Fern spore diversity and abundance in Australia during the Cretaceous

Abstract

Changes in the Cretaceous fern flora of Australia were analysed by compiling data from the palynological literature. A decline in the relative diversity and abundance of free-sporing plants during the Late Cretaceous was concurrent with a rise in angiosperms, while the relative contribution of gymnosperms was unchanged. Ferns and lycophytes were the major contributors to the free-sporing plant pattern and declined markedly from higher Early Cretaceous levels; bryophyte relative diversity peaks in the mid-Cretaceous. The fern families Schizaeaceae, Osmundaceae and Cyatheaceae/Dicksoniaceae decline only marginally, with most of the decline seen in ‘miscellaneous ferns’ (mostly comprising fern spores of unknown affinity). In contrast to the overall fern trend, the relative diversity of Gleicheniaceae increased significantly in the mid-Cretaceous. The decline in fern relative abundance, which is gradual throughout the entire Cretaceous, is equally complex. By far the largest contributor to this decline is Osmundaceae. Cyathaceae/Dicksoniaceae and Gleicheniaceae increase in relative abundance in the mid-Cretaceous, but decline in the Late Cretaceous. Matoniaceae/Dipteridaceae maintain very low and stable levels of relative diversity and abundance throughout the Cretaceous. There is no evidence that polypodiaceous ferns sensu lato underwent any significant radiation concomitant with the angiosperm radiation in the Late Cretaceous. Trends observed in the relative diversity and abundance show that patterns occurring at high taxonomic levels (e.g. all ferns) are not consistent at lower levels (e.g. families). The Australian and North American diversity data for free-sporing plants, gymnosperms and angiosperms are broadly similar. However, Schizaeaceae and Gleicheniaceae display different trends during the mid-Cretaceous, increasing in diversity in the Australian palynofloras, but decreasing in North America.