THE FOSSIL PLANT RECORD AND GLOBAL CLIMATIC-CHANGE

The generally sedentary character of terrestrial plants gives them st special dependence on their adaptation to the climate under which they live. As a consequence, plants normally show structural adaptations w hich are characteristic of their habitat, and fossil plants constitute particularly sensitive palaeoenvironmental indicators. In Quaternary pollen analysis the assumption is generally made that the species reco gnised as pollen had the same climatic constraints as their present-da y representatives. As we go back through Tertiary time, and extant spe cies become progressively rarer, we seek the nearest living relatives of the plant fossils as a basis for palaeoclimatic interpretation. Thi s approach relies on the accuracy of the taxonomic assignment of the f ossil material. Various 'non-taxonomic' features of Tertiary fossils h ave also been used in attempts to read a 'palaeoclimatic signal', inde pendent of the correctness of the taxonomic assignment. These include most notably leaf physiognomy, and growth responses to seasonality suc h as growth rings in fossil wood. When we look to Palaeozoic plants, e ven leaf physiognomic features are of limited value, but fossil plants of this age can still give us significant information about their pal aeoenvironment. The presence of charcoal (fusain) produced by wildfire puts a constraint on the level of oxygen in the palaeoatmosphere. Sto matal density and index may be used to give a proxy measure of palaeo- CO2 levels. The realisation of the link between the carbon-dioxide gre enhouse phenomenon and climate makes the use of stomatal data from fos sil plants of particular relevance to palaeoclimatic interpretation. O ur results from a study of stomatal index in plants from the Devonian to Permian interval are consistent with evidence from physical sources of major changes in global CO2 levels through that period.