Fossil evidence for the evolution of the Casuarinaceae in response to low soil nutrients and a drying climate in Cenozoic Australia

Abstract

The Southern Hemisphere family Casuarinaceae has a long fossil record, both macrofossils and pollen, none of which provides any evidence about the morphology of the precursor to the family. However, it has long been considered, from both molecular phylogenies and morphological data, that the extant genus Gymnostoma retains key ancestral states and the highly reduced leaf area is a result of a scleromorphic response to low soil nutrients. Gymnostoma has by far the earliest, most extensive and best preserved macrofossil record, beginning in the Late Paleocene. Modification of the stomatal location from superficial in Gymnostoma to encrypted in furrows in the other genera assisted in water conservation as species evolved. We conclude that the morphology of the living and fossil vegetative branchlets provides evidence that low soil nutrients (especially phosphorus) and high water availability in a relatively light limited environment were the original drivers for evolution in the Casuarinaceae. Reducing water availability (xeromorphy) in progressively higher light environments were the major drivers of post-Eocene evolution in this unique plant family.