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

Truuver K., Meidla T.

Geological Quarterly

2015

Vol. 59, No. 4

738--749

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 severly impoverished. Many of the typical pre-Hirnatian warm-water ostracod species died out, but also some distinct, cold-water species appeared. Two very different but likely coeval latest Ordovician ostracod assemblages are recorded in the Baltic countries and north-eastern 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 mixture” of typical Hirnantian cold-water and some pre-Hirnantian warm-water ostracod species is described for the first time from the Kętrzyn IG1 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 Kętrzyn borehole and elsewhere in north-eastern Poland should be re-evaluated.

The Late Glacial and Early Holocene environmental history of shal low lakes in Es to nia, re vealed from subfossil ostracod data

Sohar K., Meidla T.

Geological Quarterly

2009

Vol. 53, No 2

209-218

The Late Glacial and Early Holocene shallow lake history in Estonia is documented from the fresh water ostracod subfossil record. Three cores studied consist of Late Glacial and Holocoene sediments: gyttja, calcareous mud and peat, with ostracod subfossils being well preserved in the calcareous mud. 18 fresh water ostracod species were recorded in the cores: the most common species are Metacypris cordata, Limnocythere inopinata, Cyclocypris ovum, Cypridopsis vidua, and Candona candida. Changes in the ostracod succession of the lakes can be interpreted in the context of environmental changes that were not contemporaneous but were related to the evolution of particular water bodies. Darwinula stevensoni and Metacypris cordata do not appear together in the Late Glacial and Early Holocene lacustrine sediment records of Estonia. M. cordata appears as the water body evolves or by lowering of the water level. The appearance of Scottia pseudobrowniana in the sediments refers to the stage of an overgrown lacustrine system. M. cordata appeared in south eastern Estonia at the end of the Late Glacial (~12 800 cal. BP), when the calcareous mud started to form. The ea li est subfossil r cord of M. cordata from south wes ern Estonia is from the Early Holocene.