What role does anoxia play in exceptional fossil preservation? Lessons from the taphonomy of the Posidonia Shale (Germany)

Abstract

Konservat-Lagerstätten—deposits with exceptionally preserved fossils of articulated multi-element skeletons and soft tissues—offer the most complete snapshots of ancient organisms and communities in the geological record. One classic example, the Posidonia Shale in southwestern Germany, contains a diverse array of fossils preserved during the ∼183 Ma Toarcian Oceanic Anoxic Event. Seminal work on this deposit led to the hypothesis that many Konservat-Lagerstätten were preserved in stagnant basins, where anoxic conditions limited soft tissue degradation. To date, however, no studies have thoroughly investigated the geomicrobiological processes that drove fossil mineralization in the Posidonia Shale. As a result, the role of anoxia in its exceptional preservation remains uncertain. Here, we address these issues by reviewing the geology of the Posidonia Shale; describing the mineralization of its fossils; and synthesizing novel and existing data to develop a new model for the paleoenvironment and taphonomy of the Lagerstätte. Although shells and carbonate skeletal elements were preserved as pyritized and carbonaceous fossils, non-biomineralized tissues were primarily preserved via phosphatization (transformation of remains into calcium phosphate minerals). Unambiguous examples of phosphatization include ammonite shells, crustacean carapaces, ichthyosaur remains, coprolites, and coleoid gladii, mantle tissues, and ink sacs. Phosphatized crustaceans and coleoids contain cracks filled with pyrite, sphalerite, and aluminosilicate minerals. Such cracks were likely generated during burial compaction, which fractured phosphatized tissues, exposed their organic matter to focused microbial sulfate reduction, and thereby led to formation of, and infilling by, sulfide and clay minerals. These observations indicate that phosphatization happened early in diagenesis, prior to burial compaction and microbial sulfate reduction, beneath (sub)oxic bottom water, and corroborate the hypothesis that the animals were preserved during ephemeral pulses of oxygenation in the basin and/or within environments located along boundaries of anoxic water bodies. Overall, our findings support the view that anoxic bottom water does not directly promote exceptional preservation; in fact, it may impede it. Konservat-Lagerstätten, particularly “stagnation deposits”, tend to form in (sub)oxic depositional environments with steep redox gradients and/or high sedimentation rates. Under these conditions, organisms are rapidly buried below the redox boundary, where their mineralization is promoted by focused geomicrobiological processes, and degradation is limited by the supply of oxidants in the microenvironments around them.