A Hirnantian deep-water refuge for warm-water ostracods in Baltoscandia

The latest Ordovician is marked by a severe climate change, the Hirnantian glaciation. This climatic event affected many marine taxa including ostracods. Rich and abundant ostracod assemblages of the Baltic Palaeobasin were severely impoverished. Many of the typical pre-Hirnatian warm-water ostracod species died out, but also some distinct, cold-water species appeared. Two very diffeent but likely coeval latest Ordovician ostracod assemblages are recorded in the Baltic countries and northeastern Poland. The latest Ordovician Estonian Shelf (inner ramp) is characterized by the Medianella aequa association whilst sections in the Livonian Basin (middle to outer ramp) reveal the Harpabollia harparum association that is thought to represent a cold-water assemblage belonging to the Dalmanitina–Hirnantia Fauna sensu lato. A transitional assemblage composed of a “species mix ure” of typical Hirnantian cold-water and some pre-Hirnantian warm-water ostracod species is described for the first time from the Kêtrzyn IG 1 borehole, north eastern Poland. The assemblage is dominated by Cryptophyllus pius sp. n. The genus Cryptophyllus is rare in the two other well-known assemblages. The discovery suggests that marginal parts of the Baltic Palaeobasin could serve as a kind of refuge for the last representatives of the ostracod faunas of the inner shelf of Baltic Palaeobasin. The Hirnantian assemblage is replaced by the low-diversity recovery assemblage that is dated as late Hirnantian–Silurian in Estonia and other areas. This suggests that the position of the systemic boundary in the Kêtrzyn borehole and elsewhere in northeastern Poland should be re-evaluated.