ANALYSIS OF A RHUDDANIAN (LLANDOVERY, LOWER SILURIAN) SCLEROBIONT COMMUNITY IN THE HILLISTE FORMATION ON HIIUMAA ISLAND, ESTONIA: A HARD SUBSTRATE-DWELLING RECOVERY FAUNA

The Hilliste Formation on the island of Hiiumaa, western Estonia, is a Rhuddanian (Llandovery, Lower Silurian) sequence of limestones and shales. It represents some of the earliest Silurian rocks on the paleocontinent of Baltica. The depositional system was tropical and shallow marine with tempestites indicated by overturned and broken corals and stromatoporoids. This unit contains a recovery fauna from the Ordovician Mass Extinction. Major taxa in the Hilliste Formation include crinoids, trilobites, bryozoans, corals, stromatoporoids, gastropods, and brachiopods. Sclerobiont communities (organisms that lived on or within hard substrates) have not yet been described from Rhuddanian faunas. The Hilliste Formation contains many encrusters and a few borings on skeletal substrates (primarily corals and crinoid stems). These sclerobionts include at least three kinds of crinoid holdfasts, cornulitids, sheet-like bryozoans, runner-type bryozoans, erect bryozoan holdfasts, and auloporid corals. Most if not all of these sclerobionts inhabited dead substrates. We studied the Hilliste Formation in a small quarry near the village of Hilliste on Hiiumaa. Numerous encrusted and bored specimens were collected for analysis of sclerobiont occurrences in this rare example of a Rhuddanian hard substrate community. These encrusters and borings, along with the macrofauna, have a distinct Late Ordovician aspect.