Cainozoic ferns and their distribution

Collinson, M. E. (Department of Geology. Royal Holloway University of London, Egham, Surrey, TW20 0EX, England). Cainozoic ferns and their distribution. Brittonia 53: 173-235. 2001.-The phytogeographic distribution of Cainozoic ferns is reported based upon a critical re-appraisal of the macrofossil and mesofossil record also taking account of evidence from a few highly diagnostic spores. Well-documented circum-Arctic Cainozoic floras show ferns (Woodwardia, Onoclea, Osmunda, Coniopteris and to a lesser extent Azolla) distributed around the pole to very high paleolatitudes. Some ferns are shared between the mid-paleolatitudes of North America and Europe as would be predicted from the distributions of other biota. Evidence for the composition of Cainozoic fern floras is minimal in some regions (e.g., Antarctica, Central and South America, Africa, India, South East Asia). so the absence of fern fossils from these areas has no biogeographical significance. Matoniaceae were abundant in the preceding Mesozoic, However, the absence of Cainozoic macrofossils, nd the fact that no Cainozoic Matonisporites spores are Matonia-like, indicates that Matoniaceae had attained their modern relief distribution by. or very early in, the Cainozoic. The important Mesozoic families Marattiaceae and Dipteridaceae are also not represented by Cainozoic macrofossils. They probably also showed Cainozoic restriction but spores are not sufficiently diagnostic to enable testing of this hypothesis. Other ferns, which were also important in the Mesozoic (e.g.. Dicksoniaceae, Gleicheniaceae), have patchy, equivocal, or inadequately published Cainozoic records. The dispersed spore record may provide an opportunity to track Cainozoic Gleicheniaceae but this approach is not without problems. Most well-represented Cainozoic fern families, genera and subgenera show widespread Cainozoic ranges. typically with considerable range extensions over their living relatives, both onto other continents and north and south to higher paleolatitudes. These include Schizaeaceae (Lygodium, Anemia, and the extinct Ruffordia), Osmundaceae (Osmunda), Pteridaceae (Acrostichum). Thelypteridaceae (Cyclosorus). Lophosoriaceae (Lophosoria), Cyatheaceae (the Cnemidaria/Cyathea decurrens clade) and the heterosporous water fern Azolla (Azollaceae). A few well-represented ferns show Cainozoic distributions similar to those of the present day (e.g., Salvinia [Salviniaceae] and Ceratopteris [Pteridaceae] [the latter by the Neogene and based only on spores]) but even these had slightly broader ranges in the Cainozoic. Some Cainozoic ferns have apparently local distributions, e.g., Blechnum dentatum (Blechnaceae) in Europe: and others are so far represented at only one or few sites, e.g., Dennstaedtiopsis (Dennstaedtiaceae), Botrychum (Ophioglossales). Grammitis (Grammitidaccae) and Makotopteris and Rumohra (Dryopteridaceae). Cainozoic fossils assigned to Dryopteris (and some other dryopteroids) require revision along with those of Thelypteridaceae, the latter having high potential to provide useful paleobiogeographic evidence, at least of the Cyclosorus group. Cainozoic records of Hymenophyllaceae and Polypodiaceae are here considered unconfirmed.