As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain by a few hundred thousand years.

In North America, where the early Carboniferous beds are primarily marine limestones, the Pennsylvanian was in the past treated as a full-fledged geologic period between the Mississippian and the Permian.

In parts of Europe, the Mississippian and Pennsylvanian are one more-or-less continuous sequence of lowland continental deposits and are grouped together as the Carboniferous Period.

The current internationally used geologic timescale of the ICS gives the Mississippian and Pennsylvanian the rank of subperiods, subdivisions of the Carboniferous Period.

The collapse of the rainforest ecology in the mid-Pennsylvanian (between the Moscovian and the Kasimovian) removed many amphibian species that did not survive as well in the cooler, drier conditions.

[10][11] Most pre-rainforest collapse tetrapods remained smaller, probably due to the land being primarily occupied by the gigantic millipedes, scorpions, and flying insects.