What was the first animal to have a pentadactyl limb?

This is unlikely to ever be definitively known, as the fossil record of early tetrapods is incomplete, and the vast gulf of time that separates us from them (360+ million years) compounds the problem. However, we do have some fossils, and from them it is clear that the earliest tetrapods had more than 5 toes on each foot (they were polydactylous). Acanthostega had 8, Ichthyostega had 7 and Tulerpeton had 6 - there seems to have been a trend of reduction in the number of toes over time in the tetrapod lineage. The early Anthracosaurs seem to have had 5 toes ancestrally, a trait which they might have passed on to their amniote descendants. However, it seems as though even the early tetrapods like Acanthostega were effectively five-toed, as their forward toes were combined into a single flesh-covered digit.