During the early Palaeogene warm interval megathermal climatic regimes expanded beyond their current tropical limits. The early Palaeogene sporomorph (pollen and spore) record of the US Gulf Coastal Plain (GCP) documents an extra-tropical vegetation type that developed under these megathermal climatic conditions. It is therefore suitable to address hypotheses concerning the importance of tropical climates in controlling low latitude spatial and temporal diversity patterns. Here, I construct a new sporomorph dataset comprising 151 samples, 41831 counted specimens and 214 sporomorph morphotypes. Fifty-nine of these morphotypes were not found in the published literature and are newly described. I demonstrate that previous studies of the GCP sporomorph record that have relied on biostratigraphic datasets have underestimated the true species richness of this region. Compositional heterogeneity was important for maintaining regional species richness on the GCP. The rate and scale dependency of spatial turnover in Holocene tropical and extra-tropical sporomorphs records precluded associating the GCP vegetation more closely with any particular modern biome, however. Finally, I show that warming extra-tropical regions to megathermal levels did not stimulate increased speciation there, which does not support a direct control of temperature on speciation rate in the low latitudes.