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

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.

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 10 1 -10 2 km 2 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.
T H E Pennsylvanian-Permian transitional interval (310-290 Ma) represented a time of upheaval in the evolution of Earth's tropical biome (Bernardi et al. 2017;Gastaldo et al. 2020). Coupled with the interplay of the Pangaea assembly, oceanic circulation, atmospheric composition and polar ice-sheet extent, this environmental turnover generally provoked the contraction of the once widespread, peat-forming wetlands in favour of seasonally dry habitats (Greb et al. 2006;Schneider et al. 2006;Fielding et al. 2007;DiMichele et al. 2010;Montañez et al. 2016). The gradual aridification of the palaeotropics occurred differentiated in time and space and was initially restricted to mainland Pangaea beyond the Cathaysian realm (Rees et al. 2002;Yang et al. 2016). Although cause-and-effect relations and taphonomic biases remain However, wetlands were critical sites when long-term biosphere shifts occurred, as they provided refugia for withdrawing taxa and promoted morphological and physiological adaptations to open up ecological niches (Greb et al. 2006;Falcon-Lang & DiMichele 2010;DiMichele 2014;Pfefferkorn et al. 2017). Therefore, understanding the wetland-dryland dynamics around 300 myr ago proves essential to reconstructing the structure and evolution of the modern tropical biome and its deep-time analogues.
Coal-bearing strata from Manebach, central Germany, provide one of the most extensive records of early Permian wetland ecosystems worldwide. For more than 300 years, the deposits have been studied for their abundant and often well-preserved plant and animal fossils. Among them are vertebrate skeletons entangled in calamite stands, lm-scale fungal endophytes and cyanobacteria preserved threedimensionally in permineralized trees and stromatolites (Mylius 1709;Schlotheim 1820;M€ uller 1957;Barthel & R€ oßler 1996;Werneburg 2007;Barthel et al. 2010;Schneider et al. 2010;Krings et al. 2017). Altogether, the fossiliferous deposits not only disclose the vegetation partitioning of an early Asselian fluvial plain at various scales (Barthel 2001;L€ utzner 2001). They also provide fossil evidence of the ecosystems from nearby elevated drylands. This study documents the deposits and fossils of the hitherto barely acknowledged Manebach Lake Unit and other limnic deposits of the Manebach Formation and their enclosing fluvial strata. As a result, we draw implications on lake formation, age, geographical extent and ecology based on the combination of facies analysis, U-Pb radioisotopic dating, and the taphonomic and palaeontological characterization of the fossil biota in several sections. Results are discussed against the tectono-climatic background in equatorial Pangaea.

GEOLOGICAL BACKGROUND
Manebach is located in the southeastern part of the Thuringian Forest, a northwest-southeast-trending tectonic horst in central Germany (Fig. 1). Uplifted in the Cretaceous-Cenozoic, the Thuringian Forest exposes Variscan basement rocks overlain by an up to 2000-m-thick volcano-sedimentary succession of Late Pennsylvanian to Permian age (L€ utzner et al. 2012Fig. 2). Voluminous volcanics, stratigraphic gaps and considerable lateral variations of thickness and lithology point to deposition in a tectonically differentiated subsidence area summarized as Thuringian Forest Basin (L€ utzner & Kowalczyk 2012). This basin evolved around 302 Ma in the eastern Variscan Orogen of central Pangaea, related to the regional transition from late Variscan transpression to post-Variscan extension (Kroner & Romer 2013;Andreas 2014). Deposition generally occurred in a seasonal, tropical climate, though the succession echoes the overall aridification trend of equatorial Pangaea (Schneider et al. 2006). Despite its intramontane position, the Thuringian Forest Basin joined a drainage system connecting depocentres across the Variscan realm, as evidenced by similar aquatic assemblages (Schneider & Zaj ıc 1994;Stamberg & Tr€ umper 2021). However, based on the profile's stratigraphic range (Fig. 2), various lithologies and abundant fossiliferous beds, the Thuringian Forest Basin represents an essential continental reference section for interregional correlations of late Palaeozoic successions (Schneider et al. 2020). The strata exposed in Manebach are part of the Manebach Formation, representing the second unit of the Lower Rotliegend Rennsteig Subgroup in the Thuringian Forest Basin (Fig. 2). This formation originally existed almost in the entire basin, based on its present-day thickness variations, outcrops and subsurface distribution (L€ utzner 1969). Lithologically, the Manebach Formation contains coalbearing, greyish, fluviolimnic deposits, ranging from 10 m in the northwestern basin part to 370 m in the Thuringian Forest's southern foreland (L€ utzner et al. 2012). At the stratotype locality in Manebach, the formation is 150-180 m thick and informally subdivided into three memberlike parts (Fig. 3): the 40-50-m-thick Lower Sandstone Zone or 'Untere Sandsteinzone', the 80-m-thick Coalbearing Strata or 'Fl€ ozf€ uhrende Schichten', and the 40-mthick Upper Sandstone Zone or 'Obere Sandsteinzone' (Deubel 1960). Traditionally numbered in profile-down direction, eight major coal seams structure the Coalbearing Strata but gain limited correlative significance due to lateral facies gradients. Concerning the depositional environment, the Manebach Formation formed in a vast and, in parts, densely vegetated fluvial plain, developed during an interval of tempered volcano-tectonic activity. Braided-style channels dissected a mosaic of mineral-soil floodplains, mires, lakes and ponds (L€ utzner 2001). The lithology and sedimentary structures of the floodplain facies indicate flooding to be frequent (Tr€ umper et al. 2019). Concerning the tectonic setting, the Manebach Formation formed during an interval of peneplainization in the half-graben to graben-like Thuringian Forest Basin (L€ utzner et al. 2012). The formation of the strata ended with renewed uplifting of the footwall blocks, ultimately resulting in the deposition of the alluvial fan to lake associations known as the Goldlauter Formation.

Fossiliferous strata
To reconstruct the sedimentary environment and stratigraphic occurrence of limnic strata in the Manebach Formation, we carried out nine excavations at several sites near Manebach and Schm€ ucke between 1997 and 2021 (Table 1; Fig. 1). The excavation profiles were gained from both limnic intercalations in the Manebach Formation and their enveloping deposits. Lithological field documentation of the sections comprised the high-resolution, centimetre-scale description of the sedimentary rocks according to their grain size, fresh-rock colour, architecture, thickness, lateral extent, bedding, bounding surfaces and fossil content. In addition, representative rock samples served for preparing polished slabs and petrographic thin sections (28 mm 9 48 mm), allowing description and microfabric interpretation. Any fossils from all excavation profiles were documented according to their stratigraphic occurrence and preservation, with the final collection inventory limited to well-preserved or rare specimens. Most fossil groups were recorded semiquantitatively, classified according to the number of specimens counted per rock unit into rare (<10), moderately abundant (10-50), abundant (50-500), very abundant (>500). All rock and fossil samples are stored in the collections of the Museum f€ ur Naturkunde Chemnitz (MfNC; labelled 'F', 'P') and the Naturhistorisches Museum Schloss Bertholdsburg Schleusingen (NHMS; labelled 'Am', 'WP').

Stratigraphic correlation and radioisotopic dating
Based on their number and geographical distribution in the southeastern Thuringian Forest (Fig. 1), the excavation sites allow us to draw conclusions about the minimum area and timelines of limnic deposition. However, this demand requires the scattered profiles to be correlated; based on synchronously formed bed successions, depositional cycles at various orders, vertical profile gradients in grain size, and, to a lesser extent, lithofacies and taphofacies.
For radioisotopic dating, we employed U-Pb zircon geochronology on two tuff beds in the sections. For this purpose, tuff bed 1 was sampled in three sections and tuff bed 2 in one section of the Manebach Lake Unit, with the volume of the individual samples ranging from 1-8 kg (see Table 1). During sampling, we avoided using weathered, delithified parts of the beds to exclude alien material from the analysis. Zircon extraction and preparation took place in the laboratories for sedimentology and thin-section processing of the TU Bergakademie Freiberg. The procedure first comprised manual disintegration of the weakly lithified rocks, followed by wet screening and drying of the resulting loose material at 70°C. We mainly targeted the samples' 63-250 lm sieve fractions for further zircon extraction and only included the 50-63-lm fraction when the first-mentioned did not provide enough grains. A neodymium magnet served for removing magnetic components, while we employed density separation with bromoform (q = 2.83 g/cm 3 ) to isolate the heavy minerals. From the latter grains, zircons of most euhedral shapes were picked manually using transmitted-light microscopy. The grains were mounted in epoxy resin to form polished blocks. Cathodoluminescence microscopy ensured that altered and inherited zircons did not enter the further analysis. The previous steps left 38 to 69 zircons per sample suitable for measuring their U, Th and Pb isotopic compositions, using laser ablation sector field inductively coupled plasma mass spectrometry (LA-SF-ICP-MS) at the Senckenberg Naturhistorische Sammlungen Dresden (Museum f€ ur Mineralogie und Geologie, Sektion Geochronologie). The analyses were executed in a Thermo-Scientific Element 2 XR sector field ICP-MS, equipped with a New Wave UP-193 Excimer Laser System. A teardrop-shaped low-volume laser cell (construction: Ben J€ ahne, Dresden, Axel Gerdes, Frankfurt/Main) ensured the consecutive measurement of the zircons and the time-resolved data acquisition. Each measure comprised 15 s background acquisition and 30 s sample-spot acquisition; both were performed with a laser-spot size of 25 and 35 lm. The corresponding repetition rate ranked 10 Hz at an energy density of 5-6 J/cm 2 . The common-Pb correction followed the interference and background-corrected 204 Pb signal and a model-Pb composition (Stacey & Kramers 1975). This step was applied if the corrected 207 Pb/ 206 Pb ratios were not within the internal errors of the measured ones. Raw data faced additional correction for background signals, common-Pb, laser-induced and time-dependent element fractionation of Pb/Th and Pb/U and instrumental mass discrimination using the Excel spreadsheet of Axel Gerdes (Frankfurt/Main; for spreadsheets, see Tr€ umper et al. 2023). Reported uncertainties were propagated by quadratic addition of the external reproducibility obtained from the GJ-1 standard (c. 0.6% and 0.5-1% for the 207 Pb/ 206 Pb and 206 Pb/ 238 U ratios, respectively) during individual analytical sessions and the within-run precision of each measurement. The Ple sovice standard served for calibrating the primary GJ-1 standard at regular intervals (see Sl ama et al. 2008) and yielded an age of c. 337 Ma. Data are shown as 2r error ellipses in concordia diagrams using Isoplot Ex 2.49 (Ludwig 2001), with concordia ages calculated based on a 95% confidence level. We ranked zircons ranging in concordance from 97 to 100% concordant due to the overlap of their error ellipse with the concordia. The discordant data faced careful interpretation. Uranium and lead contents and the Th/U ratios were calculated relative to the GJ-1 standard at an accuracy of c. 10% (for U-Pb analysis raw data, see Tr€ umper et al. 2023).

RESULTS
Limnic deposits are much more common in the Manebach stratotype than their poor investigation would indicate. However, the lake strata are not evenly distributed across the stratotype, and their lithological appearance varies considerably depending on the stratigraphic occurrence. First, the known limnic deposits exclusively occur in the Coalbearing Strata, where they show a profile-up increase in thickness and extent, accompanied by a changing lithology (Fig. 3). Accordingly, lake deposits occur subordinately in the lower 56 m of the Coal-bearing Strata up to the uppermost coal seam, accounting for approximately 6-7% of the section as calculated based on L€ utzner (2001). They consist of millimetre to decimetre-thick, and a few metres wide, intercalations of greyish, laminated and massive mudstones (L€ utzner 2001;Tr€ umper et al. 2019). The limnic evidence can also be very subtle, for example, consisting of <2-mm-thick clay beds that cover bivalve thanatocoenoses in former, calamite-fringed, shallow floodplain depressions. By contrast, in the upper 24 m of the Coal-bearing Strata, the limnic record continues with decimetre-thick chert lenses and culminates in the several metres thick Manebach Lake Unit. Both the cherts and the lake unit raise the proportion of limnic strata in the upper Coalbearing Strata to about 30-40%, while coals are less important (Fig. 3). The Manebach Lake Unit occurs a couple of metres below the base of the Upper Sandstone Zone. It is the only limnic deposit to provide laminated black shales and substantial aquatic taphocoenoses (e.g. Martens 1981). Subsequent descriptions will focus on the cherts and the Manebach Lake Unit.

Cherts
Since the early 19th century, silicifications were repeatedly reported from Manebach (Voigt 1808; van Hoff 1814, Stenzel 1854; Barthel & R€ oßler 1997) and they have recently attracted increasing attention as fossil archives of early Permian plant-plant and microbe-plant interactions (Barthel et al. 2010;Krings et al. 2017). However, except for the few coal-related silicifications (Fig. 3C, unit 19), the general abundance and stratigraphic occurrence of silicifications in the Manebach Formation remained unresolved. Our excavations identified black cherts in a stratigraphic level between the uppermost coal seam and the Manebach Lake Unit (Fig. 3). These siliceous deposits only occur near the Kammerberg Fault at Manebach, where they form tectonically isolated relic occurrences partially truncated by sub-recent erosion and forest-road construction (Fig. 4A). Also, the cherts are frequently found reworked in the nearby Quaternary colluvium.
Together with the relic in situ chert occurrences, they indicate that the original siliceous lenses did probably not exceed decametre-scale lateral extents. In the following, we limit ourselves to a brief characteristic of the cherts since these deposits contribute to understand the prehistory of the Manebach Lake.

The Manebach Lake Unit
General characteristics. Outcrops of the Manebach Lake Unit occur near Manebach and Schm€ ucke, approximately 6 km apart from each other (Fig. 1). Since most documented sections have exposed this limnic unit in parts, its maximum thickness is at least 8 m (Fig. 5). However, we estimate the thickness to be less than 10 m, based on the lake unit's stratigraphic position close to the base of the Upper Sandstone Zone at Manebach (Fig. 3).
Concerning lithology, thinly bedded to laminated siltstone and fine-grained sandstone dominate the Manebach Lake deposits (Fig. 5). Mostly, the aforementioned greyish lithologies are poorly fossiliferous, and it takes hours to days of rock splitting to obtain appreciable quantities of fossilized plants and animals (Table 2; Werneburg 1997). The resulting taphocoenoses are diverse but commonly feature advanced fragmentation, disarticulation and decomposition. By contrast, fossil abundance is much higher in microbialites and black claystone that subordinately compose the sections (Fig. 5). In these lithologies, fossil preservation can be exceptional, including For geographical positions, see Figure 1; for stratigraphic positions, see Figure 3. subcellular organisms in anatomically preserved plants and mass assemblages of near-complete to partially articulated fish remains. The sections vary considerably concerning rock-type proportions, bedding, and thickness (Fig. 5). Amongst other reasons, these discrepancies reflect intercalations of a limited lateral extent and horizontal facies gradients, challenging bed-by-bed parallelization in the Manebach Lake Unit across longer distances. Nevertheless, all profiles share a black-shale unit associated with tuffs and conspicuously rich in actinopterygian remains (Fig. 5). Since this striking interval is almost identical in all profiles concerning detailed stratigraphic structure, we consider it an isochronously formed unit of major correlative significance.
The Lower Manebach Lake Unit. The Manebach Lake deposits conformably overlay fluvial, palustrine, or pedogenic deposits (Fig. 5). At Manebach, the pre-lake facies consists of decimetre-scale alternating fine-grained sandstones and horizontally stratified siltstones typical for the Coal-bearing Strata. From this alternation, the uppermost fine-grained sandstone fines upward into rooted, massive siltstone and, at Hinteres Schulzental, a 0.5-5-cm-thick bed consisting of strongly compacted, decimetre-long, coalified trunks and subordinate siltstone. At Blauer Stein, at least 30 cm thick, massive to horizontally stratified, silty limestone occurs, which provides rare, platy limestone intraclasts, autochthonous roots, millimetre-sized vertebrate and bivalve remains. This limestone grades upward into a 13 cm thick, massive, fine-sandy siltstone containing slickensides and sideritic concretions, concluding the pre-lake facies. The overlying Manebach Lake Unit generally represents a profile-up symmetric succession, manifested by initial fining and subsequent coarsening (Fig. 5). We subdivide this general structure into four intervals, the delineation of which rests upon cross-profile changes in lithology and fossil content. While intervals I and II comprise the upwards-fining, 0.4-2-m-thick Lower Manebach Lake Unit, intervals III-IV are assigned to the upwardscoarsening, nearly 5-m-thick Upper Manebach Lake Unit (Fig. 5).
Interval I starts with sharply based microbial deposits of varying appearance, ranging from decimetre-sized domal to loaf-shaped stromatolites to planar, laminated microbialites and microbialitic sandstones (Fig. 6A, B). At Hinteres Schulzental, the stromatolites often sit on top of coalified stems and form scattered clusters (Fig. 6A, B).
Generally, the microbialitic deposits show umbra brown colour, which is probably a result of recent weathering (Fig. 6C). They predominantly consist of chalcedony and, if present, sediment particles. Relic calcite-cemented domains and isolated calcite-crystal moulds in silica indicate that calcite precipitation followed primary silicification in the microbial deposits (Fig. 7A). The extent of silicification and the proportion of siliciclastic material determine whether the microbialites are preserved compact, highly porous or even loosened. Thin sections reveal that three-dimensionally preserved cyanobacteria mainly build up these biogenic rocks (Fig. 7A). Upsection, the microbialites are sharply overlain by epiclastic deposits or fine up into the latter by an increasing proportion of siliciclastic material via a planar, laminated microbialite stage (Fig. 6A). In the case of the stromatolites, stratification of the overlying strata indicates successive levelling of the biogenic topography, ultimately resulting in horizontally bedded deposits. The top of interval I coincides with the upper boundary of a few centimetres thick, black claystone that terminates upward fining in the basal deposits of the Manebach Lake Unit (Fig. 5). Interval I is among the most fossil-rich deposits of the Manebach Lake Unit regarding its fossil content. Next to the abundant to very abundant microbialites, plant remains occur rarely with isolated adpressions of isolated neuropterid and odontopterid leaflets and coniferopsid shoots. In addition, the stromatolites abundantly yield centimetre to decimetresized, permineralized and petrified gymnosperm stems exhibiting exceptional anatomical preservation of both tissues and fungi therein (Figs 6C-E, 7B). Animal remains comprise mass assemblages of ostracod casts or, in the silicified microbialites, three-dimensionally preserved ostracods showing occluded valves (Fig. 6F). From the laminated microbialites of Hinteres Schulzental (Fig. 5), there is also moderately abundant evidence of isolated scales and cranial parts of amblypterid fishes as well as rare xenacanthid shark teeth and isolated and bivalvepositioned bivalve shell valves resembling Anthraconaia Trueman & Weir (Fig. 6H).
The base of interval II corresponds to the beginning of grey to light-grey siltstones and subordinate fine-grained sandstones that form upward-fining successions (Fig. 5). At first glance, these sedimentary rocks appear horizontally bedded at a centimetre scale (Fig. 6A). However, bedding planes that cut off each other at acute angles reveal the deposits also possess low-amplitude wavy to lenticular stratification. Interval II particularly stands out F I G . 4 . Chert-bearing deposits from the upper Manebach Formation. A, excavation section and profile, Forstmeisterweg oberhalb Hirschbad. B, nodular calcrete, vertical section; MfNC P4579. C-E, chert, vertical section revealing uneven, lenticular lamination; NHMS F16500; E, compacted aerial root of a tree fern in transverse section. Grain sizes: c, clay; si, silt; fs, fine sand; ms, medium sand; cs, coarse sand. Scale bars represent: 20 mm (B, C); 2 mm (D); 500 lm (E). for its fossil deficiency, comprising tiny plant fragments, rare pteridophyll adpressions, and rare isolated amblypterid scales. Compared to interval I, microbial deposits mainly contain moderately abundant elephant-skin horizons, while thicker microbialites remain scarce (Fig. 5).
The Upper Manebach Lake Unit. Interval III introduces the Upper Manebach Lake Unit and develops from interval II through fining, with the base placed at the onset of dark grey to black siltstone and claystone, at least 10 cm thick and rich in fossil fish remains (Fig. 5). Unlike all other intervals of the Manebach Lake Unit, the roughly 1m-thick interval III is the least variable in terms of thickness, lithology and stratigraphic architecture (Fig. 5). From the base 10-20 cm up, initial fining culminates in a crucial correlative succession of at least two yellowish, fine-grained ash-tuff beds (Fig. 5). This tuff-bed succession contains a lower, 0.6-1.5-cm-thick tuff 1, which only occurs in the southwesternmost Blauer Stein section (Fig. 5). Tuff 2, by contrast, forms the more prominent pyroclastic deposit, positioned c. 20-30 cm above the base of interval III, extending across all sections and reaching a thickness of 3-15 cm ( Fig. 8A-D). At Blauer Stein, tuff 2 reaches its maximum thickness and fines upward, starting with clayand silt-sized grains that become entirely clay-sized near the top. In all the remaining sections, however, the thickness of the normally graded tuff 2 decreases to less than 3 cm (Fig. 8D), overlain by an up to 10-cm-thick claystone. This claystone shows strong silicification and often slump structures, and it includes several mm-thick, yellowish tuff laminae as a distinctive feature (Fig. 8C). Whether these slumped and disrupted tuff laminae represent genetically separate deposits or correlate with tuff 2 remains open. Further profile-up, tuff 2, or the silicified claystone, respectively, is concordantly overlain by an upwardly coarsening, laminated siltstone. That siltstone introduces a cyclic succession of three beds of siltstone to fine-grained sandstone, intercalated with finely laminated, black clay to siltstone, forming the upper part of interval III. Concerning fossil content, interval III is the most productive part of the Manebach Lake Unit. The high fossil abundance predominantly relates to the claystones between the tuff beds, previously referred to as the 'fish bed' or 'fish shale' (Werneburg 1997(Werneburg , 2003. Here, amblypterid fish remains are very abundant, peaking in the claystone underlying tuff 2 and decreasing profile-up (Figs 5, 8A). Although intense jointing prohibits large-scale sampling and examination of bedding surfaces, fossil remains mostly occur isolated and, thus, indicate scattered assemblages. Near-complete specimens remain very rare and represent juvenile to sub-adult fishes (Fig. 8E). However, the majority of finds comprise disarticulated but localized bone-and-scale accumulations ( F I G . 6 . Strata and fossils of the Lower Manebach Lake Unit. A, section, Hinteres Schulzental; arrows indicate stromatolites. B, stromatolites, top view. C, silicified stromatolite containing a permineralized Tylodendron stem (arrow), vertical section; MfNC F14419b. D-E, Tylodendron stem from C after HF etching, showing typical sclerenchyma nests (Sc) in the parenchymatous pith (Pa), transverse section; the surrounding wood (W) contains leaf traces (Lt). F, silicified ostracod accumulation in a stromatolite; MfNC F16406a. G, amblypterid scale; MfNC F16429. H, butterfly-positioned Anthraconaia-like bivalve, impression; note the presence of calcified (grey, above) and decalcified (brown, below) rock parts; MfNC F16427. Scale bars represent: 50 cm (A); 20 cm (B); 2 cm (C); 1 mm (D); 200 lm (F); 500 lm (G); 5 mm (H). the shoulder girdle can be preserved in loose contact with the body, and cranial bones maintained their relative anatomical positions (Fig. 8G, H). Finally, isolated scales and scale-roof bones contribute abundantly to the assemblages, as do articulated scale rows. The preservation quality of the fish remains hence mostly ranges from moderate to good, though the scarce presence of denticulated jaws, delicately preserved fins and articulated specimens. All fish remains strongly resemble Paramblypterus duvernoyi, with minor differences such as the shape of the postrostral's posterior margin, median-gular width and the number of branchiostegal rays ( Stamberg & Tr€ umper 2021). Other fossils remain rare to moderately abundant and comprise shark teeth and body remains, the syncarid Uronectes (Fig. 8I), isolated and, rarely, attached insect wings, ostracod casts, coprolites, pteridophyll frond remains, pinnules and stem fragments.
Interval IV concludes the Manebach Lake Unit, with the base at the top of the third greyish siltstone and its correlatives above tuff 2. This final interval continues the cyclic lithology of interval III, although claystone gets less critical and is dark grey rather than black. Consequently, laminated to horizontally bedded siltstones prevail in interval IV. In addition, sandy deposits gain importance up the profile, underscoring the general coarsening in interval IV. Asymmetric, 1.5-m-thick subcycles gradually evolving from laminated claystones into sandstones indicate deltaic progradation (Fig. 5). The upper boundary of interval IV coincides with the onset of significantly coarser-grained sandstones. As the latter psammites include thin coal beds, we consider that they may possibly represent the post-lake facies (Fig. 5). However, regarding fossil content, interval IV yields the same fossil groups as interval III, albeit in much lower abundances.
As mentioned before, intervals III and IV are the main strata to yield fossil remains of aquatic vertebrates in different preservation states. Besides moderately abundant amblypterid remains likely to be Paramblypterus cf. duvernoyi, this situation applies to moderately abundant xenacanthid sharks and the rare branchiosaurids ( Fig. 9). Xenacanthid fossils mostly comprise isolated teeth ( Fig. 9A-C), followed by rare, partially articulated and often poorly preserved body remains. Uniquely, a complete Bohemiacanthus specimen, originally embedded sidewise in laminated siltstone, was unearthed in the 1997 excavation 'Oberhalb Fischhalde' (Fig. 9D;Werneburg 1997). Here, nearly all skeletal elements remain articulated, bearing the potential for further elucidating the anatomy of this xenacanthid shark.
Evidence for branchiosaurids is based on scarce fossil remains comprising both isolated bones and partially articulated skeletons (Figs 9E-G, 10A, B). While the preservation of some remains only allows for assignments to the Branchiosauridae, other fossils have critical anatomical detail to determine the species. All these closer determinable specimens belong to Apateon dracyiensis; a branchiosaurid whose intraspecific and ontogenetic variabilities are well studied (Werneburg 2021). For instance, in one specimen showing the anterior body part (Fig. 9E, F), typal characteristics include a broad interorbital region, a short maxillary and a narrow dorsal branch of the ilium (Werneburg et al. 2022).
Rarely, the strata also provide fossil evidence of other vertebrates (Fig. 10). For instance, there is one tooth that shows a very broad base. Structures near the tooth base document a folded surface, indicating a labyrinthodont structure. The nature of this find ultimately remains in the dark because numerous vertebrate groups have evolved labyrinthodont teeth since the Devonian (e.g. Preuschoft et al. 1991). However, given that crossopterygians and amphibians were the only labyrinthodont taxa to exist in Permian freshwaters, we suggest that the tooth belonged to a representative of one of these groups. In addition, the deposits contain various digestive remains that reveal more about the ecosystem. One example includes diverse vertebrate remains accumulated in a structure that exhibits an oval outline and may be stomach contents (Fig. 10A, B). The phosphatic bones therein show various decomposition stages and belong to at least three branchiosaurid individuals, one of which is Apateon dracyiensis. Most spectacular, however, is one 2 cm long spine (Fig. 10A, B). The shape and internal structure of this partially phosphatic fossil are typical of Acanthodian spines, making this find the first evidence of this group in the Manebach Formation. Finally, there are fusiform coprolites, which are densely packed with amblypterid scales (Fig. 10D) most possibly derived from xenacanthid sharks.
The phytocoenoses are variable in terms of preservation. On the one hand, there are accumulations of tiny and indeterminable coalified plant fragments and rare F I G . 1 0 . Further evidence for tetrapods and vertebrate interactions. A-B, stomach contents containing remains of several animal groups; NHMS Am 10195. C, tetrapod tooth; NHMS Am 12319. D, fusiform xenacanthid shark coprolite containing amblypterid scales (arrow); MHNS Am 2329. Abbreviations: ac, acanthodian; art, arthropod; cl, clavicle; fem, femur; hum, humerus; il, ilium; n, nasal; pt, pterygoid. Scale bars represent: 1 mm (A); 5 mm (B); 1 cm (C, D). charcoal that mainly occur in the silty and sandy intercalations (Fig. 5). On the other hand, all lithologies yield macrofossils preserved as impressions, adpressions and compressions in overall moderate abundance (Fig. 5). These two-dimensionally preserved floras consist of isolated pinnules, fragmentary leaves and fronds that seem not to exceed a few decimetres in length. Finally, the stromatolites from the base of the Manebach Lake Unit provide exceptionally preserved, permineralized stems mostly of a few centimetres in length and diameter (Fig. 6D, E).
Remains of fossil insects are relatively rare and comprise isolated wings, wing fragments and rare nearcomplete specimens of the Blattodea ('cockroaches'), particularly the genera Phyloblatta and Anthracoblattina.

Assessing the depositional age
The Manebach Formation is a cornerstone for regional to global-scale biostratigraphic schemes on continental strata of the Carboniferous-Permian transition. Its significance results from the abundant and well-preserved fossil evidence of various aquatic and terrestrial groups in the Manebach strata and the upper Palaeozoic Thuringian Forest profile in general (e.g. Schneider et al. 2013Schneider et al. , 2015Schneider et al. , 2020. Recent radioisotopic datings constraint the biozonations developed in this profile (L€ utzner et al. 2020), although the fossil-rich Manebach Formation has remained a geochronological blind spot for a long time due to the apparent lack of volcanic deposits. Therefore, the tuffs found within the scope of this study enable us to provide an absolute age of the Manebach Lake Unit.
Preparation of the major pyroclastic beds, tuffs 1 and 2 (Figs 5, 8D), yielded subhedral to euhedral, nearbipyramidal zircons and splinters derived from once euhedral habits (Fig. 14A). The internal structure of the crystals reveals a vibrant crystallization history, roughly consisting of two growth stages. The earlier growth stage is represented by a few or even generations of crystallization nuclei, each differing in internal structure and shape from the final crystal (Fig. 14A). These nuclei can show concentric zonation or exhibit diffuse to homogeneous cathodoluminescence (CL) signals and may result from initial precipitation in the magma or were derived from crustal rocks during ascent. The later growth stage comprises the outer rim of the zircons that usually consists of nearly undisturbed concentric CL zones corresponding to the final shape of the crystal (Fig. 14A). However, resorption structures reach into the crystals and indicate that several zircons underwent corrosion in the magma as the latest event before the eruption. Therefore, we interpret the later growth zones of the zircons to date the latest preserved crystallization stage when magma temperatures and pressures fell below the thresholds required for zircon precipitation. Measurement spots were consequently positioned in the outermost growth stage, and the resulting radioisotopic data contributed to calculating the maximum ages of the tuffs.
Generally, a low proportion of the zircon measurements matched the 95% confidence level, which is to say 2% (tuff-1 sample) and 10-16% (tuff-2 samples). These concordant analyses yielded 206 Pb/ 238 U ages of 297.8 AE 2.0 Ma, 297.8 AE 2.8 Ma and 298.8 AE 4.7 Ma for tuff 2 and 298.4 AE 5.6 Ma for tuff 1 (Fig. 14C-F), thus covering a Gzhelian-Sakmarian interval. Comparing this study's radioisotopic results with other biostratigraphical and geochronological data provides constraints on the U-Pb ages of the Manebach Lake Unit. As for biostratigraphy, the upper Palaeozoic Thuringian Forest profile is among the most intensely studied continental sections, making it a globally significant reference section (Schneider et al. 2020). Most of the Thuringian Forest formations yield limnic deposits that provide conchostracans, blattoid insects, xenacanthid sharks, various amphibians and macrofloras (L€ utzner et al. 2012), groups commonly used for late Palaeozoic continental biostratigraphy and increasingly referenced by radioisotopic data (e.g. Davydov et al. 2010;Oplu stil et al. 2016;Pellenard et al. 2017). Based on conchostracans, the Manebach strata belong to the Asselian to early Sakmarian Lioestheria paupera -Pseudestheria palaeoniscorum assemblage zone (Schneider & Scholze 2018;Schneider et al. 2021). Regarding insects, scarce fossil evidence leaves the Manebach Formation poorly constrained. There is evidence of the genera Anthracoblattina (Fig. 13) and Phyloblatta, neither of which is used for biostratigraphic purposes due to their evolutionarily persistent venation patterns, weak interspecific differences, considerable individual variabilities and ecological specializations (Schneider 1978(Schneider , 1983Belahmira et al. 2019). Rather, fragments resembling the guide taxon Sysciophlebia ilfeldensis led Schneider & Werneburg (2006) to assign the Manebach Formation to what is now called the Sysciophlebia ilfeldensis -Spiloblattina weissigensis zone (Schneider et al. 2015(Schneider et al. , 2021. This assemblage zone covers the Carboniferous-Permian transitional interval, roughly 300-299.5 Ma. Biostratigraphic approaches based on xenacanthids use evolutionary changes in tooth morphology, thus defining lineage zones (Schneider 1988(Schneider , 1996. L€ utzner et al. Taking these data, and the biostratigraphical and geochronological arguments provided in this study, we consider the age of the Manebach Lake Unit and its enveloping strata likely to be 298-299 Ma or the earliest Permian (Asselian).

Morphology and formation of the Manebach Lake
Decoding the environmental and evolutionary significance of limnic taphocoenoses requires knowledge of the geological circumstances ruling their formation. Such an approach benefits from several stratigraphically correlated profiles in the lithological unit concerned, which allow deposition and fossilization to be resolved in time and space (e.g. Heimhofer et al. 2010;Buchheim et al. 2011). As for the Manebach Lake Unit, several excavations accumulated such a dataset over the years, enabling the more detailed characterization of the depositional environment. Accordingly, as evidenced by the geographical distribution of its deposits (Fig. 1), the Manebach Lake reached a minimum size in the 10 1 -10 2 km 2 range, making it a large lake, according to Verpoorter et al. (2014). However, given the abundance and thickness of dark shales in the documented sections, even larger sizes could be envisioned. With all care, this assessment possibly finds further support by the prevalence of amblypterids and xenacanthids over other (semi)aquatic fossil groups in the Manebach Lake assemblages, advocating for a large and deep lake (Schneider & Werneburg 2012;Schneider et al. 2014).
The outcrops at Manebach were probably deposited more within the lake, while the southwesternmost outcrop at Blauer Stein ( Fig. 1) appears to have been closer to the lake margin. Lithological and biofacies data supporting this more marginal position for the Blauer Stein locality include: (1) generally lower thicknesses of the lake deposits as compared to the other sections (Fig. 5); (2) coarser-grained lithofacies by trend (Fig. 5); and (3) an abundance of frond fragments of the peltasperm Autunia naumannii (Fig. 11F, G). This plant is considered to form monospecific communities in non-peat-forming alluvialplain settings, mostly above the groundwater level, such as levees, well-drained floodplain settings but also lake margins (Barthel 2009;Boyarina 2010;DiMichele et al. 2013). Its predominance and preservation in the Blauer Stein section could reflect limited transport from an A. naumannii-dominated vegetation, thus conceiving a near-shore position of these strata (see also Spicer & Wolfe (1987) for the abundance of shore-line floras in limnic taphocoenoses). Concerning lake-basin morphology, within the uncertainty of the few documented outcrops, there is geological evidence for a compact, bowl-shaped water body rather than a highly dissected, topographically differentiated lake with narrow waterways. A simple bowl shape of the limnic environment better explains the weak lateral facies gradients documented in the Manebach Lake sections (Fig. 5). A presumably gently sloping lake floor could also explain the presence of slumping structures (Fig. 8C): Lewis (1971) stated inclinations of 1°-4°to be sufficient to cause slump formation, though keeping in mind that seismic shocks could have been alternative drivers (Alsop et al. 2016).
Hydrologically, the Manebach Lake was persistent and exorheic. Both abundant fluvially influenced intercalations ( Fig. 5; Werneburg 1997), and the ichthyofauna (Figs 8,9) attest to the presence of riverine inflows and outflows. Particularly the fossil fishes needed aquatic pathways persistent enough to enter the lake and establish a population (Schneider & Zaj ıc 1994;Schneider et al. 2000;Fischer et al. 2010a). The Manebach Lake taphocoenosis thus supports the drainage-system model connecting depocentres across the Variscides; an idea that is further supported by fluvial facies analyses and palaeocurrent data, palaeotopographical reconstructions and detrital zircon and monazite provenance in late Palaeozoic strata of the Czech Republic and Germany (Gaitzsch et al. 1998;Oplu stil 2005;Z ak et al. 2018). Persistent water cover in the Manebach Lake is indicated by the continuous, subaquatically formed shale successions (Figs 5, 8A, B) that contradict episodic, subaerial exposure; although erosional structures and deposits related to dropping water levels can be challenging to identify in shaly strata (see Schieber et al. 2010;Schieber 2015).
The formation and persistence of the Manebach Lake certainly benefited from climatic humidity during the deposition of the Manebach Formation (Fig. 15), as inferred from lithological and palaeontological evidence lines in the enveloping strata: (1) the characteristic pale to greyish rock colours of the Manebach Formation, both in coarse and fine clastic rocks; (2) syndepositional bleaching of volcanic pebbles adopted by erosion of the older Ilmenau Formation; (3) coal intercalations; (4) diverse and partly dense vegetation of hygro-to mesophilous affinity in the floodplains, recorded in abundant root soils and (par)autochthonous plant-rich assemblages (Barthel 2001;L€ utzner 2001;Barthel & R€ oßler 2012;L€ utzner et al. 2012;Tr€ umper et al. 2019). However, the Manebach Lake deposits yield indications for seasonally changing precipitation rather than ever-wet conditions. On the one hand, permineralized conifer stems from microbialites document seasonally alternating water availability by growth rings (AH, unpub. data). Preservation and biostratinomy of these woody stems indicate their quick and parautochthonous burial along the lake margin (Fig. 6D, E), proving that they were growing when the lake existed. On the other hand, lamination in the Manebach Lake black shales may also attest to the former presence of rainy seasons, even if their detailed investigation remains a task for the future. Lamination also exists in the microbialites (Fig. 6C) but is not consistently developed and possibly reflects the additional impacts of other environmental parameters. Given the above arguments, we suggest a tropical-seasonal climate, with the precipitation regime ranking subhumid to humid.
The extended view of the early Permian evolution of the Thuringian Forest Basin reveals tectonics as another likely driver of the Manebach Lake formation (Fig. 15). Accordingly, the succession of the Ilmenau, Manebach and Goldlauter formations can be construed as a tectonosedimentary cycle (Figs 2, 15). Following a major unconformity at the base of the central European Permian (Rotliegend), this cycle starts with the Ilmenau Formation. Voluminous bimodal subvolcanics, extrusive volcanics and pyroclastics characterize this lithostratigraphic unit (Seyfarth 1964;Andreas 1971Andreas , 1988Andreas et al. 2005). Together with epiclastic strata and their lateral facies gradients, the rocks mirror a fluviolacustrine environment that was topographically differentiated by volcanogenic elevations (Fig. 15A; L€ utzner 1972; L€ utzner et al. 2012). Earthquakes paralleled deposition as indicated by seismites in the partially limnic Lindenberg Member, lower Ilmenau Formation (Barthel & R€ oßler 1993). In the upper Ilmenau Formation, profileup increasing intraformational reworking and the lack of effusive deposits echoes waning volcanic activity (Andreas et al. 1966;L€ utzner 1969). Intrabasinal elevations experienced denudation, nowadays recorded in localized hiatus (Andreas 2014). This development heralded an interval of peneplainization in the Thuringian Forest Basin, the deposits of which mainly comprise the Manebach Formation (Fig. 15B, C). In contrast to the underlying Ilmenau Formation, the Manebach clastics mostly consist of basement-derived detritus and, therewith, prove denudation to increasingly extend beyond the graben margins (Katzung 1964;L€ utzner et al. 2012). By its thickness variations, the Manebach Formation reflects the progressing levelling of the palaeorelief (Fig. 15B). Finally, lithofacies of the overlying Goldlauter Formation indicates that alluvial fans prograded across the near-planar basin floor from the basin margins into the depocentre, where they interfingered with fluviolacustrine environments (Fig. 15D; L€ utzner 1981). The climate became slightly drier, accompanied by the advanced red-bed formation in basin-marginal settings and the spreading of dry-adapted plant communities. Tectonic uplift of the graben flanks caused deposition of the Goldlauter Formation and initially involved some reworking of the elder Manebach clastics, albeit the corresponding hiatus remained of limited duration ( Fig. 15D; L€ utzner 1969, 1972Schneider et al. 2020). As in the Ilmenau Formation, seismite genesis again accompanied limnic sedimentation, as is recorded in the Sperbersbach Lake Unit of the lower Goldlauter Formation (Schneider & Germann 2019).
Of particular importance to this study is whether the tectonic uplifting that accompanied the Goldlauter deposition had begun earlier (L€ utzner 2004); a hypothesis that may be supported by the results of this study. Accordingly, our excavations in the upper Coal-bearing Strata of the Manebach stratotype (Fig. 3) indicate a profile-up increasing significance of limnic deposits in this lithostratigraphic unit. Following the fluvial Lower Sandstone Zone, fluviopalustrine deposits dominate the lower Coalbearing Strata up to the uppermost coal seam (Fig. 15). Here, evidence for limnic deposition is limited to laminated, light to pale-coloured mudrocks of limited, metre-scale lateral extent and up to a few decimetres in thickness, and millimetre-thick clay beds that covered taphocoenoses in once shallow floodplain depressions (L€ utzner 2001;Tr€ umper et al. 2019). Standing water bodies hence mostly comprised small lakes, ponds and puddles left from flooding. In the upper Coal-bearing Strata, the palustrine deposits considerably lose importance, usually not exceeding a few centimetres, if that, in thickness. Instead, the strata are fluviolimnic, with lake deposits being more pronounced: the cherts (Fig. 4) and, finally, the Manebach Lake Unit, which is the only deposit to provide significant amounts of limnic black shales and taphocoenoses (Fig. 15). This development indicates that the water table rose on average over the long term, increasingly impeding mire formation and culminating in the establishment of the perennial, large Manebach Lake (Fig. 15). In striking contrast to the gradually increasing role of limnic deposition in the Manebach Formation, its demise apparently came far more quickly. Fluvial deposition rapidly took over, replacing the Manebach Lake and depositing the coarse-clastic Upper Sandstone Zone. Transport competence grew and ultimately led to the alluvial-fan conglomerates of the basal Goldlauter Formation without considerable hiatus. The upper Coal-bearing Strata, and the Manebach Lake in particular, may have formed around the tectonosedimentary cycle's turning point (Fig. 15). Although this model needs verification by improved correlations of the well-studied stratotype with other sections of the Manebach Formation, more evidence argues for synsedimentary tectonics in the upper Coalbearing Strata: seismites in the cherts, tuff beds, and evidence of rapidly flooded forests in the Manebach Lake F I G . 1 5 . Model for the geological evolution of the study area. It shows that the Manebach Lake formed during an interval of sufficiently high seasonal precipitation and initial tectonic uplift of the graben flanks favouring high groundwater tables and limnic deposition. Abbreviations: base., base level; grou., groundwater table; hum., humidity. Arrow shows present-day north. For locality positions see Figure 1. (AH, unpub. data). From this, we conclude that the Manebach Lake formed in a favourable but geologically short time window when seasonal precipitation and tectonic subsidence were high enough to support a permanent, extensive water coverage (Fig. 15).

Major limiting abiotic factors
Generally, the Manebach limnic assemblages shed light on some major limiting environmental factors in the lake, together with their embedding strata. These factors surely changed in extent and significance over time, reasoned from the sedimentological and palaeontological evidence for climatic seasonality and the profile-up changing lithology reminiscent of the lake's evolution. Accordingly, the conclusions drawn below should be understood as longterm averages, and their changing role over time will be discussed if possible.
Apparently, oxygen was a significant limiting factor and its abundance followed a bathymetric gradient. This is indicated by various pieces of evidence. Primary bedding and slumps are well preserved throughout the sections (Figs 5, 8C). Therefore, with the dark to black rock colours (Fig. 8A, B) and the adpression-type preservation of plant remains (Figs 11, 12), we infer prevailing anoxia within the lake-floor deposits. Such conditions favoured the preservation of organic matter and kept infaunal macrobenthos away. However, even epibenthic life seems to have been restricted to the micro-scale in the deep water, evidenced by elephant-skin structures in some shales and the preservation of the ichthyofauna, especially in the upper Manebach Lake Unit (Fig. 5). The presence of near-complete individuals and disarticulated but localized remains of amblypterids ( Fig. 8E-G) and completely to poorly preserved xenacanthid bodies (Fig. 9D) point to more or less undisturbed decay of carcasses at the lake bottom. Although their entire preservation in quiet waters may require additional preconditions (Hellawell & Orr 2012), the fish fossils indicate that large nektonic or benthic scavengers eschewed the profundal zone of the Manebach Lake. Including the preservation of otherwise rapidly decaying cartilaginous tissues, we therefore infer oxygen deficiency in the near-bottom waters, at least temporarily (see Frey et al. 2020;Lukeneder & Lukeneder 2022).
However, the fossiliferous strata also contain indications that profundal stagnation was temporarily interrupted and that the oxycline shifted vertically over time. amongst other processes, intercalations of lenticularly bedded siltstones containing plant detritus (Fig. 5) could indicate bottom flow near riverine inlets. Finally, rising anoxia could also be indicated by butterfly-positioned bivalves from the laminar microbialites of interval I (Figs 5, 6H). As these taphocoenoses include fish remains and are sandwiched between shallow-water stromatolites below and deep-water black shale above (Fig. 5), they probably formed in considerable depths, perhaps near the oxycline. The butterfly position indicates asphyxiation, which generally results from obrution and oxygen deficiency (e.g. Schatz 2005). Given that the embedding laminar microbialites do not show evidence of episodically high sedimentation rates, we assume a rise of the oxygen mininum zone to be the likeliest reason for these taphocoenoses. An episodic or seasonal shifting oxycline may also explain the presence of abundant ostracod occurrences on bedding planes and poorly to nondisarticulated decomposed branchiosaurids (Fig. 9G).
Turbidity and sedimentation rates appear to have been low, increasing only during events such as intensified river inflow or, if present, algal blooms. This conclusion follows from the widespread elephant-skin structures in the shales and decimetre-sized, multi-annually grown stromatolites in the littoral deposits (Fig. 6C). All these microbially formed deposits prove light availability in the water column, next to low grazing pressure. In the stromatolites, thin sections reveal small proportions of epiclastic grains, supporting the model of very low average turbidity. Finally, variable degrees of decay in most amblypterids from the black shales of interval III reflect that some time must have elapsed before their burial ( Stamberg & Tr€ umper 2021).
The silicified stromatolites from the base of the Manebach Lake Unit (Fig. 5) could indicate another limiting ecological factor in the environment. These microbialites underwent silicification very early in their near-shore settings, as evidenced by the delicate three-dimensional preservation of both cyanobacteria and fungi, and rare euhedral calcite crystals within the chalcedony (Fig. 7). If silica had replaced calcite, the microorganisms would have been much more poorly preserved or even vanished, as shown by field studies on modern stromatolites (Kremer et al. 2012). Generally, recent examples of the primary silicification of stromatolites in non-marine environments are rare and occur in waters of more or less active volcanic settings, some of them additionally showing increased alkalinity (Jones et al. 2005;Cangemi et al. 2010;Kremer et al. 2012;Daza Brunet & Bustillo Revuelta 2014;Zeyen et al. 2015). If present at all, elevated alkalinity probably operated on a low level in the Manebach Lake, given the low abundance of carbonate in the deposits (Fig. 5), the abundance of aquatic metazoans (Figs 6, 8-10), evidence for fluvial inflows and indications of a subhumid to humid climate (for criteria of high-alkaline lake formation see Stollhofen et al. 2000;Zhang et al. 2018 andCao et al. 2020). As reworked igneous rocks hardly contributed to the deposits of the Manebach Formation, synsedimentary volcanism hence remains the most likely source for primary stromatolite silicification. The volcanism could have affected the lake environment in various ways, such as the lithologically evidenced deposition of pyroclastics (Figs 5, 8D) or the provision of hydrothermal fluids.

From taphonomic constraints to reconstructing the lake ecosystem
Reconstructing the compositions and functionalities of past ecosystems is the ultimate goal of palaeontological research, although this intention encounters numerous challenges. As fossilization conserves organisms, anatomical detail and biotic interactions selectively, incompletely or rarely, taphonomic and sampling biases must be evaluated to ascertain the fossil record's expressiveness (e.g. Behrensmeyer et al. 2000;Tarver et al. 2007;Walker et al. 2020). This task also applies to interpreting fossil assemblages from limnic strata, even when their formation environments have higher preservation potential than terrestrial settings (Gastaldo 1988;Boy 1998). However, proportions among fossil groups can be strongly modified in limnic taphocoenoses, depending on the structures and lifestyles of the organisms, the lake chemistry and depositional nature, and the diagenetic fate of the sediments (e.g. Martinez-Delcl os & Martinell 1993;Falk et al. 2022). In addition, some elements of the former biocoenosis could have escaped preservation, while biotas from surrounding terrestrial and aquatic environments entered the limnic setting (see Briggs et al. 1998;Voigt et al. 2014). Therefore, lakes indeed may be considered 'fossil sinks' but not in terms of flawless palaeoecological and evolutionary archivists.
During intervals of stable stratification, the presence of an oxycline divided the Manebach Lake ecosystem into two subsystems. In the anoxic bottom waters, life is likely to have operated on the microbial level, comprising detritivores, chemoautotrophic and, depending on light availability, photoautotrophic microorganisms. This mainly profundal part of the Manebach Lake ecosystem is poorly evidenced, except for rare elephant-skin-like structures in the darker shales and evidence of decay in fish carcasses that sank to the lake bottom (Fig. 8F).
By contrast, the oxygen-rich surface waters harboured a much more differentiated life throughout the year. The biotas built a diversified food chain that can be reconstructed based on the excavation results (Figs 16, 17). At the primary producer's level, photoautotrophic microorganisms played a pivotal role, evidenced by cyanobacterial stromatolites, other microbialites and elephant-skin structures in the sections (Figs 5, 6A-C, 16). Pelagic primary producers (e.g. phytoplankton) existed in the Manebach Lake, given the organic-rich shales (Fig. 8A, B) and the relative abundance of planktivorous amblypterids and other F I G . 1 6 . Trophic structure of the Manebach Lake ecosystem. consumers ( Fig. 8E-H). However, the nature of these open-water microorganisms remains unknown, and their future recognition requires methods used in palynology, organic geochemistry and organic petrology, such as fluorescence microscopy and carbon-isotope spectrometry (see Clausing 1992;Clausing & Boy 2000;Mercuzot et al. 2021). However, next to the autochthonous primary production, we also note that the biotas of the co-existing terrestrial environments contributed to the trophic structure of the lake ecosystem. Corresponding possible evidence comprises indistinguishable plant detritus, larger fragments (Figs 11, 12), fungi in the conifer stems (Fig. 7B) and insect remains (Fig. 13), although the significance of these nutrient sources probably ranked behind that of the limnic microorganisms. The floral remains mainly derived from the vegetation growing within or along fluvial channels that fed the lake. We infer this model from the cross-section dominance of mechanically robust organs of plant groups, which otherwise characterize fluvial-channel and crevasse-splay phytocoenoses in the Manebach Formation (Barthel 2001). In comparison, the selective preservation of plant remains at the lake bottom probably played a minor role, given: (1) the presence of well-preserved, more delicate plants parts (Fig. 12I); and (2) scarce evidence for advanced decay in all fossil floras (Figs 11, 12). This discussion reveals a strong bias of the Manebach Lake fossil floras toward elements adapted to temporal water deficiency. The phytocoenoses hence pretend a higher abundance of meso-to xerophilous floral elements such as conifers and peltasperms in the lake surroundings, challenging the general use of limnic plant assemblages for palaeoclimate reconstructions and biostratigraphy in the early Permian (Autunian).
Various invertebrate and vertebrate groups formed the consumer levels in the Manebach Lake (Fig. 16). Conspicuously, the resulting fossil assemblages are highly unbalanced, with remains of ostracods and amblypterid fishes being the most common. This imbalance reflects abundance patterns to some extent, although selective preservation probably occurred. For instance, the shadowy preservation of the rare conchostracan fossils indicates diagenetic dissolution, a phenomenon also known from other late Palaeozoic limnic strata. For example, Schneider & Werneburg (2006) mentioned three-dimensional and shadowy preservations of conchostracans vertically alternating in the lower Permian B€ ortewitz Lake Unit, east-central Germany. In addition, Astrop & Hegna (2015) concluded that the potential to preserve the carapaces might vary even at the family scale. Therefore, with F I G . 1 7 . Reconstruction of the Manebach Lake ecosystem (artist: Frederik Spindler). a likewise 'shadowy' appearance of Uronectes (Fig. 8I), and the imprint-like preservation of some blattoid insect wings (Fig. 13), the fossil conchostracans indicate that chitin and partly chitinous hard tissues may have experienced some dissolution in the Manebach Lake strata. These insights underline the often-highlighted role of fossil diagenesis in the long-term preservation of arthropods (see Briggs 1999; Gupta & Summons 2011) and show that members of this group may have been more abundant in the lake.
Carbonate-shelled biotas also underwent alteration during fossilization, as their predominant cast preservation in both the shales and the microbialites indicates (Fig. 6H). The timing of these dissolution processes remains open and possibly depends on the lithology considered. For instance, residual, calcified domains in the microbialites (Fig. 6H) reveal that carbonate dissolution occurred recently following weathering. This process generally raises the question of the original carbonate content in the strata, although it was probably lower than the content of epiclastic material. Nevertheless, the cast fossils document that the deposits generally allowed the preservation of carbonate-shelled organisms, limiting the possibility of lost corresponding biotas in the Manebach Lake fossil record. However, we note that not all carbonateforming biotas should be lumped together, for instance because the aragonite shells of gastropods tend to dissolve more easily (see Brachert & Dullo 2000;Jordan et al. 2015). Hence, the former presence of this group in the Manebach Lake remains somewhat open.
Lower consumer levels in the Manebach Lake may have included zooplankton, but this cannot be confirmed due to a lack of fossil evidence. Crustaceans such as ostracods, conchostracans and syncarids added to the lower-level consumers (Figs 16,17) and were probably most abundant in near-shore settings, as ostracod accumulations in the microbialites indicate (Fig. 6F). Other elements comprised the Anthraconaia-like bivalves (Fig. 6H) which remain conspicuously rare in the Manebach Lake deposits (Fig. 17). This scarcity may reflect an ecological bias, as taphocoenoses from the fluvial deposits of the Manebach Formation suggest (Tr€ umper et al. 2019). The Anthraconaia-like bivalves form mass occurrences in shallow, metre-scale depressions in these strata. Thin clay beds cover the shell beds, attesting to former ponds in the fluvial plain that obviously existed only over one or a few life cycles of the bivalves. The Manebach Formation hence provides indications that this Anthraconaia-like bivalve prefered dynamic and probably well-oxygenated fluviolacustrine settings rather than persistent, large lakes vulnerable to rising anoxia.
The planktivorous Paramblyterus cf. duvernoyi was a quantitatively important consumer in the Manebach Lake (Fig. 16). Due to their abundance, these fish probably formed the supreme food source for the apex predators, next to the less important branchiosaurid amphibians and acanthodians (Fig. 17). Xenacanthid sharks were the permanent top predators (Figs 16, 17), while coprolites, fossil teeth and stomach remains attest to the at least temporary presence of large tetrapods. The nature of these vertebrates, which may have come to the lake only to hunt, remains obscure. Amongst others, possible candidates comprise the large, terrestrially adapted eryopid Onchiodon thuringiensis and 'pelycosaurs', fossils of which are documented in the Manebach Formation (Werneburg 1999(Werneburg , 2007.

Implications for biodiversity and biogeography in the Pangaean tropics
At first glance, the Manebach Lake ecosystem (Fig. 17) is full of contrasts: microbialites that flourish in a metazoan-rich, siliciclastic lake (Fig. 6C); fungi within extraxylary tissues of stromatolite-encrusted conifers (Fig. 7B); and a lowly diverse and unbalanced aquatic community in a hydrologically open lake that is embedded in a diversified fluvial-plain ecosystem. These seeming contradictions cannot be resolved if the scope is limited to the Manebach Formation, but their driving forces become more tangible in a wider palaeogeographical and temporal context.
A striking feature of the Manebach Lake Unit is the dominance of fishes over aquatic amphibians (here branchiosaurs) throughout the sections. This prevalence implies that this lake was a fish lake and remained comparably stable as regards its large depth/size. As a result, the ichthyofauna stood in competition with the branchiosaurids and, in the case of the xenacanthids, even preyed on them, explaining the scarce branchiosaur record in the strata studied. However, besides interspecific competition, climate probably also contributed to the fish dominance in the Manebach Lake, as the evolution of limnic taphocoenoses in the Thuringian Forest Basin indicates (summarized hereafter from Schneider & Germann 2019; compare also Fig. 2). Accordingly, in the uppermost Gzhelian Ilmenau Formation (Sembachtal Lake Unit) and the lower Asselian Manebach Formation (Manebach Lake Unit), the 'fish-lake type' dominates the limnic record. Only one bed from the basal Sembachtal Lake Unit yielded numerous branchiosaurid skeletons, marking an initial 'amphibian-type' lake phase. Both formations formed under subhumid to humid conditions, especially in the case of the Manebach Formation, as coal deposits and widespread grey strata document. As discussed previously, the Asselian to lowermost Sakmarian Goldlauter Formation marks the shift toward a drier climate. The corresponding lake units are still rich in fishes, reflected by their traditional but no longer accurate designation as 'Acanthodes horizons'. However, the limnic taphocoenoses more commonly fluctuate between amphibian and fishtype stages than in the previous formations. The Goldlauter lakes were still perennial but possibly experienced more intensely alternating lake levels. Climatic aridity ultimately strengthened during the deposition of the redbed-dominated Sakmarian to lowermost Artinskian Oberhof Formation, paralleled by intense volcanism. Lake intercalations of this lithostratigraphic unit are traditionally referred to as 'Protriton horizons', with the outdated term Protriton being formerly used for several branchiosaurid genera. The Oberhof limnic systems were amphibian-type lakes and only contained depleted and scarce fish fauna concentrated in some beds of the sections. These lakes were the last perennial ones in the Thuringian Forest Basin, and they correlate with ultimate perennial-lake deposits in other basins such as the Saar-Nahe and Bourbon l'Archambault basins (Germany, France;Roscher & Schneider 2006).
The profile-up increasing abundance of aquatic amphibians over fishes in lake strata of the Thuringian Forest Basin reflects increasing aridity in the latest Gzhelian and Sakmarian, together with lithofacies. This local trend is a cut-out of the higher-rank aridification trend that characterized the European part of Pangaea from the Middle Pennsylvanian up to the late Permian. It comprised alternating drier and wetter intervals at various scales (DiMichele et al. 2006;Schneider et al. 2006;Michel et al. 2015;Oplu stil et al. 2019;Tr€ umper et al. 2022), of which the described Thuringian Forest formations documented the million year scale. This discussion reveals that peak to late icehouse climate changes shaped the fish-dominated Manebach community and possibly aquatic vertebrate faunas of tropical Pangaea. However, the low diversity of the Manebach Lake ichthyofauna may be also the result of additional factors. Exemplified by the xenacanthids, Manebach shows the lowly diverse Bohemiacanthus-Xenacanthus association that characterizes lower Permian limnic strata in several basins across Europe, except for the Saar-Nahe Basin (Schneider 1996;Heidtke 2003;Stamberg & Zaj ıc 2008). Lake units from the Carboniferous, by contrast, provide considerably more diverse shark faunas, including Orthacanthus, Plicatodus, the hybodont Lissodus, and the sphenacanthid Sphenacanthus, next to Bohemiacanthus and Xenacanthus (Schneider 1996). Schneider (1989 interpreted the freshwater-shark decline around the Carboniferous-Permian boundary as resulting from tectonic movements in the latest Gzhelian. Accordingly, late to post-Variscan tectonics involved the re-organization of fluvial interconnections between the depocentres, imposing migration barriers for some species and triggering the separation of aquatic settings. Whether such hydromorphic obstacles affect the dispersal of an organism strongly depends on its biology, in the case of freshwater sharks, especially on habitat preference during ontogeny and reproduction. Fossil elasmobranch eggs of the Fayolia and Palaeoxyris types, for instance, show different distributions in fluvial and shallow-limnic settings, respectively (Schneider & Reichel 1989;R€ oßler & Schneider 1997;Fischer & Kogan 2008;Fischer et al. 2010b). Such site-specific reproduction preferences lead to different migration behaviours of species and could therefore act as selection factors. This said, we highlight the need for more knowledge of the palaeobiology of freshwater sharks but also of the accompanying actinopterygians. Especially the latter group forms a quantitatively important proportion of late Palaeozoic aquatic faunas but remains poorly understood regarding their systematics and ecology. This demand involves the need to increasingly document limnic faunas across Europe, involving the bed-by-bed documentation of lithofacies, assemblage composition and fossil preservation. Only this approach allows for distinguishing and quantifying the impacts of abiotic and biotic factors on limnic biodiversity as the supreme goal.
The presence of stromatolites in the Manebach Lake (Fig. 17) sheds light on another aspect of deep-time limnic biodiversity. In striking contrast to the present-day situation, decimetre-sized stromatolites seem to have been almost typical elements in exorheic lakes of the latest Carboniferous and Permian seasonal tropics, at least in Europe. Domed microbialites occur in a variety of limnic strata that indicate different lake types and climate regimes based on their lithologies and ages, reaching from evaporative, carbonate-depositing settings to siliciclastic ones (Stapf 1973(Stapf , 2005Gebhardt 1988;Gand et al. 1993;Schneider & Gebhardt 1993;Kerp et al. 1996;Freytet et al. 1996Freytet et al. , 1999Freytet et al. , 2000Costamagna 2019;Schneider & Germann 2019). The Manebach Lake extends this spectrum to include the more densely vegetated fluvio-limnic plains of 'wetter' climate intervals (Fig. 15C). Today, stromatolite formation in non-marine waters focuses on ephemerally water-covered settings (Lundberg & McFarlane 2011), streams (Shiraishi 2011) and, in the case of perennial lakes, places of extreme physicochemical conditions such as hypersalinity, high alkalinity, elevated or strongly alternating temperature, strong solar irradiance, raised heavy-metal concentration, raised water turbulence and combinations of these factors (Casanova 1994;McCall 2010;Sommers et al. 2022;Far ıas et al. 2011;Ka zmierczak et al. 2011;Kremer et al. 2012). Exceptions in modern, deep, low pH-low salinity lakes comparable to Manebach are stromatolites that formed under more restricted lake conditions in the past and continue to grow slightly, such as in Lake Tanganyika, eastern Africa (Cohen et al. 1997). Consequently, and if we also consider Mesozoic and Cenozoic lake records (e.g. Awramik & Buchheim 2015), the Phanerozoic freshwater record of stromatolites follows their general decline since the Proterozoic (Pratt 1982;Proemse et al. 2017).
The 'success' of stromatolites in the late Palaeozoic lakes of Europe may be the result of both abiotic and biotic factors. Accordingly, the increasing abundance of such deposits from the Middle to Late Pennsylvanian onward parallels the spread of red beds and could relate to the gross climate aridification mentioned above. More pronounced dryness over time favoured seasonal precipitation regimes, more fluctuating lake levels and increased temporal evaporation, indicated by cross-basinal aquatic vertebrate communities and the lithofacies and geochemistry of their limnic host rocks (M€ uller et al. 2006;Zaj ıc 2014;Boy & Schindler 2012;Schneider & Germann 2019). Peat formation gradually retreated, and near-shore waters became clearer and physicochemically unstable; settings that ultimately favoured opportunistic microbial communities to form stromatolites.
However, climate alone cannot fully explain the late Palaeozoic distribution of stromatolites, given that ecologically challenging limnic settings also exist in the presentday seasonal tropics and the Permian Manebach Lake thrived under comparably stable lake conditions in a subhumid to humid climate. Given that late Palaeozoic littoral waters were likely to have been as light-filled and sediment-supplied as their modern analogues, biotic drivers come into play. One of these factors concerns the popular idea of increased grazing pressure exerted by metazoans, which, however, was relativized in more recent years based on studies of fossil and extant stromatolites (compare Walter & Heys 1985;Pratt 1982;Rishworth et al. 2016and Peters et al. 2017. Instead, the aforementioned studies consider the interplay of longterm changes in water geochemistry, biogenic deposition and interspecific competition for substrate as the reason for the Phanerozoic decline of stromatolites. As for lakes, little is known about the development of freshwater geochemistry. However, upper Palaeozoic limnic records across the world indicate that lakes of that time had a very similar chemical spectrum to modern limnic settings (e.g. Izart et al. 2012;Cao et al. 2020). This insight spotlights the role of interspecific relations that may have been different in late Palaeozoic times. For instance, quickly growing angiosperms are strong competitors for light and space in modern littoral waters due to their rapid growth, ecological modesty and diversity. Various water plants and extensive reed belts occur along present-day lake margins in various climates. These plants provide organic detritus to the water and can inhibit water movement, ultimately forming sediment traps, increasing turbidity and reducing oxygenation locally (Gabriel & Bodensteiner 2011). As a result, the slowly growing, light-dependent stromatolites are at a disadvantage. As for the late Palaeozoic, information on plants growing in shallow waters remains rare overall. This issue may also result from the challenge to preserve hydrophilous floras that lack resistant macromolecules or do not mineralize (Collinson 2010; P seni cka & Krings 2016). However, in late Palaeozoic perennial lakes, plants known to colonize littoral settings comprise noncalcareous algae (Krings et al. 2007) and sphenopsids (Pfefferkorn et al. 2001;Thomas 2014;Tr€ umper et al. 2019). These plant groups may have been as locally abundant in littoral settings as reeds or macroalgae are in recent settings. Nevertheless, the density and diversity of late Palaeozoic hydrophilous plants possibly ranged behind the abundance of recent angiosperms in those settings. Consequently, stromatolite growth in Pennsylvanian and Permian lake-littoral environments could have benefited from lower competitive stress. In search of hard substrates to colonize, the microbialites often used woody debris and accidentally submerged trees, nowadays abundantly recorded in anatomically preserved stems and trunk moulds within the stromatolites (AH, unpub. data). This wood encrustation has also been the case in the Manebach Lake (Fig. 17), where silicification even preserved various microorganisms (Fig. 7), the study of which remains a task for the future. Palaeozoic (and Mesozoic) littoral freshwaters that yield woody debris thus could have been suitable for stromatolite formation; an ecological niche that possibly disappeared when angiosperms conquered lake margins.
The dominance of the phyloblattid Anthracoblattina (Fig. 13) in the Manebach Lake entomofauna probably mirrors a habitat preference, as the spatiotemporal and stratigraphic distribution of this genus in central Pangaea indicates. Generally, Anthracoblattina occurs in Late Pennsylvanian (Westphalian-Stephanian transition) to middle Cisuralian (Artinskian-Kungurian) strata, with single finds in nearly all well-documented insect sites from Europe and North America. Within these successions, however, this insect genus shows a facies preference. While Anthracoblattina remains very rare in entomofaunas of the coal-seam roof shales, such as the Stephanian Wettin insect site in Germany (Schneider 1983), it is more common and better preserved in lake strata. Examples include the limnic deposits of Commentry in France (Stephanian B/C) and Sperbersbach (Goldlauter Formation, Asselian) of the Thuringian Forest Basin, now extended by the Asselian Manebach Lake Unit. Despite some biostratinomic bias, the associated fossil macrofloras indicate the presence of xero-to mesophilous vegetation elements near the lakeshore and in its hinterland. Consequently, Anthracoblattina may have preferred habitats near lakes rather than more or less densely vegetated swamps across Pangaea (Ricetti et al. 2016;Belahmira et al. 2019).

CONCLUSION
Lake deposits are abundant in Pennsylvanian and lower Permian successions of Europe, forming crucial stratigraphic markers and providing fossils valuable for crossbasinal correlations and environmental reconstructions. The strata are important archives for tropical biodiversity in late-icehouse Pangaea, due to their stratigraphic distribution and the high preservation potential of their depositional environments. However, to unlock this potential, we need more detailed information about the deposits' sedimentary architecture, the bed-by-bed occurrence of the fossils and their taphonomic biases. We have reconstructed the formation, ecology and trophic structure of a lake from a wetland in the peak-icehouse tropics of Pangaea based on high-resolution analyses of litho-and biofacies and radioisotopic dating of the Manebach Lake Unit, central Germany. The results and their discussion allow us to infer the following conclusions: 1. The Manebach Lake was a perennial, exorheic, intramontane water body in the 10 1 -10 2 km 2 range that existed 298 myr ago in the seasonal tropics of eastcentral Pangaea. 2. Lake formation coincided with a regionally traceable wet climate interval in the early Asselian. Post-Variscan tectonic movements favoured limnic deposition by raising the base level and groundwater tables in the half-graben-like Thuringian Forest Basin. 3. The limnic ecosystem included cyanobacterial microbialites, terrestrial-plant debris, rare bivalves, several crustaceans and a lowly diverse vertebrate fauna comprising the amblypterid Paramblyterus cf. duvernoyi, the xenacanthid sharks Bohemiacanthus and Xenacanthus and the branchiosaur Apateon dracyiensis. Coprolites, stomach remains and skeletal elements reveal the scarce presence of acanthodians and large semi-aquatic amphibians that possibly visited the lake for hunting. 4. Oxygen stratification was a limiting environmental factor in the lake, diminishing hypolimnal life and causing asphyxication near the vertically shifting oxycline. 5. Freshwater-shark diversity dropped in some European basins around the Carboniferous-Permian transition and possibly resulted from interspecific life-mode differences and tectonic reorganization of the Variscan drainage system. 6. Actinopterygians remain a blind spot in the investigation of late Palaeozoic non-marine communities, and understanding their diversity, biogeography and biology is crucial to reconstruct deep-time freshwater biosphere. 7. Woody debris in freshwater settings formed an ecological niche for colonization by stromatolites.