Dr. Gandolfo, Dr. Hermsen and other authors have published a paper on the oldest Eucalyptus macrofossils discovered from Argentina. The specimens include multiple leaves, infructescences, and dispersed capsules, several flower buds, and a single flower. The discovery is important in that these are the only illustrated Eucalyptus fossils that are definitively Eocene in age, and the only conclusively identified extant or fossil eucalypts naturally occurring outside of Australasia and adjacent Mindanao. Thus, these fossils indicate that the evolution of the eucalypt group is not constrained to a single region.
Gandolfo, M. A., E. J. Hermsen, M. C. Zamaloa, K. C. Nixon, C. C. González, P. Wilf, N. R. Cúneo, and K. R. Johnson. 2011. Oldest known Eucalyptus macrofossils are from South America. PLoS One 6(6): e21084. doi:10.1371/journal.pone.0021084.
Hermsen, E. J., M. A. Gandolfo, and M. C. Zamaloa. 2012. The fossil record of Eucalyptus in Patagonia. American Journal of Botany 99 (8): 1356–1374.