Cyclic ecological replacement of brachiopod assemblages in the top-Eifelian Dobruchna Brachiopod Shale Member (Skały Formation) of the Holy Cross Mountains (Poland)

The palaeoecology of fossiliferous shales, belonging to the upper part of the Dobruchna Brachiopod Shale Member (= set XIV) of the Skały Formation (northern Holy Cross Mountains), was studied quantitatively in a succession in the transient (1989) trench A, 5.6 m thick, near the village of Skały. The top-Eifelian strata, recording the carbonate crisis during the global Kačák Bioevent, are well known for having a particularly diverse brachiopod fauna. The four brachiopod assemblages, recognised herein, were mainly controlled by the evolving bottom-sediment properties of the outer carbonate ramp basin. Soft, unstable substrates were inhabited by the poorly-diversified Poloniproductus assemblage, associated with a distinctive, ‘incumbent’ set of largely semi-infaunal, generalist species. The pioneer community, as a result of progressive consolidation of bioclast-enriched sediment, evolved toward a more diverse biota. This consequent stabilisation of the substrate resulted in the progressive growth of crinoid thickets or bryozoan-dominated biostromes and patches, associated with rich, subordinate, sessile and vagile benthos. In this stage, diverse brachiopod assemblages were dominated by the pedunculate, eurytopic, ribbed spiriferide Eleutherokomma or specialized orthides (Aulacella, Costisorthis) in the Dobruchna Mbr, and by the expansive, large, free-lying orthotetide Xystostrophia in the overlying set XV of the Skały Fm The cyclic ecological replacement, with the characters of ecological succession in the final phase, was evidently stimulated by an irregular transition from soupy muds to a mosaic of bioclast-rich and firmer, biogenic sediments, within the cyclic pattern of distal tempestite sedimentation. The three episodes of variously reduced deposition rate, recorded in the more diverse benthos, culminated in the pioneer bryozoan/coral reef growth and abundance of epibionts, alternating with times of destructive storm activity and deposition from suspension clouds in the muddy habitats.