LANDSCAPE AND COMMUNITY ECOLOGY OF THE FAYETTEVILLE FLORA OF ARKANSAS (USA): UPPER MISSISSIPPIAN (MIDDLE CHESTERIAN/LOWER NAMURIAN A)

Structurally modern forest ecosystems, consisting of a stratified canopy, lianas, and under-story plants first evolved in the wetlands of the Mississippian. Such communities are represented by assemblages of compressed and permineralized plant remains from the Fayetteville Formation of northwestern Arkansas, USA. The Fayetteville Formation is predominantly a marine black shale unit that produces conodonts, cephalopods, foraminifera, and miospores that accurately date the strata as middle Chesterian/lower Namurian A. Permineralized stems, cones, and ovules are recovered throughout the marine black shales and represent the ecosystem at the landscape level. Within the marine shales is a terrestrial shale lens that produces compression foliage, cones, rooting organs, and axes: this terrestrial unit represents the ecosystem at the community level. Lycopsids are the most abundant specimens recovered and were the dominant canopy trees of the ecosystem. The arborescent understory was represented by the sphenopsid Archaeocalamites esnostensis and the seed fern Quaestora amplecta. Foliage from the compression flora is dominated by Sphenopteris, with Fryopsis and Rhodea present. At the landscape level, the herbaceous layer is represented by rare ferns and a diverse group of seed fern morphotaxa, that probably exhibited vine like growth architecture based on anatomical and morphological characters. These vine-like seed ferns included Lyginopteris, Rhetinangium, Heterangium, Medullosa, and an as yet un-named lyginopterid. This landscape level herbaceous layer diversity is not present at the community level, where over 95% of the recovered axes were most likely produced by a single plant species. Therefore, at Upper Mississippian time guild depth at the vine-like niche was relatively deep at the landscape level, but at the community level, guild depth was still very narrow. In addition, galleries in some permineralized stems suggest arthropod-plant interactions in the form of herbivory and wound responses had evolved.