In the official geologic time scale of the International Commission on Stratigraphy (ICS[1]), the Westphalian is placed within the Pennsylvanian Subsystem/Subperiod (323-299 Ma) of the Carboniferous System/Period.
[5] Many older scientific sources refer to the Westphalian as an epoch or series, which are higher geological ranks than its current status.
[8] Many widespread plant species first appear near the base of the Langsettian, indicating a spike of diversification in tropical coal swamp habitats.
[9][4] The lower-middle part of the Duckmantian corresponds to the Lonchopteris rugosa Assemblage-zone, the most diverse plant biozone in the Carboniferous coalfields of Europe.
[8] In a few coalfields, such as the Nord-Pas-des-Calais basin of northern France, the decline is delayed until the mid-Bolsovian, while in other areas it may begin as early as the late Duckmantian.
The overall downward diversity trend is slight, but several notable lycopsid and sphenopsid species disappear from European coal swamps at this time.
[5] U-Pb radiometric dating of tonstein beds in Spain estimate that the Asturian lasted from 310.7 to 307.5 Ma, ending just prior to the Kasimovian.
These deposits, from so-called "coal swamps" have yielded rich assemblages of fossils including spore-bearing and seed-bearing plants, fishes, and tetrapods.
The collapse of the rainforest ecology between the Moscovian and Kasimovian removed many amphibian species that did not survive as well in the cooler, drier conditions.
Reptiles, however prospered due to specific key adaptations and underwent a major evolutionary radiation, in response to the drier climate that led to the rainforest collapse.