| Summary: | Phosphatic microfossils, such as conodonts and fish microremains (dermal scales) from the upper Silurian (Pridoli) of Lithuania have been studied for their oxygen isotope composition. The conodont colour alteration index of the biogenic apatite did not exceed 1.5 reflecting only a minor thermal alteration. Conodont δ18Oapatite values range from 17.7 to 19.2‰ V-SMOW, with the average values around 18.3‰. Fossil apatite of fish exoskeleton microremains from the same samples have lower δ18Oapatite values ranging from 15.2 to 17.4‰ V-SMOW, with the average values around 16.3‰ V-SMOW. Palaeoseawater temperatures calculated from conodont apatite δ18O range from 24.8–31.5 °C, those derived from fish δ18O give approximately 10 °C higher temperatures ranging from 32.6 to 38.1 °C. The lower δ18Oapatite values of fish apatite and thus the unrealistic high palaeotemperatures are interpreted as a result of diagenetic alteration. A significant positive shift in conodont apatite δ18O of + 1.1‰ at the boundary between the Vievis and Lapės formations is observed, coinciding with a major facies change. It is interpreted as a mid-Pridoli event within the Ozarkodina eosteinhornensis Bizone, or the Ozarkodina remscheidensis Biozone reflecting a major cooling event, which may have resulted in the formation of an ice sheet in high latitudes and a glacio-eustatic sea-level fall.
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