>>11196923The Pennsylvanian and Mississippian are incredibly well studied and divided because that's where we get most of the coal from. They are actually each subdivided into dozens of other horizons.
Anyway. The plant and animal life changes though the strata, and in some cases, the disappearance of certain fossils or the sudden appearance of others is how strata are divided. One major tool is pollen and spores & other microfossils that evolved rapidly. The oil and gas industry for instance uses conodont fossils, which are incredibly small, to data and correlate the horizons in which oil may be found.
