The Systematic and Biogeographical Relationships of Ceratopetalum (Cunoniaceae) in Australia and New Guinea

We describe here the vegetative and floral morphology of the Australian and New Guinean species of Ceratopetalum. Stipule and stamen morphology are shown to be extremely variable in Ceratopetalum, and new systematic characters are identified. Nine species are recognized, including two new species, C. hylandii and C. iugumensis, from northeastern Australia, and C. tetrapterum is reinstated for material from New Guinea. Ceratopetalum gummiferum possesses a large number of unique characters for the genus, including petally, elliptical cotyledons, and hairs on stamen filaments. A phylogenetic analysis using 14 morphological characters showed that C. gummiferum is sister to all other species in the genus. The analysis also identified two clades in northeastern Australia. The C. corymbosum clade (C. corymbosum, C. macrophyllum, and C. iugumensis) is defined by large lanceate stipules and a well-developed composite intersecondary vein, while the C. succirubrum clade (C. succirubrum, C. hylandii, and C. tetrapterum) share 4-merous flowers with a strigose indumentum on the adaxial surface of the sepals. Ceratopetalum tetrapterum from New Guinea has been considered conspecific with C. succirubrum, although this study shows that it is a distinct taxon closely related to C. succirubrum and C. hylandii. The six species in northeastern Australia, with the exception of C. succirubrum, are restricted to discrete high-altitude montane habitats that are interpreted as Pleistocene refugia, which persisted during the Last Glacial. The present distribution of Ceratopetalum species in northeastern Australia probably results from relatively recent Pleistocene climate changes superimposed over much older patterns of speciation within the genus.