Site-analysis and floristics of the Late Devonian Red Hill locality, Pennsylvania, and Archaeopteris-dominated plant community and early tetrapod site
The Late Devonian Red Hill locality in north-central Pennsylvania contains an Archaeopteris-dominated plant fossil assemblage, a diverse fossil fauna, and an extensive sedimentary sequence that provide a unique opportunity to investigate the landscapes and biotic associations of the earliest forest ecosystems. Red Hill is a kilometer-long roadcut exposure of the Duncannon Member of the Catskill Formation, an upper alluvial plain facies of the Catskill Delta Complex. The Red Hill flora is typical of Late Devonian (Famennian) subtropical to tropical floodplain forest vegetation. Archaeopteris is dominant and Rhacophyton is abundant. The flora also includes cormose lycopsids, gymnosperms, Gillespiea, and Barinophyton. Sedimentological analysis of the main plant-fossil bearing layer at Red Hill indicates that it was an abandoned channel that became an oxbow lake, subsequently filled by silt deposits over a relatively short period of time. A seasonal wet-and-dry climate is indicated by well-developed paleovertisols at Red Hill and a paleolatitude reconstruction of 20° South. At least during the wet season, vegetation was lush, and grew in profusion along the freshwater littoral zone of the oxbow lake. The presence of charcoal interspersed with plant fossils indicates that low-intensity fires occurred in this landscape, perhaps as a regular event during the dry season. Fires appear to have primarily affected Rhacophyton. The specificity of the fires, the distribution profile of the plant remains in the oxbow lake siltstone layer, and additional taphonomic evidence all support the model of niche partitioning of the landscape by plants at Red Hill and perhaps for Late Devonian plant communities in general. Archaeopteris was growing in a better-drained portion of the landscape a short distance from the lake. Rhacophyton was widespread, and grew up to the edge of the lake. Cormose lycopsids grew along the edge of the lake with their rooting organs frequently submerged. Early gymnosperms appear to have been pioneer plants following fires, and Gillespiea was also likely to have been an opportunistic plant in disturbed habitats. The paleoecological analysis of Red Hill is the first systematic interpretation of a specific site that reflects the type of landscape into which the first tetrapods possibly emerged.