Early and early Middle Ordovician chitinozoans from the Baldone drill core, central Latvia

Early and early Middle Ordovician chitinozoans are relatively poorly known in Baltoscandia, thus the understanding of the early diversification of the group, as well as its biostratigraphic utility, is limited. In this paper, we document the distribution of chitinozoans in the Floian and lower Darriwilian strata of the Baldone drill core from central Latvia, a region from where very limited data on chitinozoans have hitherto been available. The Baldone section hosts one of the thickest upper Lower to lower Middle Ordovician sequences in eastern Baltoscandia and is represented by ca 85 m of red-coloured, mottled and less commonly grey marls and limestones. Chitinozoans were collected from 20 samples, revealing altogether at least 13 genera and 35 species. The lowermost productive sample, attributed to the lower Floian, represents one of the oldest chitinozoan faunas from Baltica. The upper Floian and Dapingian redbeds are barren of chitinozoans, but the lower Darriwilian Šakyna and Baldone formations contain a rich and wellpreserved assemblage with range-through taxonomic diversity reaching up to 19 species per sample. The Baldone section provides new insights into the distribution of the regional biozonal species Cyathochitina regnelli, which is characteristic of the Kunda Regional Stage in Estonia and Sweden. Several other biostratigraphically significant species of Cyathochitina, Conochitina and Lagenochitina that are typical of other regions were also recovered. They suggest links with chitinozoan assemblages of other continents during the early Darriwilian. Based on the rich collection at hand, chitinozoan diversity estimates for early Darriwilian are increased, and four new chitinozoan species are described: Bursachitina baldonia, Conochitina ulsti, Eremochitina? procera and Sphaerochitina? latviensis.