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

Abstract

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.

Paleocene-Lower Eocene carbonate platforms of westernmost Tethys

“…The additional presence of planktonic foraminifera indicates open sea conditions into the photic zone, like the solitary corals, hampered of algal symbionts. Solitary corals usually thrive in soft fine-grained substrates both in shallow turbidwater habitats (Sanders and Baron-Szabo, 2008) and in deeper habitats in meso-to oligophotic marine environments (Serra-Kiel et al, 2003;Astibia et al, 2014).…”

Attachment structures in Rhizotrochus (Scleractinia): macro- to microscopic traits and their evolutionary significance

“…A wide variety of unique skeletal structures have evolved in free-living corals to adapt to soft-substrate environments (e.g. Gill & Coates 1977;Elias 1984;Neuman 1988;Hoeksema 1993;Plusquellec et al 1999;Sanders & Baron-Szabo 2008). In contrast, little is known about skeletal adaptations of corals living on hard substrates.…”

“…Gill & Coates ; Elias ; Neuman ; Hoeksema ; Plusquellec et al . ; Sanders & Baron‐Szabo ). In contrast, little is known about skeletal adaptations of corals living on hard substrates.…”