WHAT IS THE TEMPO OF TURNOVER? A COMPARISON OF OBSERVED AND NEUTRALLY SIMULATED CHANGES IN THE LATE CARBONIFEROUS WETLAND FORESTS OF ATLANTIC CANADA

The late Pennsylvanian of Atlantic Canada records a major transition in the evolution of fossil plants. This transition has been associated with global floral turnover, broadly characterized by the disappearance of the arborescent Lycopsida (giant club mosses), predominance of seed ferns, and widespread aridification. In recent years it has become apparent that there are significant regional differences in the tempo of floral turnover, with Western Pangaean rainforests appearing to have experienced collapse earlier than those in Eastern Pangaea. What remains unclear is the specific tempo of floral community change within Atlantic Canada. This study explores the transition in fossil plant community structure at the regional scale, focusing on Middle Pennsylvanian localities from Nova Scotia, New Brunswick, and Prince Edward Island. In particular, we focused on comparing the changes in plant community composition to those expected under a simulated neutral model of community assembly. Our neutral model simulates progressive, stochastic changes in community composition by assuming all organisms have equal likelihood of dispersal and extirpation. Deviation from this model therefore indicates the action of ecological filters which either accelerate or decelerate the rate of change in community composition. We compared our model to a dataset of 733 unique fossil plant assemblages, compiling species and genus occurrences from museum repositories, field collections, and records from the published literature, representing plant fossil occurrences primarily spanning the Westphalian stage European chronostratigraphy. We used these occurrence data to perform multivariate analyses of site similarity among localities from 5 successive time bins. Our results show some deviation from the neutral model, with accelerated change in the middle Westphalian, followed by significantly less change than expected through the latter part of the studied interval. These results, in tandem with estimates of species origination and extinction rates through the same interval, indicate a shift towards a hyper-conservative wetland system in the in the latest Westphalian of Atlantic Canada, slightly delaying the dismantling of lycopsid dominance in regional wetland rainforests.