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The paper discusses the reliability of different biozones in terms of their synchroneity when crossing facies boundaries within a sedimentary basin. Graptolite biozones are the most trusted ones, but also biozones based on conodonts, chitinozoans and less ‘authoritative’ groups like ostracodes and vertebrates are used. The integration of bio- and chemostratigraphy aids the understanding of the pattern and timing of fossil distribution. Despite different environments most of the analysed biozones are in general synchronous units and in the majority of cases their time signals do not contradict each other. The much discussed Ozarkodina crispa (and its two morphs) has a stratigraphical range in the East Baltic that is longer than commonly recognized elsewhere. Its first occurrences in Estonia have been reported from shallow-water facies below the Mid-Ludfordian Carbon Isotope Excursion. The geographical distribution of Oz. crispa is much wider in the upper Ludfordian open shelf rocks and a few occurrences are also known in the lowest Přídolí in the East Baltic. Analogous ecostratigraphical trends are characteristic of some other fossil species. The analysis demonstrates that, when properly studied, all of the fossil groups considered can provide useful biostratigraphical information. The subdivision of the Přídolí Series into two stages is discussed. Bio-chemostratigraphical data confirm the late Ludfordian age of the Kuressaare Formation and its correlatives. The bio- and chemostratigraphical testing  of biozonal indices suggests some tentative correlations with other areas, in particular the type Ludlow area, and enables identification of the Silurian–Devonian boundary in the East Baltic

proceedings of the estonian academy of sciences. geology/estonian journal of earth sciences

A bio-chemostratigraphical test of the synchroneity of biozones in the upper Silurian of Estonia and Latvia with some implications for practical stratigraphy