The Norian fish deposits of Wiestal („Seefeld Member“, Northern Calcareous Alps, Salzburg, Austria) – taxonomy and palaeoenvironmental implications

Abstract

Abstract The Alaunian Seefeld Member of the Upper Triassic, a dark grey laminated and bituminous dolomitic limestone succession outcropping near the Wiestal-reservoir lake northeast of Hallein (Salzburg, Austria) is known for its extremely rich ‘ganoid’ fish fauna since more than a century. A privately initiated excavation that took place from 2012 to 2014 yielded far more than a thousand well-preserved fish fossils recovered largely from five mm-thin fossil horizons. The actinopterygian assemblage is dominated by several growth stages of the highly variable ginglymodian Paralepidotus ornatus, allowing for a documentation of ontogenetic transformations in cranial and postcranial morphology, dentition and squamation patterns, associated with habitat and dietary shifts. Small-sized swarm-fishes such as the macrosemiid Legnonotus and the teleost Pholidophorus are rather common members of the assemblage, while the occurrence of the ginglymodian Semiolepis, the dapediid Dandya, the dwarfish pycnodont Eomesodon and the large predatory ‘palaeopterygian’ Saurichthys is restricted to rare individuals. A single scale of a large-sized coelacanth, a well-preserved, small lobster-like decapod, plant remains and coaly gagate derived from disarticulated driftwood belong to rare associated finds. Both the perfect preservation of all fossils and the bituminous laminated dolomitic limestones barren of microfossils argue for a deposition under anoxic conditions, most probably due to salinity stratification. The occurrence of complete swarms, partly showing isoorientation of fish carcasses in distinct layers, speaks in favour of recurrent and rapid mortality events triggered by upwelling anoxic bottom water, most likely released by severe tropical storms.