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

Bone beds represent key archives for reconstructing coastal ecosystems, yet their depositional environments are often debated. Here we present an integrated sedimentological, palaeontological, ichnological, palynological and geochemical study of the Late Triassic Niederschönthal bone bed (Belchen Member, Klettgau Formation) in northern Switzerland. The bone bed occurs as a thin, laterally discontinuous conglomeratic layer resting on an erosive surface. It is dominated by coprolites, vertebrate remains, reworked lithoclasts and micritic pebbles. The vertebrate assemblage is diverse and dominated by osteichthyans and chondrichthyans, with subordinate phytosaurs and temnospondyl amphibians, alongside abundant charcoal and plant debris. Sedimentological features, including bored micritic pebbles with pyrite-rich rims, combined with stable isotope data, indicate deposition under sulfate-rich, at least polyhaline conditions. Palynological assemblages dominated by hygrophilous spores and characteristic Rhaetian dinoflagellates constrain the age to the middle Rhaetian and document a diverse lowland vegetation adapted to a humid environment. The faunal composition, coprolite assemblage and sedimentary structures point to repeated reworking and concentration of vertebrate remains during short-lived high-energy events. We interpret the Niederschönthal bone bed as having formed in a muddy coastal plain to lagoonal environment with fluvial-estuarine to marine channels having been periodically affected by small-scale transgressive pulses. This study reconciles earlier conflicting interpretations by demonstrating that the bone bed records a complex interplay of terrestrial, freshwater and marine influences rather than a fully marine or purely continental setting.