Arborescent lycopod reproduction and paleoecology in a coal-swamp environment of late Middle Pennsylvanian age (herrin coal, Illinois, U.S.A.)

Abstract

The arborescent lycopods dominated many coal-swamp plant communities of the Middle Pennsylvanian. A relatively small number of important species occurred in coal swamps, each with distinctive ecological requirements reflected in their reproductive biology. The major genera in the late Middle Pennsylvanian age Herrin Coal of the Illinois Basin were Lepidophloios, Lepidodendron sensu L. scleroticum and L. dicentricum, and Paralycopodites, which are dealt with in this study. Sigillaria, and Lepidodendron sensu L. hickii (true Lepidodendron), were minor parts of the vegetation. The species of these genera conform to three ecological strategies. Opportunists include Lepidophloios hallii, Lepidodendron dicentricum, and Lepidodendron hickii. These species had determinate, dendritic crowns and each tree apparently reproduced during a short, unrepeated interval late in determinate growth. They grew in areas that were disturbed or with high abiotic stress. Paralycopodites brevifolius was a colonizing species, rapidly occupying sites where peat formation was irregular, and perhaps locally disturbed by clastic influx. Paralycopodites had straight trunks with rows of deciduous lateral branches; cones were borne at the tips of the branches conferring individuals with continuous, high levels of reproduction. Displacement from sites occurred as edaphic conditions changed. Lepidodendron scleroticum and Sigillaria were site occupiers. Lepidodendron scleroticum trees individually produced the most massive wood and periderm of coalswamp lycopod species, and also had deciduous lateral branches and low but continuous reproductive output. Locally, L. scleroticum was very abundant, and such areas occurred irregularly. None of the lycopod trees had vegetative reproduction. Some Sigillaria may have been apomictic, although the major circumstantial evidence supporting this is from Upper Pennsylvanian specimens of Mazocarpon oedipternum.