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 lycopsid s is recorded based on a set of material including megafossils, cuticular s tructures and dispersed megaspores, which were found from the lower and upp er Upper Permian Sections, representing about a 14 myr time-span, in the Ba ode and Linxian districts NW Shanxi and adjacent areas. These materials ind icate that the Palaeozoic arborescent lycopsids in North China might have p ersisted up to the end of the Permian, when the terminal Permian mass extin ction took place and that the replacement of the Palaeozoic forest by small Mesozoic Isoetales had occurred ahead of the extinction. At the lower Uppe r Permian Baode Section, an alternate sequence of megafossils from Synchysi dendron via Paralycopodites back to Synchysidendron is discerned, roughly c ompatible with the sequence in dispersed megaspores from Sublagenicula via Laevigatisporites to Sublagenicula at the same Section. This indicates a ma jor climatic fluctuation at that time. Anomalous results of stomatal indice s from the lycopsids in five horizons provide some evidence that the Permia n 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 lycop sid in association with Triangulatisporites type-megaspore, from the upper Upper Permian has been regarded as probable precursor of the Mesozoic Isoet ales. Special attention is paid to the appearance of charcoal-beds and fusa inized 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 disappeara nce of the lycopsid forests through disrupting their reproductive functions . Some discussions on 'dieback of forest vegetation' after the P-Tr event a re presented. Known megafossil evidence is not fully consistent with the sc enario 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-sp an during Permian and Triassic, and took place diachronologically everywher e. In North China, there were two steps for the dieback. First, there was a n arborescent lycopsid dieback in the mid-stage (Kazanian) of the Late Perm ian. The main extinction mechanisms are thought to be water-stress and high -frequency wildfires. Second, sparse trees or small population of conifer w oods locally replaced the lycopsid forest and soon went to extinction. No r esurgence of conifer forest occurred in N. China and diverse horsetails mig ht be the pioneers of the Triassic vegetation recovery. (C) 2001 Elsevier S cience B.V. All rights reserved.