Conservatism of Late Pennsylvanian vegetational patterns during short-term cyclic and long-term directional environmental change, western equatorial Pangea

The fossil record offers a unique opportunity to track species pools through time and to examine their responses to long-term, directional changes in physical conditions. The ability to estimate environmental variables, such as temperature, rainfall amount (and its seasonal distribution) and atmospheric CO2, is improving continuously. Links between proxy measures of such physical factors and quantitative data on changes in ecosystem composition and dynamics, formerly the purview of Pleistocene palaeoecology, are now available, even in the deep past, for both terrestrial (e.g. Wing et al. 2005; Falcon-Lang et al. 2006; Montanez et al. 2007; Secord et al. 2008; Currano et al. 2008; Tabor et al. 2011) and marine (Palaeozoic examples: Olszewski & Erwin 2009; Ivany et al. 2009; Fall & Olszewski 2010; Bonelli & Patzkowsky 2011; Holland & Zaffos 2011) systems. Plant fossils themselves can be especially useful as palaeoclimate proxies, even in the distant world of the Palaeozoic (e.g. Falcon-Lang et al. 2009), because of the strong sensitivities of plants to habitat and regional climate. As sessile organisms, plants are directly and inextricably linked to climate at the level of individual, population, local community and biome. Quantitative floristic studies of vegetation from the Late Palaeozoic are numerous (e.g. Scott 1978; Pfefferkorn & Thomson 1982; Bartram 1987; Eble et al. 1994, 2006; Pryor & Gastaldo 2000; DiMichele et al. 2007; Oplustil et al. 2007; Simůnek & Martinek 2009; Libertin et al. 2009; Bashforth et al. 2010), although few have had a framework with which to link plants and their associated environments to changing global climate (e.g. Phillips & Peppers 1984; Falcon-Lang 2004). It is a particular challenge to find a long, relatively uninterrupted stratigraphic section that contains both a common species pool or pools and a clear signal of an overall, directional and interpretable environmental change. The latest Pennsylvanian–earliest Permian Markley Formation of north–central Texas provides such an opportunity. The Markley Fm was positioned in the western Pangean tropics during the Pennsylvanian–Permian transition, a time of global climate change that accompanied the onset of Permian glaciation (Fielding et al. 2008). Within the Markley Fm, its partial equivalent – the Harpersville Fm – and into the base of its successor – the Archer City Fm – there are distinct packets of lithofacies that repeat in the stratigraphic section. Each lithofacies contains a distinctive fossil-plant assemblage, characterized and described by Romanchock et al. (in press), which can be followed through the Markley Fm. The goal of this paper is to go beyond such a qualitative flora-to-facies correlation and to examine quantitatively the floristic similarity within and among these facies in space and through time in order to investigate, and in this instance document, the coherence of species assemblages in the face of short-term and long-term environmental changes, and to evaluate spatio-temporal scales on which niche versus neutral biological community assembly patterns may operate (e.g. Alonso et al. 2006; Weiher et al. 2011). In addition, we wish to examine the conformance of the Markley Fm data with interpretive models that call for climate change as the controlling variable on repeating patterns of lithofacies and plant distribution through time, in the short and long term (allocyclicity), v. models that call for changes in sedimentary environments as the driver of temporally recurrent floristic change under a relatively constant background climate (autocyclicity).