Regional-scale spatial heterogeneity in the late Paleocene paratropical forests of the US Gulf Coast

The study of spatial patterns in biotic compositional variability in deep time is key to understanding the macroecological response of species assemblages to global change. Globally warm climatic phases are marked by the expansion of megathermal climates into currently extra-tropical areas. However, there is currently little information on whether vegetation in these "paratropical" regions resembled spatially modern tropical or extra-tropical biomes. In this paper we explore spatial heterogeneity in extra-tropical megathermal vegetation, using sporomorph (pollen and spore) data from the late Paleocene Calvert Bluff and Tuscahoma Formations of the formerly paratropical U.S. Gulf Coast (Texas, Mississippi, and Alabama). The data set comprises 139 sporomorph taxa recorded from 56 samples. Additive diversity partitioning, nonmetric multidimensional scaling, and cluster analysis show compositional heterogeneity both spatially and lithologically within the U.S. Gulf Coastal Plain (GCP) microflora. We then use sporomorph data from Holocene lake cores to compare spatial patterns in the late Paleocene GCP with modern tropical and extra-tropical biomes. Distance decay analysis of the Holocene data reveals a higher rate of spatial turnover in tropical versus extra-tropical vegetation types, consistent with a latitudinal gradient in floral compositional heterogeneity. The specific combination of rate and scale dependency of distance decay in the Holocene assemblages prevented us from associating the late Paleocene GCP with any particular modern biome. Our results demonstrate the importance of spatial scale, taphonomy, and lithology in determining patterns of spatial heterogeneity, and show the potential of the fossil sporomorph record for studying spatial patterns and processes in deep time.