We report extant ostracods from the Co To Islands in northeastern Vietnam and identified 77 ostracod species. On the basis of the comparison of ostracod species reported from adjacent areas, the assemblages from the Co To Islands have a relationship with those of the Kymerian Province, especially with that of South China, and have little relationship with the East Indian Province. Our result is useful for the reconstruction of the dispersal of the Kymerian ostracods during the Quaternary.

Microscopic-sized fossils adapted to a particular chemosynthetic environment (such as cold methane seep) are poorly known. Here we report a new ostracod (small crustacean with high fossilization potential) species probably endemic to a cold methane seep environment. This new discovery is important because there is a wealth of microscopic-sized fossils in geological records and this species can be used as an indicator fossil for past cold methane seep environment.