Cainozoic ferns and their distribution

Collinson, M. E. (Department of Geology. Royal Holloway University of Londo n, Egham, Surrey, TW20 0EX, England). Cainozoic ferns and their distributio n. Brittonia 53: 173-235. 2001.-The phytogeographic distribution of Cainozo ic ferns is reported based upon a critical re-appraisal of the macrofossil and mesofossil record also taking account of evidence from a few highly dia gnostic spores. Well-documented circum-Arctic Cainozoic floras show ferns ( Woodwardia, Onoclea, Osmunda, Coniopteris and to a lesser extent Azolla) di stributed around the pole to very high paleolatitudes. Some ferns are share d between the mid-paleolatitudes of North America and Europe as would be pr edicted from the distributions of other biota. Evidence for the composition of Cainozoic fern floras is minimal in some regions (e.g., Antarctica, Cen tral and South America, Africa, India, South East Asia). so the absence of fern fossils from these areas has no biogeographical significance. Matoniac eae were abundant in the preceding Mesozoic, However, the absence of Cainoz oic macrofossils, nd the fact that no Cainozoic Matonisporites spores are M atonia-like, indicates that Matoniaceae had attained their modern relief di stribution by. or very early in, the Cainozoic. The important Mesozoic fami lies Marattiaceae and Dipteridaceae are also not represented by Cainozoic m acrofossils. They probably also showed Cainozoic restriction but spores are not sufficiently diagnostic to enable testing of this hypothesis. Other fe rns, which were also important in the Mesozoic (e.g.. Dicksoniaceae, Gleich eniaceae), have patchy, equivocal, or inadequately published Cainozoic reco rds. The dispersed spore record may provide an opportunity to track Cainozo ic Gleicheniaceae but this approach is not without problems. Most well-repr esented Cainozoic fern families, genera and subgenera show widespread Caino zoic ranges. typically with considerable range extensions over their living relatives, both onto other continents and north and south to higher paleol atitudes. These include Schizaeaceae (Lygodium, Anemia, and the extinct Ruf fordia), Osmundaceae (Osmunda), Pteridaceae (Acrostichum). Thelypteridaceae (Cyclosorus). Lophosoriaceae (Lophosoria), Cyatheaceae (the Cnemidaria/Cya thea 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 [Pterid aceae] [the latter by the Neogene and based only on spores]) but even these had slightly broader ranges in the Cainozoic. Some Cainozoic ferns have ap parently local distributions, e.g., Blechnum dentatum (Blechnaceae) in Euro pe: and others are so far represented at only one or few sites, e.g., Denns taedtiopsis (Dennstaedtiaceae), Botrychum (Ophioglossales). Grammitis (Gram mitidaccae) and Makotopteris and Rumohra (Dryopteridaceae). Cainozoic fossi ls assigned to Dryopteris (and some other dryopteroids) require revision al ong with those of Thelypteridaceae, the latter having high potential to pro vide useful paleobiogeographic evidence, at least of the Cyclosorus group. Cainozoic records of Hymenophyllaceae and Polypodiaceae are here considered unconfirmed.