New species of xeromorphic Banksia (Proteaceae) foliage and Banksia-like pollen from the late Eocene of Western Australia

Abstract

Banksia microphylla leaf fossils and Banksieaeidites zanthus pollen are newly described from late Eocene lignite of the Zanthus-11 borehole, drilled east of Norseman in Western Australia. The leaf fossils are the first known in Banksia to show extreme narrowness (<1.5 mm wide) combined with the xeromorphic trait of margins rolled onto the lower surface so that the diffusely placed stomata are exposed to the outside environment only via grooves on each side of a thick, abaxial midrib. Both this Banksia leaf type and another with encrypted stomata evolved before the widespread initiation of severe climatic aridity in the late Neogene, likely in regions of edaphic infertility and periodic water stress. New interpretations of leaf morphology and foliar evolutionary pathways in Banksia are proposed. Banksia microphylla probably belongs to subgenus Spathulatae, where it strongly resembles many species in the large, wholly Western Australian clade that includes most species in section Oncostylis, series Abietinae. Banksieaeidites zanthus is morphologically consistent with Banksia pollen, and its extremely small size also suggests placement in Spathulatae. The new fossils and other evidence from Zanthus-11 indicate the local presence of quite open, sclerophyll vegetation with conifers, which was unlikely to have been frequently burnt.