Climate signals in Palaeozoic land plants

Abstract

The Palaeozoic is regarded as a period in which it is difficult to recognize climate signals in land plants because they have few or no close extant relatives. In addition early, predominantly axial, representatives lack the features, e.g. leaf laminae, secondary growth, used later 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 complex anatomy attempt to disentangle evolutionary, taxonomic, habitat and atmospheric effects on stomatal frequencies. Ordovician–Silurian vegetation is represented mainly by spores whose widespread global distribution on palaeocontinental reconstructions with inferred climates suggest that the producers were independent of major climate variables, probably employing the physiology and behavioural strategies of extant bryophytes, further characterized by small size. Growth–ring studies, first possible on Mid–Devonian plants, have proved most informative in elucidating the climate at high palaeolatitudes in Late Permian Gondwana. Changes in the composition of Carboniferous–Permian low–latitude wetland vegetation are discussed in relation to tectonic activity and glaciation, with most confidence placed on the conclusion that major extinctions 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.