Oyster–microbial rolling stones (ostreoliths) from the Upper Jurassic (Kimmeridgian) of Sulejów and Małogoszcz, Poland, are described and interpreted in terms of their origin and development. The ostreoliths are composed mainly of small-sized oysters (Nanogyra nana) that initially encrusted bivalve or ammonite shells. These structures were frequently overturned during their further growth by hydrodynamic agents. The oyster shells are covered by a thin crust that is suggestive in appearance of a microbial mat. We postulate that the mats not only bound the shells, but also induced early cementation of the sediment infill, thereby triggering the rapid lithification and rigidity of ostreoliths. This interpretation is supported by the presence of bivalve borings that cut the shells, microbial crust and internal sediment infill. The oyster–microbial association provided favorable conditions under which hardground biota thrived in a soft-bottom setting which was otherwise inimical to such organisms. These hardground biota are represented by a variety of encrusters (foraminifera and foraminifera-like problematica, plicatulid bivalves, cyclostome bryozoans, serpulids, and thecideid brachiopods) and borers (bivalves, bryozoans, and phoronids). Ostreoliths from Sulejów are of smaller size, and with a less-developed oyster cortex, than those from Małogoszcz. This may be due to a higher frequency of physical disturbance (storms) at Sulejów which halted ostreolith development at earlier phases than at Małogoszcz. A large oyster buildup recorded previously from Brzegi, close to Małogoszcz, suggests that, under favorable conditions, ostreoliths could eventually have transformed into stationary, patch-reef–type structures.

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