Diverse sclerozoan assemblages encrusting large bivalve shells from the Callovian (Middle Jurassic) of southern Poland

A diverse sclerozoan assemblages consisting of both encrusting and boring biota are described from the large limid bivalves Ctenostreon from the Callovian hardground setting of Zalas in southern Poland. At least 27 encrusting and seven bioerosion taxa are reported here, which makes this assemblage not only one of the most diverse in the Middle Jurassic, but the richest in encrusting taxa from the Callovian. The encrusters consist of cryptic biota of which sedentary polychaetes and cyclostome bryozoans dominate with respect to both species number and abundance. The bioerosion traces are dominated by tiny pits referred to the ichnogenus Oichnus, probably made by some soft-bodied biota in the present case, followed by the borings of acrothoracican barnacles (Rogerella). The first colonizers of the bivalve shells probably were borers as they only occur in the host shells. The encrusting pioneers presumably were oysters and oyster-like bivalves, followed by opportunistic serpulid/sabellid polychaetes and cyclostome bryozoans. The last colonizers were calcisponges and thecideide brachiopods. In comparison to the only known Late Callovian shallow and reef-associated, tropical sclerozoans of Israel, the assemblage from the open-marine, deeper setting of Poland is much richer in encrusting taxa. Such a surprising high encruster diversity in the marine northern paleo-latitude may have resulted from the deeper and calmer environment with a both reduced sedimentation rate and algal cover, and without any significant salinity changes, the factors that are thought to have impacted the tropical and shallow-marine sclerozoans from Israel.

► Callovian (Middle Jurassic) sclerozoan assemblages are poorly known world-wide.
► The most diverse encrusting assemblage from the Callovian from Poland is reported.
► Callovian sclerozoans from Poland are much diverse than tropical ones from Israel.
► High diversity relates to deeper, calm environment with a low sedimentation rate.
► It also relates to reduced light hampering a development of intensive algal cover.