The ecology of Cainozoic ferns

Abstract

The ecology of Cainozoic ferns is documented (excluding that based only on nearest living relatives). Free-floating water ferns (of the modern genera Azolla and Salvinia) are widespread in the Cainozoic. They are represented by whole plants and dispersed or interconnected megaspores and microspore massulae in freshwater facies associated with a range of aquatic angiosperms. Acrostichum (a fern characteristic of mangroves today) was clearly associated with lakes and freshwater marshes in the Cainozoic. In southern England an Acrostichum/ Typha association existed comparable to that which is rare today, e.g. in the Florida Everglades. Other Cainozoic ferns also grew at the margins of lakes and in mires, especially well-represented by the Princeton Chert flora (Dennstaedtiaceae, Dryopteridaceae, blechnoids and Osmunda). In North America ferns such as Onoclea and Osmunda were associates of freshwater swamp forests dominated by taxodiaceous trees. These ferns, along with Woodwardia and the extinct Coniopteris, had a Cainozoic circum-Arctic distribution to very high palaeolatitudes. The Eocene fern flora of Yellowstone National Park, USA, grew in a disturbed volcanogenic terrain but the same ferns also occurred in backswamp settings. Gleicheniaceae were part of a fire-prone vegetation in the Miocene of Australia but other Cainozoic Gleicheniaceae are very poorly understood. Relatively little is known about the Cainozoic ecology of the Marattiaceae, Matoniaceae, Dipteridaceae, Dicksoniaceae and Cyatheaceae despite their Mesozoic importance. The Cainozoic record of tree ferns (proven by stem fossils) is very patchy but does include members of the Cyatheaceae, Dicksoniaceae and Osmundaceae ( Aurealcaulis, which grew in swampy floodplain forests). Although the epiphytic habit had evolved in extinct families of ferns in the Carboniferous there is no convincing evidence for fossils of epiphytic ferns in the Cainozoic. The fern Lygodium (for which a climbing habit is often inferred from morphological similarity with modern Lygodium) was widespread in the Cainozoic in North America, Chile, Europe, Australia and probably China. However, there are no rachis fossils to confirm or refute the interpretation that Palaeogene to Miocene Lygodium was a climber.