Taxonomic and ecological variations of Permian-Triassic transitional bivalve communities from the littoral clastic facies in southwestern China
Abstract Immediately after the latest Permian mass extinction, bivalve faunas colonized the Permian-Triassic Transitional Beds (PTTB), especially in littoral clastic facies of South China. The Permian-Triassic (P-Tr) transitional bivalve fauna is composed of holdovers, long-term survivors and newly evolved taxa, and characterized by moderate diversity and high dominance. However, the taxonomy and ecology of this bivalve fauna of littoral clastic facies have not been well understood, especially in comparison with offshore marine records. This paper presents a study on a P-Tr transitional bivalve fauna from five sections of littoral clastic facies in southwestern China, and as such it contributes to a better knowledge of the ecological processes of benthic communities during the P-Tr transition. In total, 20 species in 13 genera of bivalves are identified from the PTTB in the lower part of the Kayitou Formation and the basal part of the Feixianguan Formation of the five sections. Cluster analysis suggests that the transitional bivalve fauna could be further divided into two successive communities (early and late communities), and each is composed of five assemblages. Promyalina–Neoschizodus community (early community) is characterized by presence of holdover taxa, high taxonomic diversity and evenness, and more abundant shallow infaunal suspension feeders. In Pteria community (late community), taxonomic diversity and evenness are much lower, and stationary epifaunal suspension feeders boost at the expense of shallow infaunal suspension feeders compared with the Promyalina–Neoschizodus community. Furthermore, the Promyalina–Neoschizodus community shows a clear spatial variation along environmental gradient from brackish lagoon to shallow marine settings, while the Pteria community is more uniform and homogeneous laterally. The Promyalina–Neoschizodus community in the studied region is of latest Permian age while the Pteria community should belong to the earliest Triassic. Consequently, a significant taxonomical and ecological turnover of bivalve communities occurred between the Promyalina–Neoschizodus community and the Pteria community during the extinction interval in southwestern China.