Mycorrhizal associations are ancient, with different types varying in their ecological characteristics and evolutionary history. It is widely appreciated that vegetation history provides context for modern plant distributions, but the integration of paleoecological data into mycorrhizal studies remains relatively new. The Early Eocene Climatic Optimum (EECO) between approximately 53-51 million years ago was characterised by globally warm and equable temperatures, elevated atmospheric CO2, and the absence of sea ice and polar ice caps. It provides a historical model for the extremes to which Earth’s climate can change the distribution of organisms. The predominantly frost-free climates during the EECO led to fossil co-occurrences of host plant lineages and insects that are currently distributed in temperate to tropical biomes, in places such as the Okanagan Highlands of British Columbia and Washington. We considered that examination of data on past distributions of mycorrhizal hosts and symbionts may reveal important patterns, or elucidate deep time events that make little sense when viewed from the present. Here we combined paleobotanical data on the co-occurrence of tree genera in North America during the EECO, with the best available dated phylogenetic data on the evolution and current distribution of different lineages of ectomycorrhizal fungi.
Results/Conclusions
Paleobotanical evidence suggests that Eocene floras of North America were substantially different from today and many of today’s major host lineages of EMF, or their direct ancestors, co-occurred during the EECO. According to recent comparative phylogenetic analyses, the EECO matches the middle of the period (57-45 Mya) in which the /sebacina EMF lineage appears to have first evolved from a saprotrophic ancestor, most likely in North America. We posit that the evolutionary origins of this fungal lineage during this period of increased host co-occurrence, presents a parsimonious explanation for its present day generalist habit and wide distribution. Co-migration of fungi with hosts during and following the Eocene, coupled with limited instances of long distance dispersal, provides a likely scenario for the phylogenetic clustering observed in present-day biogeographical traits. Significant EMF species radiations in other major lineages of EMF (Hebelomateae and Inocybaceae), which evolved in earlier and later periods, also correspond well with the EECO, as does the first appearance of certain EMF genera (Boletus and Puberulum). Deep time co-occurrence patterns are just one way in which paleoecological data can help to inform hypotheses about how the distributions of ectomycorrhizal hosts and fungi may be affected during the Anthropocene.