A cluster of the brachiopod Magas chitoniformis (von Schlotheim, 1813) represents settling of a single spatfall on a soft-bodied substrate, probably a sponge: instantaneous burial by a chalk ooze debris flow (Maastrichtian, Denmark).

A well-defined monospecific cluster of the brachiopod Magas chitoniformis (von Schlotheim, 1813) was found in uppermost Maastrichtian chalk at Stevns Klint, Denmark. It comprises a sheet-like accumulation of 53 well-preserved adult specimens with an average length of 7.0 mm. The stratigraphical level is the ‘Sponge Bed’ named so by fossil collectors because of the wealth of imprints or skeletal elements of large sponges, commonly showing 3D-preservation. The unimodal size-frequency distribution of the brachiopod shells indicate that they represent a single successful spatfall most likely on a soft-bodied sponge. There are no signs of transport or winnowing of the chalk and the cluster is interpreted as formed by instantaneous burial of the sponge and its epifauna by a chalk ooze debris flow. The triggering mechanism was probably a major earthquake caused by movement on a nearby syndepositional fault. Another cluster dominated by Magas chitoniformis was found in the Nørre Flødal quarry in northern Jylland in an upper Maastrichtian chalk rich in imprints after sponges. A similar burial mechanism is envisaged based the position of the locality in the tectonically very active Sorgenfrei-Tornquist Zone. The clusters are excellent examples on brachiopod attachments to a soft-bodied organism, which has left no direct trace in the sediment.