Evolution, migration and radiation of late Mesozoic conchostracans in East Asia

Late Mesozoic deposits in East Asia are mainly of continental origin and contain an abundance of conchostracans, whose rapid evolution and radiation makes them biostratigraphically useful in subdividing and classifying non-marine strata in the region. In this paper we discuss the evolution and radiation of conchostracan faunas during the late Mesozoic in East Asia. Pseudograpta, known from Callovian deposits in north-west Scotland, migrated to and flourished in East Asia during the early Oxfordian (Tuchengzian: early Late Jurassic), where it not only diversified into about eight species, but also led to the evolution of Monilestheria and Nestoria by adding a row of tubercles on growth lines and losing the very fine radial lirae on growth bands near the venter of carapaces, respectively. Nestoria reached the acme of its development during the Kimmeridgian (Dabeigouan: mid Late Jurassic), forming a major component of the Nestoria-Keratestheria fauna. By the beginning of the Yixianian (Tithonian or early Cretaceous: correlation with the standard geological time scale is controversial), an Eosestheria fauna began to flourish, occupying Transbaikalia in Russia, south Mongolia, and north and north-east China in the ancient Amur River drainage system. By the Early Cretaceous Eosestheria and Diestheria, two dominant genera of the Eosestheria fauna, evolved into Yanjiestheria and Neodiestheria respectively, through a reduction in the size of the reticulation of the carapace and an increase in the density of the radial lirae. The Yanjiestheria fauna flourished. Consisting of some 100 species in 16 genera, its distribution expanded westwards to the Junggar Basin and eastwards to coastal regions of south-east China, and to Japan. During the Cenomanian a homogeneous Nemestheria fauna was widely distributed in East Asia. Soon after, however, this evolved into four faunas within three palaeogeographic provinces: a Euestherites fauna in the ancient Amur River-Songhua Lake drainage system in north-east China; Linhaiella and Tenuestheria faunas in the Yunmeng Lake system in south-cast China; and an Aglestheria fauna in the palaeo-lakes of south-west China. Copyright (c) 2007 John Wiley & Sons, Ltd.