CLIMATE SIGNALS IN PALEOZOIC LAND PLANTS

The Palaeozoic is regarded as a period in which it is difficult to rec ognize climate signals in land plants because they have few or no clos e extant relatives. In addition early, predominantly axial, representa tives lack the features, e.g. leaf laminae, secondary growth, used lat er as qualitative and quantitive measures of past climates. Exceptions are stomata, and the preliminary results of a case study of a single taxon present throughout the Devonian, and analysis of stomatal comple x anatomy attempt to disentangle evolutionary, taxonomic, habitat and atmospheric effects on stomatal frequencies. Ordovician-Silurian veget ation is represented mainly by spores whose widespread global distribu tion on palaeocontinental reconstructions with inferred climates sugge st that the producers were independent of major climate variables, pro bably employing the physiology and behavioural strategies of extant br yophytes, further characterized by small size. Growth-ring studies, fi rst possible on Mid-Devonian plants, have proved most informative in e lucidating the climate at high palaeolatitudes in Late Permian Gondwan a. Changes in the composition of Carboniferous-Permian low-latitude we tland vegetation are discussed in relation to tectonic activity and gl aciation, with most confidence placed on the conclusion that major ext inctions at the Westphalian-Stephanian boundary in Euramerica resulted from increased seasonality created by changes in circulation patterns at low latitudes imposed by the decrease of glaciations in most parts of Gondwana.