Yuzhou Flora—A hidden gem of the Middle and Late Cathaysian Flora

Abstract

The Yuzhou Flora of the southern North China Platform possesses the typical regional features of the Middle and Late Cathaysian Flora. For a long time during the Permian, this area was located on shoreline marine tidal flats and deltas at low latitudes with a warm and humid climate. Consequently, many successive layers with abundant well-preserved plant fossils and a complete evolutionary sequence were developed. This is a unique and ideal place to study the Middle and Late Cathaysian Flora. We have recognized 111 genera and 307 species from this flora. The rapid morphological evolution and uninterrupted stratigraphic succession of Lobatannularia (Sphenophytes) make this group one of the bases for biostratigraphic divisions of the Permian System of the North China Platform. Ninety percent of Pteridophytes belong to the order Marattiales, which are well developed during the early and middle stages of the Yuzhou Flora but experience a sudden decline during the late stage. Ancient Pteridospermatophytes only flourished during the Middle Permian whereas advanced Peltaspermales are extremely well developed at the end of the early Late Permian. Among them Shenzhouphyllum is one of characteristic elements in the late stage of the Yuzhou Flora. The Cycadophytes evolved relatively early in this area and they differentiated and proliferated during the late Middle to early Late Permian. The number of species of Ginkgophytes is small but this group is extremely abundant, especially in the early Late Permian. The Gigantopteridales (preangiosperms) is a very important and unique group in the Yuzhou Flora. Based upon their venation and leaf architecture, this group can be divided into three evolutionary stages, i.e., appearance and differentiation, first flourishing, and second flourishing and sudden disappearance, which form the basis for biostratigraphic division and correlation of the Permian System of the North China Platform. Based upon the evolutionary trend of venation pattern and leaf architecture, we propose some taxonomic changes at the generic level. The Yuzhou Flora emerged at the middle Early Permian (Zisongian) and suddenly disappeared at the early Late Permian (Wujiapingian). It is later replaced by the Euro-American Zechstein flora in the late Late Permian. The Yuzhou Flora is divided into three (early, middle and late) stages based upon the cyclic development of its plant groups. Five fossil plant assemblage zones are recognized from the Yuzhou Flora based upon the succession of each group and their combined characters in different layers.