Affinities and palaeobiogeographical significance of some Ordovician bivalves from East Yunnan, China

The four poorly known Ordovician bivalve genera erected by Guo (1988) from the Lower Ordovician (Floian) Hongshiya Formation of East Yunnan, China are redescribed, based on a re-examination of the type material, and their taxonomic position is clarified. This paper places the four genera in the superfamily Cycloconchoidea. Reinterpretation of the hinge structure of Baidiostraca allows us to recognize it as a cycloconchoid rather than a myophorid; the new family Baidiostracidae is proposed. Zhenxiongella gen. nov. is described based on Taselasmodum septatum Guo, 1988, because this species differs significantly from the type species Taselasmodum decussatum Guo, 1985. These two genera differ fundamentally from other cycloconchids in possession of a strong posterior internal septum in each valve and a pedal retractor scar at the posterior of the hinge-plate; they are included in the new subfamily Taselasmodinae. The poorly understood family Nyassidae Hall, 1885 is enlarged and discussed herein; it seems that this family forms a unique lineage in the Cycloconchoidea. Zadimerodiidae Guo, 1988 is a junior subjective synonym of Nyassidae Hall, 1885. On the basis of the bivalve faunas, East Yunnan was sufficiently far from West Yunnan to prevent interchange of most species, although one species is common to both plates. On the other hand, a common genus and the similarities between two cycloconchids suggest proximity of East Yunnan and Australia, although these regions have no species in common. New information on the duplivincular ligament of glyptarcids allows these to be classified with the superorder Pteriomorphia, rather than the Heteroconchia, as previously proposed. The comparisons between the eastern West Yunnan fauna and those of Western Europe, where glyptarcids are also important members of the fauna, suggest that the eastern West Yunnan fauna lay at a much higher palaeolatitude than the East Yunnan fauna.