Constraints on Siluran and Early Devonian phytogeographic analysis based on megafossils

Abstract

Abstract The detection of phytogeographic patterns in Silurian and Early Devonian land vegetation is hampered by the dearth of occurrences of megafossils, the absence of consistently precise correlation, unreliability of identification, and the lack of sufficient detailed sampling and information on sediments to permit evaluation of taphonomic influences on composition of assemblages. Interpretation is further complicated by the exceptional evolutionary position of the plants themselves and the lack of extant representatives. There is little information on whole plants, their life histories, their ecological and climatic tolerances. The composition of Ludlow, Pridoli and Lochkovian/Gedinnian assemblages is analysed. Those from Kazakhstan and Siberia are particularly enigmatic. The assemblages are plotted on the appropriate continental reconstructions. Most are clustered on the southeast margin of Laurussia and very few occur at high latitudes. In Ludlow time, Baragwanathia dominated assemblages in Australia in contrast with the rhyniophytoid assemblages of Laurussia. New information from Kazakhstan and northwest China adds to the data base in the Pridoli. Differences in Gedinnian assemblages from South Wales, the Welsh Borderland and Scotland, where sediments have been intensively studied and correlation is based on fish faunas and, more usefully, on palynomorphs, are related to taphonomy, to local vegetation distribution pattern, and to evolutionary changes. World-wide analysis of occurrences shows four distinct assemblages, based on Laurussia, Kazakhstan, Australia and Siberia, the latter being a rare example of high latitude vegetation.