Polydactyly in early tetrapod aquatic animals was not polydactyly in the common sense, i.e., it was not an anomaly in the sense it was not a condition of having more than the typical number of digits for a given taxon.
The increasing number of such discoveries in paleontology has led to the challenging of the hypothesis that pentadactyly, as displayed by most modern tetrapods, is plesiomorphic. The number of digits may have reduced in amphibians and reptiles independently.
- Coates, M.I. and Clack, J.A. (1990) Polydactyly in the earliest known tetrapod limbs. Nature, 347, pp.66-69.