Fossil eucalypt species older and more broadly distributed than previously thought

Fossils are the only concrete evidence of ancient life available to scientists. They allow us to assess homology, understand distribution patterns, give clear morphological characters upon which to base phylogenies, and enable the calibration of molecular phylogenies. However, fossils are frequently incomplete and disassociated from the larger organism, as in the case of dispersed pollen, making them difficult to draw conclusions from.

In a new article published in the American Journal of Botany, lead author Maria C. Zamaloa and colleagues report on pollen found in an Argentinian fossil Eucalyptus flower first described in 2012, as well as on the flower itself. The group examined the fossils using both light and scanning electron microscopy, described them, and assessed the variability seen among the more than 60 pollen grains, which presumably came from the same individual.

The researchers erected the new species Eucalyptus xoshemium for the whole fossil flower. The donated pollen found within the flower comes from the fossil species Myrtaceidites eucalyptoides. Previously, M. eucalyptoides was known only from dispersed pollen, which prevented palynologists from assessing intraspecific variability in the pollen grains and affected confidence in its phylogenetic placement. In fact, the variability amongst the pollen grains was such that, had they been found in isolation, at least two different species might have been assumed. Finding the grains within a Eucalyptus flower confirms M. eucalyptoides’ place within the Eucalyptus sensu lato lineage.

This finding pushes back the age of Eucalyptus considerably, suggesting that it had begun to diversify even before the early Eocene. It also provides clear evidence for the occurrence of M. eucalyptoides outside Australia, which had been controversial. “The finding of M. eucalyptoides within the anthers of Eucalyptus xoshemium sp. nov. represents the oldest and, so far, the only confirmed record for South America, expanding its previously known stratigraphical distribution and definitely setting its age to at least the early Eocene,” write the authors.