Epifauna as a tool in autecological analysis of Silurian brachiopods

The epifauna (mostly bryozoans) of some large brachiopod populations from the uppermost Wenlock Mulde Marl (Gotland, Sweden) have been analysed to obtain information on autecology and especially on the life position of the species. After separation of the post-mortem element of the epifauna, an attempt was made to discriminate between forms that settled above or below the water-sediment interface, and between those that settled on the exposed or the shaded side of the brachiopods. The results were tested against observations on a large number of brachiopods found in presumed life position in one bed. The size distribution in the supposed thanatocoenosis was also analysed. In some forms (dalmanellids, Atrypa, Striispirijer) the life position appears to be the 'normal' one, obliquely, with the beak down in the sediment. Two species gave unexpected results; Leptaena depressa, which seems to have lived with the discs in nearly vertical position, and Meristina obtusa, which appears to have been freely suspended by the pedicle, like many recent terebratuloids. Size distributions showed that some forms (the dalmanellids, Atrypa sp., and M. obtusa) had a 'normal' population structure, with many more larval and young valves than adults. Others, such as Striispirijer sp. and especially L. depressa had an abnormally low number oflarval and juvenile valves, indicating special ecological and reproductive conditions.