Latest Mid-Pennsylvanian tree-fern forests in retrograding coastal plain deposits, Sydney Mines Formation, Nova Scotia, Canada

摘要

A latest Mid-Pennsylvanian (early Cantabrian) tree-fern forest is reported from Cape Breton Island, Nova Scotia. The fossil forest is rooted below the Point Acom coal seam of the Sydney Mines Formation at Cranberry Head. The tree-ferns are preserved in the deposits of a coastal plain, subject to brackish incursions. They were constructed of psaronid-type trunks and massive pecopterid fronds, and locally intergrew with sigillarian lycopsids, neuropterid pteridosperms, and calamiteans. Basal stump diameter measurements, including the root mantle, range from 12 to 89 cm, indicating that the ferns were mostly large forest trees, rather than shrubs. Stump distribution measurements on palaeosols indicate that localized patches of tree-fern-dominated forest attained densities of c. 3850 trees per hectare when scaled up to standard forestry units, much denser than typical Mid-Pennsylvanian lycopsid forests. Tree-ferns dominantly grew in aggrading floodbasin settings, and a few of the largest trees show evidence for post-burial regeneration. Being approximately coeval with the late Mid-Pennsylvanian extinction event, which resulted in tree-fern forests rising to dominate tropical lowlands for the first time, the Cranberry Head fossil forests provide insight into community composition and structure during a critical phase of ecosystem reorganization.