CAN EVOLUTIONARY MECHANISMS BE DETECTED IN THE FOSSIL RECORD? A CASE STUDY FROM THE EOCENE REPUBLIC FLORA

Paleogene hyperthermals, most notably the Paleocene-Eocene Thermal Maximum (PETM, ~ 55. 5 Ma) and Eocene Thermal Maximum (ETM, ~ 53. 2 Ma), had a profound impact on regional plant communities and ecosystems, resulting in rapid evolution and shifts in geographic ranges for many taxa. The Okanagan Highlands floras of northeastern Washington and central British Columbia provide an interesting snapshot of plant communities present after these major perturbations of earth systems. Within this floristic region that occurs between the western Interior Basin and the western coastal assemblages, Republic (~ 49 Ma) provides clues to the origin of deciduous floras. The Republic flora also is significant for documenting the exchange of taxa between Asia and North America. Since this flora has been well collected, and many taxa assignable to modern lineages are present, Republic has provided key fossil material making it possible to explore the evolutionary and distributional responses after major hyperthermals. One family well represented in these floras is the Rosaceae, today possess an array of evolutionary mechanisms ranging from vegetative reproduction to complex breeding systems and hybridization. The rapid evolutionary rates in this family apparently have been so profound that identifying a single morphological synapomorphy is impossible. One lineage, the Dryadoideae, consists entirely of taxa with a symbiotic relationship with the root nodule forming actinomycete Frankia. Other taxa present in the Republic flora have modern species with this same symbiotic relationship (e.g. Alnus, Comptonia). Other evolutionary innovations, including specialized modes of asexual reproduction and shoot differentiation, demonstrate a set of novel responses to environmental change. Understanding these floras provides us with a unique perspective towards identifying what mechanisms modern plants could employ in response to climate fluctuations.