Editing Eucalyptus (section)

==Fossil record==
The oldest definitive ''Eucalyptus'' fossils are from [[Patagonia]] in [[South America]], where eucalypts are no longer native, though they have been introduced from Australia. The fossils are from the early [[Eocene]] (51.9 Mya), and were found in the [[Laguna del Hunco Formation]] in [[Chubut Province]] in [[Argentina]].<ref>{{cite journal |last1=Gandolfo |first1=MA |last2=Hermsen |first2=EJ |last3=Zamaloa |first3=MC |last4=Nixon |first4=KC |last5=González |first5=CC |display-authors=etal |year=2011 |title=Oldest Known Eucalyptus Macrofossils Are from South America |journal=PLOS ONE |volume=6 |issue=6|page=e21084 |doi=10.1371/journal.pone.0021084 |bibcode=2011PLoSO...621084G |pmid=21738605 |pmc=3125177|doi-access=free }}</ref><ref>{{Cite journal |last1=Giraldo |first1=L. Alejandro |last2=Wilf |first2=Peter |last3=Donovan |first3=Michael P. |last4=Kooyman |first4=Robert M. |last5=Gandolfo |first5=Maria A. |date=28 November 2024 |title=Fossil insect-feeding traces indicate unrecognized evolutionary history and biodiversity on Australia's iconic Eucalyptus |journal=[[New Phytologist]] |language=en |volume=245 |issue=4 |pages=1762–1773 |doi=10.1111/nph.20316 |issn=0028-646X |pmc=11754931 |pmid=39605238 }}</ref> This shows that the genus had a [[Gondwana]]n distribution. Fossil leaves also occur in the Miocene of New Zealand, where the genus is not native today, but again have been introduced from Australia.<ref>{{cite web|url=https://mikepole.wordpress.com/2014/09/22/eucalyptus-fossils-in-new-zealand-the-thin-end-of-the-wedge/|title=Eucalyptus fossils in New Zealand - the thin end of the wedge - Mike Pole|date=2014-09-22}}</ref>

Despite the prominence of ''Eucalyptus'' in modern Australia, estimated to contribute some 75% of the modern vegetation, the fossil record is very scarce throughout much of the [[Cenozoic]], and suggests that this rise to dominance is a geologically more recent phenomenon. The oldest reliably dated macrofossil of ''Eucalyptus'' is a 21-million-year-old tree-stump encased in basalt in the upper Lachlan Valley in [[New South Wales]]. Other fossils have been found, but many are either unreliably dated or else unreliably identified.<ref name="Hill, R. S 1994">{{cite book |first=R.S. |last=Hill |chapter=The history of selected Australian taxa |chapter-url={{GBurl|loBrTOJDojoC|p=390}} |title=History of the Australian vegetation: Cretaceous to Recent |publisher=Cambridge University Press |date=1994 |isbn=978-0-521-40197-5 |pages=390 }}</ref>

It is useful to consider where ''Eucalyptus'' fossils have not been found. Extensive research has gone into the fossil floras of the [[Paleocene]] to [[Oligocene]] of South-Eastern Australia, and has failed to uncover a single ''Eucalyptus'' specimen. Although the evidence is sparse, the best hypothesis is that in the mid-Tertiary, the continental margins of Australia only supported more mesic noneucalypt vegetation, and that eucalypts probably contributed to the drier vegetation of the arid continental interior. With the progressive drying out of the continent since the [[Miocene]], eucalypts were displaced to the continental margins, and much of the mesic and rainforest vegetation that was once there was eliminated.<ref name="Hill, R. S 1994"/>

The current superdominance of ''Eucalyptus'' in Australia may be an artefact of human influence on its ecology. In more recent sediments, numerous findings of a dramatic increase in the abundance of ''Eucalyptus'' pollen are associated with increased charcoal levels. Though this occurs at different rates throughout Australia, it is compelling evidence for a relationship between the artificial increase of fire frequency with the arrival of Aboriginals and increased prevalence of this exceptionally fire-tolerant genus.<ref name="Hill, R. S 1994"/>