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This study focuses on sclerobionts from a large collection of epibenthic echinoids (>2,000 specimens) of the genera Conulus and Camerogalerus. Samples were collected from five localities in southern Poland (Polish Jura and Miechów Trough), where Turonian carbonates with terrigenous input are exposed. Low intensity (mean ca. 5 %, maximum ca. 10 %) and slight encrustation ('loosening effect') exclusively by episkeletozoans probably resulted from low productivity of encrusters while the importance of other factors cannot be excluded unambiguously. Echinoids served as a main substratum and after death formed shellgrounds ('echinoid carpet') offering abundant benthic islands for encrusters in an otherwise soft-bottom environment. The moderate abundance but low-diversity assemblage is represented by bivalves, sedentary polychaetes, foraminifera, bryozoans, corals, and sponges. This assemblage is similar to a nearly contemporaneous assemblage from the Bohemian Basin. The presence of numerous spirorbins offers insights into their early evolution and may indicate that their first peak in abundance after origination was not prior to the earliest Turonian. This is regarded as one of the important ecological steps towards the rise of modern sclerobiont communities. Encruster diversities are independent of their abundance and, as shown in our novel planar projections, lateral parts of tests were preferentially encrusted. This pattern is explained by the combination of largest flat area and stable orientation. Encrusting bivalves and serpulids dominated hard substrate environments in the Turonian of Poland. [ABSTRACT FROM AUTHOR]

Bored and encrusted carbonate concretions, termed hiatus concretions, coming from the Middle Jurassic (Upper Bajocian and Bathonian) siliciclastics of the Polish Jura, south-central Poland, have been subjected to detailed paleoecological investigation for the first time. The concretions possess variable morphology and bear distinct traces of bioerosion and encrustation as a result of exhumation on the sea floor during intervals of low sedimentation and/or erosion. The borings are dominated by Gastrochaenolites and Entobia. Epilithozoans, represented by at least 26 taxa, are dominated by sabellid/serpulid worm tubes and bryozoans, while sponges and corals are minor. No relationship between the concretion size and the number of encrusters has been found, suggesting that concretion size was not the primary factor controlling diversity. Stable isotope analyses and the presence of crustacean scratch marks and Rhizocorallium traces on many of the hiatus concretions indicate that they formed just below the sediment-water interface, within the sulfate reduction zone. Moreover, crustacean activities may have been a prelude to their origin, as shapes of many concretions closely resemble thalassinoidean burrow systems. It is also possible that crustacean activity around the concretions promoted their exhumation by loosening the surrounding soft sediment. The presence of borings and encrusters on different concretion surfaces, as well as truncated borings and a number of abraded epilithozoans, indicate that after the concretions were exhumed they were repeatedly overturned and moved on the sea floor, probably due to episodic storm-related bottom currents in shallow subtidal environment. [ABSTRACT FROM AUTHOR]

Depositional redox conditions of the uppermost Bajocian–Bathonian (Middle Jurassic) ore-bearing clays of the Gnaszyn/Kawodrza area in the Polish Jura have been determined using an integrated geochemical (Th/U and U/Th ratios, degree of pyritisation (DOP), sulphur stable isotopes, biomarker analysis) and petrographic approach (measurements of pyrite framboid diameters, and microfacies analysis). The Th/U and U/Th ratios indicate that oxic conditions prevailed on the sea-floor during this interval, and 34S isotopes suggest open-system conditions. DOP values, however, are rather scattered, and may reflect oxic, dysoxic, or anoxic conditions. We consider that the DOP values result from reducing conditions within the sediment and the chemistry of the pore-waters, rather than true sea-floor redox conditions. Pyrite framboid populations also indicate that dysoxic conditions prevailed within the sediment, beneath an oxygenated water column. Biomarker data did not provide any evidence of water column stratification or anoxia during sedimentation of the Middle Jurassic clays. [ABSTRACT FROM AUTHOR]