Polydactyly in stem-tetrapods

Polydactyly in stem-tetrapods should here be understood as having more than five digits to the finger or foot, a condition that was the natural state of affairs in the earliest stegocephalians during the evolution of terrestriality.

"SHH is produced by the mesenchymal cells of the zone of polarizing activity (ZPA) found at the posterior margin of the limbs of all vertebrates with paired appendages, including the most primitive chondrichthyian fishes.

[10] The increasing knowledge of labyrinthodonts from Romer's gap has led to the challenging of the hypothesis that pentadactyly, as displayed by most modern tetrapods, is plesiomorphic.

The number of digits was once thought to have been reduced in amphibians and reptiles independently,[1][11] but more recent studies suggest that a single reduction occurred, along the tetrapod stem, in the Early Carboniferous.

[12][13] Even the early ichthyostegalians like Acanthostega and Ichthyostega appear to have had the forward ossified bony toes combined in a single stout digit, making them effectively five-toed.