Material and methods

All hardground samples were collected from the Ohesaare cliff (Saaremaa, Estonia) (Figure 1). All samples originate from the top 40 cm of the section, about at the level of the previously reported hardground (Mõtus and Hints 2007) (Figure 2 and Figure 3). Sample orientations were marked during collecting. Thereafter, they were manually cleaned of clay with water and a soft brush. Trypanites borings and microconchids were counted in a 5x5 cm grid. All encrusting fossils were drawn on a transparent plastic film, and their surface area was estimated using a millimeter grid. A centimeter grid was used to calculate the total surface area of the hardground samples. We were able to identify microconchid species in the few cases when shells were completely preserved. Most of the spirorbiform shells were partially broken or preserved as spiral attachment scars. They were identified at group level as microconchids if smaller than 3.0 mm in diameter. Juvenile growth stadia of Anticalyptraea calyptrata are very similar to those of microconchids, so the actual number of Anticalyptraea calyptrata may have been slightly higher and the number of microconchids slightly lower in the association than estimated here. Fragments of encrusting graptolites were very incomplete and too poorly preserved for measuring the surface area of encrustation. Conchicolites sp. and Aulopora sp. were also too incompletely preserved to measure area of encrustation. Remains of probable cementing brachiopods were too poorly preserved for certain identification. Figured specimens are deposited at the Geological Museum, Museum of Natural History, University of Tartu (TUG).