Persistence of Late Carboniferous tropical vegetation during glacially driven climatic and sea-level fluctuations

The Late Carboniferous glaciation reached its peak during the latter part of the Westphalian (late Desmoinesian). In the tropical Illinois basin this was the time of deposition of the Carbondale and lower Modesto Formations, characterized by the cyclic repetition of major coal deposits, black shales, limestones, gray shales and sandstones. These lithological changes evidence repeated major fluctuations in climate and sealevel. Fossil floras from the tropical ever-wet to seasonally wet terrestrial lithofacies, coals and shales above coals, remain compositionally similar throughout the 1–1.5 m.y. time interval. Floras from coals, known from coal balls and palynology, characteristically were composed of three major plant communities: the wettest sites were dominated by monocarpic lycopsids, intermediate sites were dominated by polycarpic lycopsids, and areas subject to fires and intermittent flooding were dominated by medullosans and the small lycopsid Paralycopodites. Clastic-substrate environments were dominated by ferns and pteridosperms and conform to a single biozone, indicating their compositional unity and distinctness from earlier and later assemblages. The composition of lowland floras is poorly known from times between the deposition of coals and associated terrestrial rocks, intervals during which most of the lowlands were flooded and marine rocks were being deposited. Consequently, the temporally intermittent recurrence of lowland floras despite repeated, widespread environmental disruption may be explained either by vegetational persistence and migration of floras, or repeated disruption and reassembly. Several lines of evidence suggest persistence as a likely explanation.