Ancient Swiss Bone Bed Reveals a Drowned Coastal World from the Late Triassic

On the banks of the small river Ergolz in northern Switzerland, a layer of rock no thicker than a few centimeters has puzzled geologists for more than 170 years. This lens-shaped bone bed at Niederschönthal, first encountered in 1854 when the pioneering Swiss geologist Amanz Gressly unearthed the bones of a gigantic reptile, has now been subjected to a full forensic investigation spanning sedimentology, palaeontology, trace fossils, pollen analysis and geochemistry. The verdict, published in the Swiss Journal of Geosciences, resolves a long-running dispute: the bone bed is neither purely marine nor purely continental, but a record of a muddy coastal plain swept by fluvial-estuarine channels as the sea gradually pushed inland during the middle Rhaetian, roughly 205 million years ago.

The site itself is steeped in history. Gressly, the scientist who coined the very term facies, discovered bones of a large reptile that Rütimeyer soon named Gresslyosaurus. Karl Strübin opened six trenches at the locality in 1901, and in 1972 local geologist Hans Peter Mohler rescued further bones from a construction pit, most likely belonging to an early sauropodomorph dinosaur. Despite this rich legacy of discovery, interpretations of the bone bed’s origin remained contradictory. Some researchers argued for a storm-reworked shallow-marine setting, while others saw a lagoonal or terrestrial deposit. The new study set out to settle the question once and for all.

The bone bed itself sits within the Belchen Member, the uppermost part of the Klettgau Formation, resting on a sharp erosive surface. It is a discontinuous conglomeratic layer ranging from zero to five centimeters in thickness and extending laterally for just 1.8 meters. Its composition is extraordinary: coprolites, or fossilized droppings, dominate, together with vertebrate bones and teeth, reworked limestone clasts and large laminated sandstone pebbles. Below it lies a channel deposit of reworked caliche containing temnospondyl remains, and above it a cross-bedded sandy layer with uni-directional wave ripples, marine bivalves and resting traces of starfish. The whole succession documents a transgression, the slow drowning of a coastal landscape by an encroaching sea.

The vertebrate assemblage is remarkably diverse. Quantitative analysis of a 7.7 cubic decimeter sample shows that bony fish make up about 49 percent of the remains and sharks and their relatives about 43 percent. The predatory ray-finned fish Sargodon tomicus is the most frequent single taxon, followed by scales of Gyrolepis albertii and the sharp teeth of Severnichthys acuminatus, a hunter that grew up to a meter long. The small shark Lissodus minimus, a resident of lacustrine to brackish waters, dominates among the chondrichthyans, with the euryhaline hybodontids Hybodus cloacinus and Rhomphaiodon minor appearing more rarely. Tetrapods account for about 2.5 percent, including teeth of the giant temnospondyl amphibian Mastodonsaurus, an apex predator reaching five to six meters, and abundant remains of semi-aquatic phytosaurs.

Coprolites are the true stars of the deposit. They range from half a centimeter to ten centimeters across, and many show spiral structures that betray their producers. Some contain fish scales and small bones, while others include charcoal, linking their makers directly to the fire-swept land nearby. Jelly roll-like spiral specimens are attributed to sharks, and ribbed, arcuate forms match the ichnotaxon Alococopros, thought to derive from proterosuchid or prolacertid reptiles. The many different states of abrasion among the coprolites and bones, from pristine to heavily rounded, tell a story of repeated reworking: fossils that formed in a soil during a dry phase were eroded and concentrated during short-lived high-energy events as conditions grew wetter.

The pebbles embedded in the bone bed provided the decisive geochemical clues. Two types occur, arenitic and micritic, and both carry dark rims just below their surfaces composed of pyrite, an iron sulfide. Pyrite forms during early diagenesis in sulfate-rich waters, and because river water is normally poor in sulfate, the rims point to at least polyhaline, and possibly fully marine, salinity at the time of final deposition. Several micritic pebbles are also riddled with U-shaped borings matching the ichnogenus Rogerella, produced by acrothoracican barnacles known from marine to brackish settings. Stable isotope data reinforce the picture: most oxygen isotope values sit slightly below Late Triassic seawater values, indicating modest freshwater influence, while one pebble with values around minus seven per mil appears to be a caliche nodule eroded from land, perfectly consistent with the terrestrial plant and animal remains in the same layer.

Palynology pinned down the age and the landscape. Samples from the Strübin excavation yielded 32 and 36 sporomorph taxa respectively, including spores of bryophytes, lycophytes, sphenophytes and fifteen fern taxa, alongside a rich pollen flora of seed ferns, cycads, bennettitales and conifers. Crucially, both samples contain the diagnostic Rhaetian dinoflagellate cysts Rhaetogonyaulax rhaetica and Dapcodinium priscum, and marker taxa such as Rhaetipollis germanicus, Limbosporites lundbladii and Cingulizonates rhaeticus constrain the deposit to the middle Rhaetian, corresponding to the Limbosporites lundbladii Subzone of European biostratigraphic schemes. The abundance of hygrophilous spores reveals a lush, humid lowland vegetation, at odds with conventional reconstructions of a hot and arid Rhaetian climate and suggesting either a humid pulse linked to sea-level rise or a broader humid phase in the Central European Basin.

Among the most striking discoveries are charcoal fragments, the first Rhaetian charcoal ever recorded from Switzerland and the Alps. Microscopic anatomy shows pycnoxylic gymnosperm wood with uniseriate bordered pits, direct evidence of palaeo-wildfires. The strongly rounded edges of the larger fragments indicate long transport, probably as bed load in rivers draining forested hinterlands, before the charcoal was washed into the coastal plain. Wildfires are most frequent in climates with pronounced dry seasons, hinting at rainfall seasonality within an otherwise humid setting, and the permineralized conifer stem in the same beds corroborates the palynological evidence for rich conifer forests in the region.

Taken together, the evidence paints a vivid picture of a drowned world. Late Triassic Switzerland lay at the southern margin of the Central European Epicontinental Basin, receiving sediment from the Vindelician-Bohemian High and the Central Massif. The Niederschönthal succession begins with alluvial-plain clays, passes through a fluvial channel deposit and overbank sediments bearing the sauropodomorph Gresslyosaurus, then reaches the bone bed itself, an allochthonous mixture of freshwater, brackish and rare marine elements concentrated by transgressive pulses in channels cutting across a muddy coastal plain. Above it, marine bivalves and starfish traces announce full marine conditions, a trend that culminates in the ammonite-rich Lower Jurassic limestones higher in the cliff. The study demonstrates that such bone beds are not simple archives of one environment but complex palimpsests, where terrestrial, freshwater and marine influences interleave, offering a uniquely detailed window into the ecosystems that flourished just before the dawn of the dinosaurs’ dominance.

Subject of Research: Sedimentology and palaeoecology of a Rhaetian bone bed in northern Switzerland

Article Title: Sedimentology and palaeoecology of a Rhaetian bone bed in Northern Switzerland formed during transgression of a muddy coastal plain setting with fluvial-estuarine channels

Article References: Meyer, C. A., Wetzel, A., Roghi, G., & Kustatscher, E. (2026). Sedimentology and palaeoecology of a Rhaetian bone bed in Northern Switzerland formed during transgression of a muddy coastal plain setting with fluvial-estuarine channels. Swiss Journal of Geosciences, 119, 85-110. https://doi.org/10.3897/sjg.119.187486

Image Credits: AI Generated

DOI: 10.3897/sjg.119.187486

Keywords: bone bed, Rhaetian, Late Triassic, northern Switzerland, coastal plain, transgression, phytosaurs, coprolites, palynology, pyrite, charcoal, palaeoecology