How old are the eucalypts? A review of the microfossil and phylogenetic evidence

Molecular age estimates for the Eucalypteae (family Myrtaceae) suggest that the eucalypts, possibly associated with fire, have been present for ∼65million years. In contrast, macrofossils and fossil pollen attributable to three important eucalypt genera (Angophora, Corymbia and Eucalyptus) in the Eucalypteae date to ∼51-53million years ago (mid-Early Eocene) in Patagonia, eastern Antarctica and south-eastern Australia. At present, there is no fossil evidence to show that eucalypts had evolved before this epoch, i.e. when Australia was part of eastern Gondwana, although this seems probable on the basis of molecular-dated phylogenetic analyses. The primary reason is the absence of macrofossils, whereas the earliest fossil eucalypt-type pollen recorded (Myrtaceidites tenuis) is attributed to Angophora and Corymbia, not Eucalyptus. This pollen type is recorded in Australia and Antarctica but not in New Zealand or South America. The only Myrtaceidites morphospecies found in Upper Cretaceous and Paleocene deposits in Australia is M. parvus, whose affinity lies with multiple extant Myrtaceae groups other than the Eucalypteae. In the present paper, we review current phylogenetic and microfossil databases for the eucalypts and assess this evidence to develop a 'consensus' position on the origin and evolution of the eucalypts in the Australian region.