Decoding the drivers of deep‐time wetland biodiversity: insights from an early Permian tropical lake ecosystem

Abstract

Wetlands are important to continental evolution, providing both arenas and refugia for emerging and declining biotas. This significance and the high preservation potential make the resulting fossiliferous deposits essential for our understanding of past and future biodiversity. We reconstruct the trophic structure and age of the early Permian Manebach Lake ecosystem, Germany, a thriving wetland at a time when the tropical biosphere faced profound upheaval in the peaking Late Palaeozoic Icehouse. Nine excavations, high‐resolution spatiotemporal documentation of fossils and strata, and U–Pb radioisotopic dating of tuffs allow us to distinguish autogenic and allogenic factors shaping the limnic biocoenosis. The Manebach Lake was an exorheic, oxygen‐stratified, perennial water body on the 101–102 km2 scale, integrated into the catchment draining much of the European Variscides. Lake formation paralleled an Asselian regional wet climatic interval and benefited from rising base level due to post‐Variscan half‐graben tectonics. Stromatolite‐forming cyanobacteria, bivalves, several crustaceans, amblypterids and xenacanthid sharks formed a differentiated biocoenosis in the lake. Fossil stomach remains and teeth prove the rare presence of acanthodians, branchiosaurs and large amphibians. The results indicate woody‐debris‐bearing lake littorals devoid of semi‐aquatic and aquatic plants as places suitable for stromatolites to grow, underpin the model of declining freshwater‐shark diversity in most Permian Variscan basins, demonstrate fish/amphibian ratios in limnic assemblages to measure lake perenniality and reveal taphonomic biases in lake taphocoenoses. Our outcomes call for more knowledge about the diversity, ecology and fossilization pathways of past limnic biotas, particularly microorganisms and actinopterygian fishes, to reconstruct deep‐time continental ecosystems.

Freshwater Stromatolites From an Early Permian Wetland (Manebach, Thuringian-Forest Basin, Germany): Structure, Development, and Paleoenvironmental Context

“…Magmatic zircons from a tuff bed in the lacustrine sequence were used to obtain U-Pb radioisotopic constraints from the Manebach Fm. (Trümper et al 2023). The LA-ICP-MS age of 297.8±2 Ma concurs with the biostratigraphic correlations.…”

“…The Manebach scenario described in this study strongly suggests that an intrabasinal, coniferdominated type of vegetation thrived somewhere in the vicinity of the lake. Several previous studies also mention the occurrence of fern and seed fern fossils in the lacustrine succession (Werneburg 1997(Werneburg , 2003Trümper et al 2023). However, we found very little evidence of typical floodplain elements in the strata containing stromatolites.…”

“…Most stromatolite-encrusted axes exhibit randomly distributed brittle deformation with microcrystalline quartz filling cracks and cavities that may suggest incipient wood decay or some transport prior to stromatolite formation. On the other hand, the stromatolitic layer itself shows only minor sedimentary input, and thus argues against extensive transport of even the smaller axes (Trümper et al 2023). The extraxylary tissues of detached woody plant axes are typically lost shortly after they enter water, in part because wood rapidly loses volume due to post-mortem shrinkage, whereas the extraxylary tissues are far less prone to shrinkage (Rößler et al 2021).…”

“…The scarcity of carbonaceous deposits in the Manebach sequence, the presence of both fish and mollusk fossils, and a subhumid to humid climate setting argue against high alkalinity and terminal lake conditions. Instead, the Manebach lake was a low-pH, low salinity lake with a certain fluvial inflow and prevailing deposition of black shales, rather than a carbonate-dominated environment (Trümper et al 2023).…”

“…In the latest Carboniferousearly Permian seasonal tropics, stromatolites were common elements of limnic strata (e.g., Stapf 1973Stapf , 2005Gand et al 1993;Schneider and Gebhardt 1993;Kerp et al 1996;Freytet et al 1996Freytet et al , 1999Freytet et al , 2000Schneider and Germann 2019). Their distribution during this interval of geological time was affected by both aridification resulting in pronounced seasonality and increased evaporation (see Trümper et al 2023, and references therein) and competitive stress imposed by fast-growing plants in the habitats of growth.…”