Flora of the Kazanian–Urzhumian boundary in the middle Permian of the Russian Platform

Abstract

Rich fossil plant localities in the upper Guadalupian and the lowermost Lopingian of the Russian Platform are confined to several discrete “levels” or “horizons” divided by intervals that are almost devoid of plant remains. One such “horizon” is located at the boundary between the Upper Kazanian and the Urzhumian of the regional stratigraphic scale. Numerous localities of this level can be grouped into two geographical clusters, the northern one being confined to the Kama River Basin, and the southern one to the Orenburg Region and Southern Bashkortostan. Regarding the southern localities, three floras that are seemingly successive in time can be distinguished. Against a common background of articulates (Paracalamites and Equisetites), the lowermost flora is characterized by the dominance of leaves of Rufloria (Cordaitanthales) with very rare conifers. Conifers (Quadrocladus, Sashinia, Dvinostrobus) and leaves of Phylladoderma (Peltaspermales) are the most abundant elements of the middle assemblage, whereas Rufloria leaves are very seldom found together with the peltasperm Ginkgophyllum and the conifer-like Steirophyllum. The uppermost flora is dominated by Odontopteridium and Ustyugia, two closely related genera of peltasperms, whereas cordaitaleans are totally absent. Comparison with the northern localities, which can be linked to the type sections of the Upper Kazanian and the Urzhumian, enables the dating of these assemblages in terms of the regional stratigraphic scale. All three floras prove to be confined to the uppermost Kazanian, and only the youngest one could also occur in the lowermost Urzhumian. As the stratigraphic ranges of all observed plant taxa exceed the total stratigraphic interval under study, the sequence of the floras seems to be caused by ecological (most likely climatic) factors rather than the actual evolution of plants. In particular, the observed gradual elimination of cordaitaleans confirms the general view on the extinction of this group on the Russian Platform. The southwestern boundary of the occurrence of cordaitaleans on the Russian Platform stretched from the southwest to the northeast, approximately parallel to the palaeolatitudes reconstructed on the basis of palaeomagnetic data. During the Guadalupian it moved to the northeast, which is to the north considering the position of the North Pole of that time. Cordaitaleans were the main peat-forming plants in the Late Palaeozoic of northern Pangea (in the Kuznetsk, Tunguska and Pechora Basins). So their retreat to the north was most likely a consequence and a reflection of the warming and drying of the climate.