Late Anisian microbe-metazoan build-ups (“stromatolites”) in the Germanic Basin – aftermath of the Permian – Triassic Crisis

The so-called Permian – Triassic mass extinction was followed by a prolonged period of ecological recovery that lasted until the Middle Triassic. Triassic stromatolites from the Germanic Basin seem to be an important part of the puzzle, but have barely been investigated so far. Here we analyzed late Anisian (upper Middle Muschelkalk) stromatolites from across the Germanic Basin by combining petrographic approaches (optical microscopy, micro X-ray fluorescence, Raman imaging) and geochemical analyses (sedimentary hydrocarbons, stable carbon and oxygen isotopes). Paleontological and sedimentological evidence, such as Placunopsis bivalves, intraclasts and disrupted laminated fabrics, indicate that the stromatolites formed in subtidal, shallow marine settings. This interpretation is consistent with δ13Ccarb of about -2.1 ‰ to -0.4 ‰. Remarkably, the stromatolites are composed of microbes (perhaps cyanobacteria and sulfate reducing bacteria) and metazoans (non-spicular demosponges, Placunopsis bivalves, and/or Spirobis-like worm tubes). Therefore, they should more correctly be referred to as microbe-metazoan build-ups. They are characterized by diverse lamination types, including planar, wavy, domal and conical ones. Microbial mats likely played an important role in forming the planar and wavy laminations. Domal and conical laminations commonly show clotted to peloidal features and mesh-like fabrics, attributed to fossilized non-spicular demosponges. In the light of our findings, it appears plausible that the involved organisms benefited from elevated salinities. Another possibility is that the mutualistic relationship between microbes and non-spicular demosponges enabled these organisms to fill ecological niches cleared by the Permian – Triassic Crisis and maintain their advantage until the Middle Triassic.