The Upper Triassic Polzberg palaeobiota from a marine Konservat-Lagerstätte deposited during the Carnian Pluvial Episode in Austria

A rich assemblage of various marine taxa from the lower Carnian Polzberg Konservat-Lagerstätte near Lunz am See (Northern Calcareous Alps, Lower Austria) is described for the first time in detail. The fossiliferous layers were deposited during the Julian 2 Ib (Austrotrachyceras austriacum Zone, Austrotrachyceras minor biohorizon). The fine-laminated Reingraben Shales comprise abundant and well-preserved members of the marine Carnian food chain. Invertebrates with the bivalve Halobia, the ammonite Austrotrachyceras and the coleoid Phragmoteuthis dominate over vertebrate actinopterygian fishes. Fragile groups such as polychaetes and isopods are entirely preserved as soft body fossils. The diverse assemblage comprises ammonites (Austrotrachyceras, Carnites, Sageceras, Simonyceras), coleoids (Phragmoteuthis, Lunzoteuthis), bivalves (Halobia), gastropods (caenogastropods/heterobranchs), one echinoid, thylacocephalan arthropods (Austriocaris), crustaceans (the decapod Platychela and isopods such as Obtusotelson, Discosalaputium), polychaetes (Palaeoaphrodite sp., eunicid polychaete), acytinopterygians (Saurichthys, Polzbergia, Peltopleurus, Habroichthys), cartilaginous fishes (Acrodus), coelacanth fish (“Coelacanthus”), a lungfish (Tellerodus), and a conodont cluster (Mosherella). Regurgitalites produced by large durophagous fish and coprolites produced by piscivorous actinopterygians accompany the Polzberg palaeobiota along with rare plant remains (Voltzia). The entire fauna of Polzberg and the excellent preservation of the specimens present a window into the Upper Triassic assemblage and palaeoenvironment during the so-called Carnian Pluvial Episode (CPE) in the early Mesozoic. The occurrence of the freshwater lungfish Tellerodus and the branchiopod Eustheria, a member of brackish to freshwater environments, points to the influence of occasional freshwater pulses or sediment transport events on the marine environment. The Polzberg palaeobiota was deposited during the global CPE, triggering the environmental conditions of the Polzberg Basin and resulting in the formation of the Reingraben Shales with the Polzberg Konservat-Lagerstätte.

www.nature.com/scientificreports/ the Austrotrachyceras abundance zone vary between 1.4 and 0.3%. The total sulphur (TS) content ranges between 1.8 and 0.3%. The laminated appearance of the rock is a result of wispy, discontinuous, flaser-like laminae of dark, amorphous organic material and pale-coloured laminae comprising masses of halobiid shells composed of light grey to whitish calcite. The laminae and layers range in thickness from 0.1-0.2 mm to 10-25 mm. The contact surfaces between the layers and laminae are gradational to sharp. Phosphatic debris is abundant and consists mainly of actinopterygian fish scales, bones and teeth. The dominant benthic bivalves form shell pavements of juvenile to adult Halobia rugosa.

Biostratigraphy: the Austrotrachyceras minor abundance zone
The Austrotrachyceras minor abundance zone is bordered by biohorizons which are characterized by a sharp and significant biostratigraphic change within the fossil assemblage and/or a change in the frequency of its members, as observed at Polzberg 2,39 . The lower Carnian fossiliferous deposits at Polzberg appear to be deposited during the Julian 2 Ib (Austrotrachyceras austriacum Zone, Austrotrachyceras minor biohorizon). The Austrotrachyceras minor biohorizon is underlain by the A. triadicum biohorizon and overlain by the Neoprotrachyceras oedipus Subzone with the basal Austrotrachyceras n. sp. 1 biohorizon 40 . Such biohorizons are very important for lateral correlations. The presence of abundance zones ("ammonite beds"; characterized by abundance or massoccurrence of ammonites) is exceptionally valuable for the interregional correlation of the Late Triassic. Such uniformity beds are formed by a monotonous ammonite assemblage from at least a single bed up to few metres thickness. The appearance of the abundant index ammonite A. minor within the fossiliferous interval is crucial for the understanding of the biostratigraphical and interregional linkage of the lower Carnian (Julian) Polzberg Konservat-Lagerstätte.

The Polzberg Konservat-Lagerstätte linked to the Carnian Pluvial Episode
During the Carnian (Late Triassic), the Polzberg area was located at the north-western rim of the Tethys in an area of 15° N to 30° N 19,40 (Figs. 1B, 2B). The dry Middle and Upper Triassic climate was interrupted by a middle Carnian global phase of increased humidity in the western Tethys and hence in Europe. This episode is characterised by a worldwide decrease of platform inhabitants and reef demise known as the Carnian Pluvial/Humid Episode (CPE 19,20,[41][42][43]. The episode was a longer and multi-phased process rather than a single event 44 . This humid phase was also termed the "Middle Carnian Pluvial Event" 45 and is characterized by abundant siliciclastics transported by large rivers from the Baltic Craton towards the north-western branch of the Tethys. The sudden increase in siliciclastic input explains the breakdown of the carbonate factory 45 . The "Reingraben turning point" 46 is reflected in biofacies, lithofacies, and in evolutionary events and is mirrored in all facies belts of the entire NW Tethyan continental margin 45 . The humidification is reflected by a change in lithology and facies. The basal Julian sequence in the Polzberg area is characterized by nodular limestones of the Reifling Formation deposited on the palaeoslope. At the base of the Julian 2 Austrotrachyceras austriacum Zone (A. austriacum Subzone, biohorizon of A. triadicum), the Reifling Formation is replaced by the limestone deposits with organic-rich mudstones of the Göstling Member to terrigenous siliciclastic deposits of the Reingraben Formation. Global warming, combined with enhanced humidification during the Early Carnian, and the eruption of large amounts of volcanogenic

The palaeoenvironment of the Carnian Polzberg deposits
The laminated deposits of the Reingraben Shales were formed in a relatively deep marine environment within an intra-platform basin, as inferred from the dominance of a nektonic fauna 3,5 and references therein, 14 . The well-preserved soft bodied fauna (carbonisation, phosphatisation), the abundance of organic material in the sediment, the presence of common framboidal pyrite crystals, the absence of sessile organisms, and the lack of bioturbation point to dysoxic to anoxic bottom conditions during the deposition of the Reingraben Shales 5,30 . The Polzberg sub-basin within the Reifling Basin was mainly normal marine with ephemeral and limited freshwater input 14 . Low energy on the sea floor (absence of bottom currents) and dysaerobic conditions, which prevented predators from separating the ammonite shells from the jaw apparatuses, led to the extraordinary preservation of the Polzberg palaeobiota with entire fish carcasses, fragile taxa, ammonite conch-jaw association, and abundant double-valved bivalves. These exceptional preservational features of articulated hard parts and soft body preservation are typical for Konservat-Lagerstätten 1 . Bottom-water dysoxia-anoxia is known to be connected to increased levels of fossil preservation, for example as articulated hard parts of multi-element skeletons such as arthropods, echinoderms and vertebrates or in the form of preserved soft tissues. When oxygen concentrations drop below a critical threshold level (0.1 ml/l dissolved oxygen), bioturbation virtually ceases and laminated, organic-rich deposits accumulate 49 as observed in Polzberg with the Reingraben Shales. Subsequently, oxygen-related changes in benthic and endobenthic layers yield different communities and ichnocoenoses. The absence of bioturbation in the dark-laminated layers of Polzberg within the Reifling Basin appears to be controlled by oxygen conditions in the substrate. Oxygen availability was highly variable during the deposition of the Carnian sediments of this basin, depending on the climate changes and subsequent adaptation of the palaeoenvironments in the Polzberg area. Species abundance is a simple feature to measure relative palaeooxygen levels. Thus, the non-genetic, oxygen-restricted biofacies (ORB) scheme has been proposed 50 . ORBs are defined simply by their number of species and the sediment fabric. ORB 3 and 4 contain only a few benthic species which can be either very (ORB 3) or prolifically abundant on some bedding planes (ORB 4 50 ), as observed at Polzberg with Halobia rugosa mass occurrences. The situation within the shales here fits best with an ORB 4 biofacies. Under totally anoxic conditions, trace fossils are absent or rare 51 . The position of the redox boundary fluctuated. Oxygen concentrations changed and the duration of oxygenated phases varied. Short-term low oxygenated conditions with low oxygen values in the bottom water favoured the colonialization of monotonous but abundant benthic epifauna. There were long phases with opportunistic benthic taxa or no benthic fauna at all, where the nekton such as fish and ammonoids dominated the macrofauna (i.e. no oxygen near the sea floor).
No sorting due to sedimentological or biological effects is visible; fossil alignments or concentrations triggered by bottom current transport are lacking in the Polzberg assemblage. An enrichment by redeposition by currents or turbidites can be clearly ruled out based on the autochthonous character of the nearly monospecific benthic macrofauna with its dominant element, the thin-shelled halobiid bivalve Halobia rugosa. Except for the transported skull of the freshwater inhabitant Tellerodus and of terrestrial plant material (Voltzia), the autochthonous character of the Polzberg deposits is strengthened by the preservation of fragile parts and the extraordinary preservation of in situ buccal (anaptychi) associations within or close to the body chambers of Austrotraychyceras minor. The geochemical results, along with the laminated fabric and facies, abundant organic matter and high amounts of sulphur indicate that the assemblage was deposited under conditions of intermittent oxygen depletion associated with stable water masses. A dynamic environment, controlled by short-and long-term fluctuations in oxygen levels, along with poor circulation of bottom-water currents within an isolated, basin-like region, led to the accumulation of the Austrotrachyceras abundance zone (= "Trachyceras Schichten"). The lamination generally indicates a very quiet depositional environment undisturbed by currents. Within the Reingraben Shales, dysaerobic (not anaerobic [49][50][51] ) conditions prevailed, allowing endobenthic colonization of the incompletely bioturbated sediment. Decreasing levels of dissolved oxygen in bottom waters over time are suggested by thin, black, laminated limestones ('black shales'). The Austrotraychyceras abundance zone is situated in the laminated deposits. The following features are observable: (I) high TOC, (2) high sulphur content, (3)   Fig. 6). Such excellent preservational deposits (i.e. Konservat-Lagerstätten) are rare in the European Triassic marine record. Only few comparable sites are known from Seefeld in Tyrol 9 or Wiestal in Salzburg 10 , both within the Norian Seefeld Formation, and from the Middle Triassic (late Anisian to Ladinian) Konservat-Lagerstätte of Monte San Giorgio (Ticino, Switzerland 7,52 ). Special conditions are required for the formation of such conservational deposits with fragile and well-preserved fossil remains. Stagnation of water masses along with terrigenous influx by enhanced runoff resulting in accumulations of organic material led to dark-laminated, pyrite-rich deposits that promoted the soft tissue preservation of fishes 54 and other fossil taxa. The Polzberg palaeobiota was deposited in an intraplatform basin, which intensified these conditions, as the Reifling Basin was surrounded by the Wetterstein platform. The demise of platforms with a co-occurring carbonate breakdown was a worldwide phenomenon at that time. This promoted the sedimentation of argillaceous sediments, and the shale deposits were influenced by the enhanced runoff from emerged land, representing former shallow submarine platforms. The enhanced run-off was triggered by the increasd humidity during the Carnian Pluvial Phase (CPP; 40,[42][43][44] ). The Carnian was characterized by a worldwide humidification in the Carnian Pluvial Episode and by a sea level regression. Both events enhanced the stratification effect in the newly formed intraplatform basins. This also helps to explain the origin of fossil remains in such distinct Fossil Lagerstätten comprising a variety of marine and freshwater taxa accompanied by plant remains. The preservation of a palaeocommunity including benthic (epifaunal and infaunal) and nektonic taxa point to a deposition within the inhabited palaeohabitat where the organisms primarily lived, with minimal post-mortem drift or transport. Interestingly, the occurrence does not show densely packed ammonoid shells as expected for sudden-death events in which fossils are preserved in accumulations or mass-occurrences. Such mass mortality events are preserved in Wiestal, where masses of fishes www.nature.com/scientificreports/ are accumulated in very thin layers (i.e., mm thickness, 5 fish layers 10 ). Nektonic fish remains and ammonite shells in the host rock do not exhibit any size sorting (shell diameter from 4 to 80 mm) and lack preferential orientation by bottom water currents. Taphonomic evidence suggests that the Polzberg palaeobiota was formed by stagnant, oxygen-depleted basinal waters without major transport or reorientation of fossil carbonate shell material or fish carcasses. The main diagnostic criteria for the presence of a Konservat-Lagerstätte are fulfilled 3,5,14,16 . More specifically, such unique windows into Earth history contain entire fossil remains, grouped fossil parts, in situ preservation, soft tissue preservation and/or normally rarely preserved fossil remains of numerous fragile fossil taxa. Such palaeocommunities mirror the trophic conditions of the palaeo-food web at the time of deposition (Fig. 6). Fossil remains are not significantly affected by benthic scavengers or bacterial decay. Contrastingly, frequent shell fragments, crushed by nektonic predators, and well-preserved bromalites are main but little-known constituents of the fossil record in the Polzberg palaeobiota 5 . Bromalites consist mostly of fish coprolites and rare regurgitalites. The evidence suggests that Polzberg locality preserves two types of bromalites: coprolites incorporating fish remains with fish scales, and regurgitates with ammonites and coleoid hooks and cartilage masses. Additional recent findings show bromalites with only one constituent, i.e. dominated either by hooks from teuthids, ammonite shells or fish scales. The predators here were therefore apparently specialized on different diet strategies and prey. Most likely, different actinopterygiid fishes equipped with various dentitions fed on cephalopods or other fishes. Additional feeding types occurred at the sea floor in the form of scavengers, harvesting organisms or decomposition of organic metarial.
A single specimen of the lungfish Tellerodus sturii was also found here 26 . Mesozoic dipnoans were restricted to freshwater environments and their remains found in marine deposits are commonly interpreted as a result of post mortem transport from freshwater ecosystems. Conchostraca appear frequently with Eustheria in the upper, more argillaceous part of the Polzberg section. Eustheriids typically inhabit freshwater or at least brackish environments. Both elements-the vertebrate lungfish Tellerodus and the conchostracan shells-indicate a sporadic influx of freshwater or sedimentation from surrounding shallow-water or terrestrial areas into the restricted Reifling Basin (incorporating the Polzberg zone). A possible adaptation of these new cocnchostracan species to marine environments cannot fully be excluded but requires more detailed investigations on their distribution based on bed-by-bed sampling. Plant remains with foliated trunks of the Coniferophyta member Voltzia support this interpretation.
Food web of the Polzberg palaeobiota. Over the last 140 years, 6397 fossils have been collected here during several excavation campaigns and by citizen scientists. The amount and variety of fossil remainsenable conclusions on the palaeo-food web of the Upper Triassic Polzberg deposits (Fig. 6), dominated by benthic halobiid bivalves, nektonic actinopterygiid fishes and ceratitid ammonites 5 (Fig. 7).
The primary producers in this food web are represented by algae and bacteria, grazed by primary consumers such as gastropods and arthropods and filtered by benthic bivalves. The low-level consumers are preyed upon by secondary consumers or predators including different actinopterygiid fishes and ammonites. The latter groups served as prey for larger cartilaginous fish (Acrodus) and the actinopterygiid fish Saurichthys. The presence of a top predator of the ichthyosaur group is speculative but probable. Thylacocephalan arthropods, represented by Austriocaris, are thought to have had either a scavenging mode of life near the sea floor, as shown for Ostenocaris 55,56 , or as actively hunted in the water column for conodonts, small cephalopods 57 and other small members of the palaeobiota see also 57 . Ostenocaris was found in the lower Jurassic of Osteno (Italy) with stomach contents or regurgitalites comprising fish vertebrae or scales and coleoid arm hooks 55,56 . Late Devonian thylacocephalans from Maïder (Morocco) with Concavicaris were assigned as predatory carnivors 57 , as also shown for Dollocaris, a middle Jurassic thylacocephalan from La Voutle (France) preying on other arthropods 59 . They, in turn, were hunted themselves by chondrichthyans or other large fishes 57 , hence serving as an important food source for numerous fish taxa.
Starting at the base of the trophic pyramid at the upper Triassic sea floor from Polzberg basin, and extending across the entire food web, this system is similar to others from the Permian to Triassic marine palaeoworld, with comparable trophic levels of invertebrate and vertebrate members 10,52,58-61 .
Important evidence for food webs here is gained from bromalites, represented by coprolites and regurgitalites 5 and references therein. As reported by Lukeneder et al. 5 , regurgitalites were produced by large durophagous predators. The cephalopods and arthropods here appear to be too small to produce bromalites up to 100 mm in size. The rich ichthyofauna and lack of reptile remains point to probable bromalite producers among predatory fishes. There is evidence that Palaeozoic and Mesozoic shelled cephalopods were preyed upon by sharks and actinopterygian fishes 41,43-45 . Krystyn 2 and Lukeneder et al. 5 noted the occurrence of the cartilaginous fish Acrodus along with the actinopterygiid predators Elpistoichthys, Gigantopterus, Saurichthys, Thoracopterus, Habroichthys, Nannolepis and Peltopleurus. Griffith 14 stated that the Upper Triassic ichthyofauna of the Polzberg region is characterized by abundant flying fish, which, according to that author, suggests strong predation pressure in this marine ecosystem. Furthermore, 55% of the genera of marine fish known from Polzberg were predatory 5,14 . The largest specimens described by Griffith 14 belonged to Saurichthys (50 cm in length). Saurichthys is an ambush predator 62 targeting other actinopterygiid fishes, was also shown in the Norian fish assemblages from Wiestal in Salzburg 10 . Hornung et al. 10  The marine predatory vertebrates in Polzberg that potentially produced the regurgitalites desribed herein are Acrodus and Saurichthys 5 . Durophagy sensu lato (the ability to consume hard prey 5 and references therein) is possible with numerous dental types, especially when dealing with thin-shelled prey such as the small ammonites in the present regurgitalites 5 . We assume that various durophagous actinopterygiids hunted and crushed their prey, including Elpistoichthys, Gigantopterus, Saurichthys, Thoracopterus, Habroichthys, Nannolepis, and Peltopleurus. Triassic species of Saurichthys are characterized by monognathic heterodonty-the teeth in one jaw differ in size and shape. Given the above, we argue that a durophagous shark such as Acrodus, which was equipped with a typical durophagous dentition (crushing or grinding) with blunt and broad teeth, most likely produced the studied regurgitalites 5 As noted by Lukeneder et al. 5 , the more abundant but smaller and longitudinal coprolites, containing masses of almost exclusively fish scales, were most likely produced by medium-sized piscivorous actinopterygians common in the Polzberg palaeobiota: Elpistoichthys, Gigantopterus, Saurichthys, Thoracopterus, Habroichthys, Nannolepis, and Peltopleurus.
No evidence is currently available of a possible top "predator x" (Fig. 7) from the late Triassic ichthyosaur-or nothosaur-group see [66][67][68][69] in the Polzberg basin, but fossil finds are expected in upcoming excavation campaigns. This would enable testing the top predator hypothesis.

Conclusions
This is the first report on the discovery of historical and recent findings of the palaeobiota from the Polzberg Konservat-Lagerstätte to a broader, international scientific community. The Upper Triassic (Carnian) Polzberg locality from the Austrian Alps yieded producers, consumers, as well as small and large predators within the frame of the Reifling intraplatform basin during the Carnian Pluvial Episode (CPE). This worldwide humidification in the Carnian caused the deposition of the dysoxic sediments of the Reingraben Shales at the epipelagic to upper mesopelagic sea floor of the basin, which was periodically disconnected from oxygenated bottom currents. In the low-oxygen ecosystems at Polzberg, bivalves of the genus Halobia were the dominant epifaunal elements, at least near the sea floor and/or within the carbon-rich and laminated sediment. In the overlying oxygenated water column, ceratitid nektonic/nektobenthic ammonites (Austrotrachyceras) and nektonic actinopterygiid  www.nature.com/scientificreports/ fishes prevailed. The occasional freshwater influx from the surrounding Wetterstein Platform resulted in terrigenous and argillaceous sedimentation, This was accompanied by terrestrial plant material (Voltzia) and the freshwater dipnoid lungfish Tellerodus. The entire fossil assemblage provides new insights into Upper Triassic trophic interactions and the food chains of this Carnian marine ecosystem. We highlight the importance of multi-pronged analyses-taxonomy, geochemistry and palaeoecology-of such conservation "Lagerstätten" to extract the entirety of hidden information in these special deposits. The presence of fragile nektonic and benthic taxa points to unique palaeoenvironmental conditions in the Carnian dysoxic bottom water of the Reifling Basin. Triassic invertebrates (e.g., ammonites, phragmoteuthids, bivalves, gastropods, crustaceans, polychaetes) and vertebrates (actinopterygiids, sarcopterygiids, chondrichtyiids) made up the marine benthic and nektic communities. Our report underlines the diverse palaeobiota including new taxa from the Triassic Polzberg Konservat-Lagerstätte. Our study also confirms the presence of an isolated marine intraplatform basin. During the humid and warm Carnian Pluvial Episode, that basin was affected by enhanced freshwater influx and terrigenous input of siliciclastic sediments from the surrounding emerged platform highs. Our report marks the starting point for future descriptions of numerous taxonomic members of the Polzberg palaeobiota, both vertebrates and invertebrates. This would be an important step forward in improving our knowledge of Upper Triassic marine ecosystems. Our approach also highlights the cooperation between citizen scientists and professional researchers because private collectors have sampled the Polzberg fossil site over decades. Over the next two years, further excavations are planned, and the expected findings will no doubt shed new light on the palaeobiota of the Polzberg Konservat-Lagerstätte deposited during the Carnian Pluvial Episode and help test several hypotheses presented here.

Material and methods
6397 fossil remains stem from the ravine Polzberggraben (Lunz Nappe, Northern Calcereous Alps) near Polzberg (= Schindelberggraben; or given as Polzberg locality in numerous collections), between mount Föllbaumberg (1014 m) to the west and mount Schindelberg (1066 m) to the east. The investigated fossil material is housed in the collections of the NHMW and the GBA. The material was collected over the last 140 years (field campaign GBA 1886 and NHMW 1909), with a focus over the last 10 years by the private collectors Birgitt and Karl Aschauer (both Waidhofen and der Ybbs, Lower Austria). The authors contribute to these extensive collections with their own findings over the last 20 years. The fossil remains recorded herein have been investigated with a variety of analytical tools and electronic instruments. Macro-photographs were done with a Nikon Digital Camera, D 5200 SLR, lens Micro SX SWM MICRO 1:1 Ø52 Nikon AF-S, processed by the free graphic software tool digiCamControl version V.2.1.2.0 at the NHMW. Digital high-quality photomicrographs were taken using a Discovery.V20 Stereo Zeiss microscope. The magnifications were × 10 × 20 and × 40 in incident light mode. Data from the AxioCam MRc5 Zeiss were processed and documented using the AxioVision SE64 Rel. 4.9 imaging system at the NHMW.
Thin sections of rock samples were made in the NHMW laboratories. Samples were embedded in Araldite epoxy resin, sectioned, mounted on the microscope slides and polished with silicon carbide and aluminium oxide powders to a thickness of about 19 μm.
Sulphur (% S), total organic carbon (% TOC) and total carbonate content (% CaCO 3 ) were measured at the Institute for Earth Sciences (Karl-Franzens-University, Graz, Austria). Calcium carbonate content was measured using a carbonate bomb technique. Total carbon (TC) content was measured using a LECO WR-12 analyser, and TOC content was calculated as the difference between TC and CaCO 3 , assuming that all carbonate is pure calcite.