Diversity and richness of the Devonian terrestrial plants in the Southeastern Mountainous Altay (Southern Siberia): Regional versus global patterns

Abstract

Devonian deposits of the Southeastern Mountainous Altay, a large region in Southern Siberia, contain abundant remains of terrestrial plants. A semi-quantitative analysis of this regional palaeobotanical record reveals how both the diversity (number of taxa) and richness (number of taxa with an account of their abundances) of floristic assemblages changed during the Emsian–early Famennian time interval. A total of 60 species, representing 42 genera, are known from 8 regional assemblages. Changes in diversity of species and genera occurred simultaneously. The number of taxa was high in the early Emsian, declined in the late Emsian, rose again in the Eifelian–middle Givetian, dropped in the late Givetian, reached the highest values in the early Frasnian, and experienced the greatest decline taking place in the late Frasnian–early Famennian. The standing diversity (number of taxa crossing the time boundaries) was maximal in the Middle Devonian. The dynamics of terrestrial plant richness was similar to that of diversity with an exception of middle Givetian decline in richness despite growth of diversity. The floras were dominated by pteridophytes. Propteridophytes were less abundant, and pinophytes were very rare. Propteridophyte extinctions were high in the middle Givetian, concurrent with a brief decline in pteridophytes. Some regional and global patterns of floral dynamics were similar. However, propteridophyte decline was not abrupt globally. The main abiotic driving factor influencing species richness and diversity appears to have been regional shoreline shifts. It does not appear that climate changes was important for regional changes in phytodiversity, although both regional and global phytodiversity was at its high during the Middle Devonian cooling phase. A comparison of palaeobotanical data from the Southeastern Mountainous Altay and Kazakhstan suggests palaeogeographic proximity and proves an idea of Altay-Mongolian terrane wandered between Gondwana, Kazakhstan, and Siberia. The early Emsian and the early Frasnian, when floras of the study region were diverse and rich, are characterized by the very high degree of similarity. Thus, interregional floral exchanges would facilitate plant radiations.