PALEOECOLOGY OF THE HILLISTE FORMATION (LOWER SILURIAN, LLANDOVERY, RHUDDANIAN) HIIUMAA ISLAND, ESTONIA: AN EXAMPLE OF A SHALLOW MARINE RECOVERY FAUNA

The Hilliste Formation (Lower Silurian, Llandovery series and Rhuddanian stage) is well exposed in a quarry in western Estonia. During the deposition of this unit, Estonia was part of the paleocontinent Baltica, which was located near the equator. The Hilliste Formation thus records the recovery of tropical invertebrate marine communities following the mass extinction at the end of the Ordovician. Globally, pre-extinction levels of marine diversity were not met until the Wenlock, about 15 million years after the end of the Ordovician; this formation was deposited about three million years following the event. The Hilliste Formation contains a diverse fauna including brachiopods (orthids, atrypids, rhynchonellids, pentamerids, and strophomenids), corals (favositids, halysitids, heliolitids and rugosans), stromatoporoids, bryozoans, gastropods, crinoids, ostracodes and trilobites. We measured, described and sampled the Hilliste Formation at Hilliste Quarry on Hiiumaa Island, western Estonia. The unit records a regression from depths between normal and storm wavebase to depths at or above normal wavebase. The evidence for this paleoenvironmental interpretation includes more argillaceous beds in the bottom two-thirds of the formation and more biosparite/grainstone upwards. The top third of the formation consists of massive biosparite/grainstone with little clay and overturned and fragmented corals and stromatoporoids indicating high depositional energy. The fauna changes stratigraphically upwards from one dominated by brachiopods and gastropods to a community primarily of corals, stromatoporoids and crinoids. This fauna provides additional information about biotic recovery in eastern Baltica and its implications for the migration of Early Silurian Baltic taxa into other regions.