Abstract
Following the end-Permian crisis, microbialites were ubiquitous worldwide. For instance, Triassic deposits in the Germanic Basin provide a rich record of stromatolites as well as of microbe-metazoan build-ups with nonspicular demosponges. Despite their palaeoecological significance, however, all of these microbialites have only rarely been studied. This study aims to fill this gap by examining and comparing microbialites from the Upper Buntsandstein (Olenekian, Lower Triassic) and the lower Middle Muschelkalk (Anisian, Middle Triassic) in Germany. By combining analytical petrography (optical microscopy, micro X-ray fluorescence, and Raman spectroscopy) and geochemistry (δ13Ccarb, δ18Ocarb), we show that all the studied microbialites formed in slightly evaporitic environments. Olenekian deposits in the Jena area and Anisian strata at Werbach contain stromatolites. Anisian successions at Hardheim, in contrast, host microbe-metazoan build-ups. Thus, the key difference is the absence or presence of nonspicular demosponges in microbialites. It is plausible that microbes and nonspicular demosponges had a mutualistic relationship, and it is tempting to speculate that the investigated microbial-metazoan build-ups reflect an ancient evolutionary and ecological association. The widespread occurrence of microbialites (e.g., stromatolites/microbe-metazoan build-ups) after the catastrophe may have resulted from suppressed ecological competition and the presence of vacant ecological niches. The distribution of stromatolites and/or microbe-metazoan build-ups might have been controlled by subtle differences in salinity and water depth, the latter influencing hydrodynamic processes and nutrient supply down to the microscale. To obtain a more complete picture of the distribution of such build-ups in the earth’s history, more fossil records need to be (re)investigated. For the time being, environmental and taphonomic studies of modern nonspicular demosponges are urgently required.
Highlights
Microbialites represent benthic microbial communities fossilized through trapping and binding of detrital sediment and/or localized mineral precipitation [1–4]
The name derives from the term ‘Stromatolith’, which was coined by Kalkowsky [21] to describe layered carbonate structures in the Lower Triassic
Olenekian stromatolites in the Jena area exhibited planar to wavy laminations, while
Summary
Microbialites represent benthic microbial communities (i.e., biofilms or microbial mats) fossilized through trapping and binding of detrital sediment and/or localized mineral precipitation [1–4]. In the Phanerozoic, they showed a marked decline [4], the concept of decline is considered too simplistic [8]. Microbialites display marked reoccurrences at certain times in the Phanerozoic, as for instance in the aftermath of the Permian–Triassic crisis (e.g., [9–19]). The temporary proliferation of biofilms/microbial mats in this critical interval likely resulted from a suppressed ecological competition and/or seawater chemistry at that time [2,3,20]. Depending on their characteristics, microbialites can be classified into different types (e.g., stromatolites, thrombolites, dendrolites, and leiolites: [1,21–24]). The name derives from the term ‘Stromatolith’, which was coined by Kalkowsky [21] to describe layered carbonate structures in the Lower Triassic
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