The evidence that we have which has been assigned to the Pennsylvanian period is the total destruction of great swamps. They were buried and preserved, not formed. That's why they're now coal.
Formation of the Great Coal Swamps
Spread of Amphibians
Amphibians were buried in a swamp. Does that mean that amphibians were spreading? No, it doesn't, it means that amphibians lived in the swamp that got buried. It's popular science inductive doublespeak (SciPop).
Extensive Swamps in Lowlands
Most of the sedimentary deposits assigned to the Mississippian are coal beds in the Mississippi valley. The coal is the remains of swamp habitat that was buried in a world-wide flood.