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The Dangers of High-Rise Living on a Muddy Seafloor: An Example of Crinoids from Shallow-Water Mudstones (Aalenian, Northern Switzerland)

¹ Geologisch-Paläontologisches Institut, Universität Basel, Bernoullistrasse 32, CH-4056 Basel, Switzerland; Andreas.Wetzel{at}unibas.ch
² Naturhistorisches Museum, Augustinergasse 2, CH-4001 Basel, Switzerland

A crinoid Lagerstätte of Chariocrinus württembergicus (Opalinuston Formation, Aalenian, Middle Jurassic) was formed under storm conditions. The Lagerstätte is oriented parallel to paleowave crests, and exhumed concretions indicate sediment reworking. Paleoflow was nearly perpendicular to (storm-) wave ripple-crests and was directed towards a depocenter to the south-southeast. Both waves and currents affected crinoid parts, such as stems, and document combined flow conditions. The crinoids lived on a low-relief swell somewhat above storm wave base in a muddy environment, and caused their own in-situ burial. During a storm, wave agitation led to suspension of sediments extending into the habitat of the crinoids. The crinoids provided additional friction within the near-bottom, suspension-rich, agitated water body, thereby reducing the speed of water currents. This led to sediment deposition, and the crinoids were buried in fine-grained sand and silt. In this way, they induced their own burial; therefore, the Lagerstätte is the product of a feedback process. Rapid burial of the crinoids led to their excellent preservation. Articulated preservation is possible only if the flow was too slow to remove the crinoids from their substrate. Some crinoids tried to escape these hostile conditions by autotomizing their crowns, but these were pushed down to the seafloor by oscillatory water movement, and were immediately covered, mouth upward, by sediments. The preserved crowns provide an estimate of the population density in the range of 110 animals m^–2, which is a minimum value because some crowns might have been washed away before burial.