Regional-scale trends in the composition of late Pennsylvanian palaeobotanical communities from Atlantic Canada.

The late Pennsylvanian of Atlantic Canada records a major transition in the evolution of fossil plants. This transition, broadly characterized by the replacement of the arborescent Lycopsida (giant club mosses) and Cordaitales (early gymnosperms) by Mariattales (tree ferns) and Coniferales (early modern conifers), has been known globally as the "Carboniferous Rainforest Collapse", and is commonly associated with widespread aridification amid fluctuations between glacial and interglacial climate regimes. In recent years it has become apparent that there are significant regional differences in the tempo of floral turnover, while studies of single locality sequences have revealed significant cyclical changes in plant community composition over short timescales. What remains unclear is the specific tempo of floral turnover within Atlantic Canada. This study explores the transition in fossil plant community structure at the regional scale, focusing on the late Pennsylvanian localities from Nova Scotia, New Brunswick, and Prince Edward Island. Fossil macrofloral occurrence data were compiled from museum repositories and records from the published literature, representing plant fossil occurrences spanning the middle Bashkirian through the end of the Kasimovian. Species and genus presences and absences were used to conduct multivariate analyses of similarity between localities. Preliminary results show a gradual shift in species composition over time, with the greatest degree of turnover occurring in the late Moscovian. Conversely, analysis of community composition at the genus level revealed little change in composition between time bins, but an increase in variance between sites within time bins. These results indicate that the shift in palaeofloral community composition in Atlantic Canada may be best described as a gradual increase in the breadth of uniquely composed communities as opposed to a turnover pulse, as assemblages similar in composition to those of the middle Bashkirian persist throughout the studied interval. [ABSTRACT FROM AUTHOR]