THE FAYETTEVILLE FLORA OF ARKANSAS (USA): STRUCTURALLY MODERN WETLAND COMMUNITIES FROM THE UPPER MISSISSIPPIAN (MIDDLE CHESTERIAN/LOWER NAMURIAN A)

Structurally modern forest communities, consisting of a stratified canopy, lianas, and under-story plants first evolved in the wetlands of the Mississippian. Examples of such complex communities are represented by assemblages of compressed and permineralized plant remains from the Fayetteville Formation of northwestern Arkansas, USA. The Wedington sandstone member is a deltaic unit that produces compression foliage, cones, rooting organs, and stems of wetland plants. The lower marine shale unit produces allochthonous, permineralized stems, cones, and ovules with excellent cellular preservation: many of which can be directly correlated with the deltaic plant remains. Conodonts, ammonoids, foraminifera, and miospores that co-occur with plants in the marine shale accurately date, and globally correlate the strata as middle Chesterian (lower Namurian A). Lycopsids were the dominant trees of the community and are represented by stems that may be identified as Lepidodendron and much-branched forms of Lepidophloios, plus foliage, cones, and rooting organs assignable to Cyperites; Lepidostrobus, Lepidocarpon; and Stigmaria respectively. Foliage including the Sphenopteris, Sphenophyllum and Rhodea types are preserved in the Wedington Sandstone member. Zygopterid ferns are represented by the rachis Etapteris. A diverse group of seed fern morphotaxa is present in the assemblage including the ovule Rhynchosperma, the cone Telangiopsis, and stems assignable to Quastora, Lyginopteris, Rhetinangium, Megaloxylon, Medullosa, and an as yet un-named lyginopterid. Morphology of the Medullosa and the un-named lyginopterid suggests they were lianas. Therefore, not only were lianas present in the ecosystem, but guild depth at the clinging/climbing niche was relatively deep for an ecosystem at this time in the evolution of plant communities. In addition, fusain from the Wedington Sandstone suggests fire played a role in the ecosystem, and insect galleries in some permineralized stems suggest insect-plant interactions in the form of herbivory and wound responses.