Early Palaeozoic (Ordovician-Silurian) Ostracoda of northeast Vietnam

Ostracods are the most abundant fossil arthropod group, present in the geological record since at least the Early Ordovician. Despite a global distribution in marine shelf facies almost from their first appearance in the fossil record, studies on their palaeogeographical distribution have been dominantly in North America and Europe. Here I address one of the regional knowledge gaps, describing the palaeobiogeographical and palaeoecological significance of Late Ordovician and late Silurian ostracods from northern Vietnam, an area which was part of the South China palaeoplate through the early Palaeozoic.

This thesis presents a detailed systematic palaeontology for an assemblage of Late Ordovician (Katian) ostracods from the Phu Ngu Formation, Na Ri District, Bac Kan Province. The assemblage yields nine species from seven genera, including two new palaeocopine species, five binodicopine species (three new, including one new genus, and one indeterminate species), one new leiocopine species and a possible metacopine.

The palaeobiogeographical significance of the ostracod genera from the Phu Ngu Formation is explored. A novel approach combines palaeogeographical reconstructions of ostracod locations with palaeoclimate simulations of ocean temperatures. The deep-marine palaeo-depositional setting of the Phu Ngu assemblage, alongside general circulation model simulations of Ordovician ocean-temperatures, and global patterns of ostracod distribution, suggests the possibility of early radiation of benthic ostracods into the deeper, colder ocean below the thermocline during the Late Ordovician, and perhaps earlier.

The thesis documents late Silurian (Pridoli) ostracods from the Si Ka Formation of northern Vietnam. The occurrence of two, low diversity but distinct ostracod assemblages associated with estuarine deposits, coupled with previous incidental reports of ostracods occupying marginal and brackish settings through the late Silurian and Devonian suggests ostracods were pioneering the occupation of marginal marine and estuarine settings 60 Myr before their proposed colonisation of such settings in the Carboniferous Period.