End-Ordovician ostracod faunal dynamics in the Baltic Palaeobasin

The Late Ordovician Baltic Palaeobasin (BPB) offered a favourable environment for a diverse and abundant ostracod fauna to thrive across the basin. A short period of unstable environmental conditions at the end of the Ordovician, the Hirnantian glaciation, and the concurrent extinction event completely rearranged the ostracod associations. Statistical analyses reveal temporal (diverse pre­glacial, more uniform glacial and poorly diverse post­glaciation faunas) and geographical (upper shelf and middle shelf faunas) separation of ostracod associations in the BPB. The geographical division applies to the pre­glacial and glacial faunas only; the scanty post­glacial fauna found in deeper­water sections is uniform. Ostracod associations are distinct both in a temporal and geographical sense. Juxtaposing ostracod data with δ13C curves shows that the typical Hirnantian Harpabollia harparum fauna in the BPB appeared near the peak of a δ13C excursion. This might explain why the H. harparum fauna seems to appear first in deeperwater sections and is absent or appears later in shallower­water sections. The majority of the nearshore sections in Estonian and Lithuanian shelves lack most or all of the Hirnantian strata. The lowermost Hirnantian is preserved in only a few sections (e.g. Männamaa and Puhmu in Estonia). The appearance of the post­glacial ostracod fauna, which most probably was a survival fauna rather than a recovery fauna, is related to the falling limb of the δ13C curve. The appearance of this fauna was previously considered as a marker of the Ordovician–Silurian boundary in the region but, according to the present understanding, took place in the late Hirnantian.