ECOLOGICAL SUCCESSION IN CRYPTIC SCLEROBIONT COMMUNITIES FROM THE CALLOVIAN (MIDDLE JURASSIC) NEAR KRAKÓW, POLAND

A cryptic Callovian (Middle Jurassic) sclerobiont community is well represented in southern Poland at Zalas Quarry near Kraków. It is found associated with a complex sedimentary sequence which begins with a sandy crinoid-rich biosparite. This unit is deeply eroded into a ferruginous hardground capped with stromatolites (from a few mm to up to 40 cm thick) succeeded by pinkish marls and limestones rich in iron ooids and oncoids. The hardground is probably Middle Callovian whereas the stromatolitic level and lower part of the pinkish marls are uppermost Callovian. The lack of Middle Callovian ammonoids in this interval probably shows a significant stratigraphic gap (the Jason and Coronatum Zones) between the biosparite and the stromatolites. The marls and limestones above are rich in ammonites of the Mariae Zone of the lowermost Oxfordian (Upper Jurassic). Two types of cavities developed on the Callovian hardground surface. The first formed when aragonitic ammonoid shells were buried and then dissolved, leaving narrow voids in the cemented sediments. The second cavity type was the underside of the ventral valve of the calcite-shelled clam Ctenostreon proboscideum, which did not dissolve. Both cavities were occupied by similar encrusting cryptic sclerobionts. Serpulids (Glomerula, Serpula, Cycloserpula and Dorsoserpula) and cyclostome bryozoans (Hyporosopora, Reptomultisparsa and Stomatopora) dominated much of the encrusting space, but other encrusters included oysters (Liostrea), plicatulids (Plicatula), calcareous sponges, foraminiferans (Nubecularia) and thecideidine brachiopods. Our paleoecological analyses of overgrowth data show that bivalves and serpulids were the opportunistic first encrusters in the community succession, followed by cyclostome bryozoans, thecideidine brachiopods, foraminiferans and sponges. This pattern is similar to what has been observed in other Jurassic cryptic sclerobiont communities in Britain, France, Israel and Utah. These communities provide important insight into the paleoenvironmental transition near the Middle-Late Jurassic Transition (MLJT) in south-central Europe. The succeeding Callovian stromatolites are unusual in that they were apparently formed in deep water as shown by a pelagic fauna associated with them.