Development of faunal diversity during the late Llandovery–early Wenlock in the easternmost part of the Baltic Palaeobasin – implications for the Ireviken Event

The composition, diversity and distribution of various groups of organisms in the late Llandovery (Telychian) and early Wenlockian (Sheinwoodian) interval of the Pahapilli 675 and Kõrkküla 863 sections (western Estonia) were studied. Due to the small size, fragmentation and disarticulation of the specimens, the studied paleontological material could, in the major part, be tentatively considered as “biodetritus” or “bioclast material” occurring in the carbonate and siliciclastic lithologies. The recognized conodont biozonation provides a detailed stratigraphic framework for this study. The brachiopod fauna is represented by two different Dicoelosia–Skenidioides communities, comprising diverse associations of non-brachiopod fauna, whose composition changes over time. Successive associations of trilobites are based on dominating taxa among odontopleurids, encrinurids, calymenids and aulacopleurids. The distribution and taxonomic composition of Silurian echinoderms (cyclocystoids, bothriocidarid echinoids, pisocrinid crinoids) in the East Baltic area are documented for the first time. New data on bryozoans, corals, sponges and other fossils are provided, improving the knowledge of the whole biota of the open shelf and transition to basinal environments. The distribution of faunas is affected by different changes in the ecosystem. The highest diversity was recorded in the Upper Pterospathodus a. amorphognathoides Conodont Zone, before the main faunal turnover caused by the Ireviken Event, which was associated with rapid sea-level fall and climate changes. The faunal recovery started at the early Wenlockian in the second half of the Ireviken Event, above the base of the Lower Kockelella ranuliformis Conodont Zone.