Ostracods from the Upper Silurian Si Ka Formation, Northern Vietnam, and Their Paleobiogeographical Significance

Abstract. The first detailed, systematic record of Silurian ostracod crustaceans from Vietnam is presented. Ostracods from the upper Silurian (upper Ludlow–lower Pridoli) Si Ka Formation of Ha Giang Province, northern Vietnam comprise ca. 10 species, including two new species of hollinoideans, two beyrichioideans, three species of eridostracine, at least one putative paraparchitoidean, and two indeterminate palaeocopid species. The fauna co-occurs with macroplant and fish fragments, and pterineid bivalves, in lithofacies that are interpreted as estuarine. The presence of beyrichioideans is consistent with their reported occurrence in marine, marginal-marine and estuarine ostracod assemblages elsewhere in the late Silurian and Devonian. One of the beyrichioidean species possibly represents Qujingsia nonaculeata, a species known from the Ludlow–Pridoli of South China, thus endorsing the supposed late Silurian age of the Vietnam fauna and its paleogeographical position on the South China paleo-plate. The other beyrichioidean is Beyrichia (Beyrichia), a globally distributed taxon in the Silurian and Early Devonian. Three species of eridostracine are referred to Cryptophyllus, a genus with a global distribution and temporal range from the Ordovician to Carboniferous. Hollinoidean taxa, whilst endemic at the genus-level, show affinities with glossomorphitine and sigmoopsinae taxa from Europe.