Traces of arborescent lycopsids and dieback of the forest vegetation in relation to the terminal Permian mass extinction in North China

An attempt to trace the last descendants of Palaeozoic arborescent lycopsids is recorded based on a set of material including megafossils, cuticular structures and dispersed megaspores, which were found from the lower and upper Upper Permian Sections, representing about a 14 myr time-span, in the Baode and Linxian districts NW Shanxi and adjacent areas. These materials indicate that the Palaeozoic arborescent lycopsids in North China might have persisted up to the end of the Permian, when the terminal Permian mass extinction took place and that the replacement of the Palaeozoic forest by small Mesozoic Isoetales had occurred ahead of the extinction. At the lower Upper Permian Baode Section, an alternate sequence of megafossils from Synchysidendron via Paralycopodites back to Synchysidendron is discerned, roughly compatible with the sequence in dispersed megaspores from Sublagenicula via Laevigatisporites to Sublagenicula at the same Section. This indicates a major climatic fluctuation at that time. Anomalous results of stomatal indices from the lycopsids in five horizons provide some evidence that the Permian lycopsids were capable of CAM photosynthesis. Synchysidendron baodeense (sp. nov.), the last descendant of the Carboniferous lepidodendrids has been described from the lower Upper Permian, and Chaloneria? sp., a small lycopsid in association with Triangulatisporites type-megaspore, from the upper Upper Permian has been regarded as probable precursor of the Mesozoic Isoetales. Special attention is paid to the appearance of charcoal-beds and fusainized lycopsid remains, which provide significant evidence for palaeofire activity on the eve of the terminal Permian mass extinction. This could be the main extrinsic factor to trigger the sudden decline and even disappearance of the lycopsid forests through disrupting their reproductive functions. Some discussions on 'dieback of forest vegetation' after the P-Tr event are presented. Known megafossil evidence is not fully consistent with the scenario that has been drawn based on palynological data of West Europe. The dieback is a long-term evolving process that occurred over a 20 myr time-span during Permian and Triassic, and took place diachronologically everywhere. In North China, there were two steps for the dieback. First, there was an arborescent lycopsid dieback in the mid-stage (Kazanian) of the Late Permian. The main extinction mechanisms are thought to be water-stress and high-frequency wildfires. Second, sparse trees or small population of conifer woods locally replaced the lycopsid forest and soon went to extinction. No resurgence of conifer forest occurred in N. China and diverse horsetails might be the pioneers of the Triassic vegetation recovery. (C) 2001 Elsevier Science B.V. All rights reserved.