The Fayetteville flora of Arkansas (USA); a snapshot of terrestrial vegetation patterns within a clastic swamp at Late Mississippian time

The Fayetteville Formation of northwestern Arkansas (upper Mississippian/middle Chesterian) contains two compression plant fossil assemblages (one in situ) that represent plant communities, and an allochthonous permineralized assemblage recovered from marine strata that represents the landscape. This preservation of spatial ecological subunits (communities) nested within a larger subunit (landscape) provides a snapshot of vegetation patterns within a Late Mississippian clastic swamp. Fifteen whole plants are recognized. Seed ferns are the most speciose group and lycopsids account for most biomass. Seed fern taxa known only as permineralized specimens include one canopy tree (Megaloxylon), two understory trees, and five herbaceous layer plants. Two herbaceous layer seed ferns are observed only as compressions. Lycopsids are represented as two canopy trees that are known from both permineralizations and compressions. Archaeocalamites is also known from both permineralizations and compressions but was an understory tree. Ferns are rare and are preserved only as fragments of permineralized rachises from two species. As revealed by the in situ compression assemblage, the two species of lycopsid canopy trees co-occur and they formed communities that occupied ever-wet bottomlands, with Archaeocalamites occupying the understory, and a single species of seed fern comprising the herbaceous layer. Lycopsids do not co-occur with Megaloxylon. Megaloxylon probably formed a second community type in somewhat water-stressed areas of the swamp with an understory of small arborescent seed ferns, some Archaeocalamites, and an herbaceous-layer seed fern. Ferns probably formed a third type of community in disturbed sites.