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Palaeoecology of solitary corals in soft-substrate habitats: the example of Cunnolites (upper Santonian, Eastern Alps)




Diethard Sanders [], Institute of Geology and Palaeontology, Faculty of Geo- and Atmospheric Sciences, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria/EU; Rosemarie Baron-Szabo [], Department of Invertebrate Zoology, Smithsonian Institution, W-329, MRC-163, Washington DC 20560, USA; manuscript received on 23/01/2006, manuscript accepted on 25/05/2007.


The upper Santonian Hofergraben Member (Eastern Alps) provides an example of a soft-substrate habitat suited mainly for solitary corals (Cunnolites), for colonial forms of solitary coral-like shape (Placosmilia, Diploctenium), and for colonial corals of high sediment resistance (e.g. Actinacis, Pachygyra). The Hofergraben Member consists mainly of silty-sandy marls of wave-dominated, low-energy shore zone to shallow neritic environments. Substrates of soft to firm mud supported level-bottoms of non-rudist bivalves, gastropods, solitary corals, colonial corals, rudists, echinoids, and benthic foraminifera. Boring and/or encrustation of fossils overall are scarce. In the marls, Cunnolites is common to abundant. Both a cupolate shape and a lightweight construction of the skeleton aided the coral to keep afloat soft substrata. Cunnolites taphocoenoses are strongly dominated by small specimens (about 1–3 cm in diameter). Cunnolites was immobile and mostly died early in life upon, either, smothering during high-energy events, rapid sedimentation associated with river plumes, or by toppling and burial induced by burrowing. Comparatively few large survivor specimens may show overgrowth margins interpreted as records of partial mortality from episodic sedimentation or tilting on unstable substrate. Scattered pits and scalloped surfaces on large Cunnolites may have been produced, in some cases at least, by predators (durophagous fish?). Post-mortem, large Cunnolites provided benthic islands to corals, epifaunal bivalves and bryozoans. In a single documented case of probable in vivo contact of Cunnolites with the colonial coral Actinastraea, the latter prevailed.