ABSTRACT: Peat-swamp vegetation of the late Middle Pennsylvanian differed markedly from that of the Late Pennsylvanian. Dominating the former was a variety of arborescent lycopsids, medullosan pteridosperms, and increasing numbers of marattialean tree ferns. In contrast, marattialean tree ferns and medullosan pteridosperms were dominant in the Late Pennsylvanian whereas lycopsids were of lesser importance, although much species turnover occurred in each of these groups. The most severe turnover was among the lycopsids; the Sigillariaceae were the only major lycopsid lineage to survive, thrive, and attain wide distribution throughout Euramerica, represented almost exclusively by Sigillaria brardii Brongniart (possibly a species complex). The nature of the ecological transition that brought the marattialean vegetation to dominance is not well understood, even though the pattern has been documented across Euramerica. In a study of terminal peat-swamp vegetation preserved at the coal-bed to roof-shale transition in three major late Middle Pennsylvanian age coals of the Illinois Basin, USA, we have identified two major plant assemblages. The first, preserved at the coal-to-roof transitional interface, is autochthonous to minimally parautochthonous and appears to have been buried rapidly during the introduction of sediment to the swamp habitat, burying the vegetation close to its site of growth. This vegetation is similar in taxonomic composition to that documented from the peat swamp (based on coal balls), although the quantitative abundances of the dominant species are different from most coal-ball assemblages. In contrast, the second major plant assemblage occurs in tidal channels that traversed actively developing mudflats flanking major estuaries on the lowland landscape. These channels primarily contain the arborescent lycopsid Sigillaria Brongniartwith scattered remains of marattialean tree fern and pteridosperm foliage. This second assemblage more closely resembles the peat-swamp flora of the Late Pennsylvanian than that of the Middle Pennsylvanian andmay presage this younger flora. The pattern suggests that following the ecological disruption during the Middle-Late Pennsylvanian transition, vegetation formerly dominant in Middle Pennsylvanian clastic-swamp habitats colonized the Late Pennsylvanian peat swamps and surrounding swampy lowlands. This clastic-swamp vegetation thus appears to have harbored lineages (even if not individual species) that survived the turnover and were then able to move into the disrupted landscape from which the formerly dominant forms had been removed.