and the commonly-accepted ideas presented in elementary textbooks. Only three Devonian tetrapods are known.
None has five toes.
They bear, respectively, six, seven, and eight digits on their preserved limbs. As with so many other actual facts of nature, we point to the obvious intelligence of the designer. For example, look at the many types of cars that have four wheels. Designers utilize a proven principle. We believe that all the creatures around us were created by a super-intelligent Designer.