PALEOBOTANICAL AND PALEOECOLOGICAL CONSTRAINTS ON MODELS OF PEAT FORMATION IN THE LATE CARBONIFEROUS OF EURAMERICA

The dominant plants of the Late Carboniferous lowland tropics were tax onomically and structurally distinct from those of any later time peri ods. Dominance was distributed among lycopsids, ferns, sphenopsids, pt eridosperms and cordaites, and each of these groups had distinctive an d different ecological preferences and amplitudes. Peat-forming habita ts were dominated by lycopsids throughout the Westphalian, with a sign ificant cordaitean element in the middle Westphalian; during the Steph anian tree ferns were dominant, following major extinctions near the W estphalian-Stephanian transition. Each of the major plant groups had d istinctive architectures and tissue composition. Trees contributed up to 95% of the peat biomass and tree forms of lycopsids, Psaronius and Medullosa lack good modern analogues. The cordaites were the only wood y plant group to contribute significantly to peat, and then only durin g the mid-Westphalian. Structurally wood-like lycopsid bark is the maj or ''woody'' tissue encountered in most Westphalian coals. Tree ferns and pteridosperms were largely parenchymatous in construction; the sti gmarian root systems of lycopsids also were largely parenchymatous. Th e tissue structure of these dominant plants suggests the need for extr eme caution in the inference of mire ecological conditions or vegetati onal structure from coal petrographic data. Peat formed under arboresc ent ferns or pteridosperms, or peat repeatedly exposed to decay and re rooting by stigmarian root systems of lycopsids, would have a distinct ly non-woody signature and yet would have formed in a forested environ ment. A summary is presented of the autecology and synecology of mire plants, emphazing the structural framework provided by lycopsids durin g the Westphalian. Certain constraints in the links between peat bioma ss and miospore palynology are discussed in terms of over-representati on, under-representation and non-representation. The formulation of Sm ith's four-phase hydroseral model is discussed and compared with more recent data available from plant paleoecology.The current debate over an ombrotrophic vs. rheotrophic origin of Late Carboniferous peats rel ies in large part on paleobotanical data, almost entirely palynologica l, in combination with petrographic analyses. Ecological studies of mi ospores and of coal-ball and compression macrofossils, and the linkage of miospores to source plants, permit the re-evaluation of mire succe ssional models. Evidence for tree lycopsids, sphenopsids, pteridosperm s and cordaites suggests growth mainly in rheotrophic mires. Tree fern s are likely candidates for growth in domed mires, although evidence i s ambiguous and some tree ferns clearly grew under rheotrophic conditi ons. Densospores, produced by at least Sporangiostrobus lycopsid subtr ees, have been considered diagnostic of ombrotrophic conditions; abund ant evidence refutes this simplistic interpretation and suggests broad ecological amplitudes for densospore producers, including growth unde r rheotrophic conditions. Although plant fossils alone can not resolve most of the major debates in modern coal geology, paleobotany does co ntribute significantly to our understanding of ancient mires. An appro ach combining paleobotanical data with petrography, sedimentology and geochemistry, on a case by case basis, is most likely to produce a cle ar picture.