Monospecific mass occurrence of the dimerelloid brachiopod Halorella amphitoma in a shallow marine environment of the Late Triassic Dachstein platform (Bakony Mountains, Hungary)

Dimerelloid brachiopods with patchy but cosmopolitan geographical distribution, including the Late Triassic (Norian) Halorella amphitoma , offer a unique opportunity to identify and reconstruct specific palaeo-microenvironments in the geological past. The group and this species are commonly associated with hydrocarbon seepage in Palaeozoic and Mesozoic records. In addition, mass occurrences of H. amphitoma are also known from neptunian dykes and palaeokarst cavities on the vast Late Triassic Dachstein carbonate platform. It has been suggested that the shells were either washed into these structures or the animals were living directly there in a dark environment, but normal marine conditions were implied for their habitat. However, here we report a new record of the mass occurrence of this species from the Bakony Mts. (central Transdanubian Range Unit) in Hungary that cannot be reconciled with any of the previously suggested palaeoenvironments. A combination of palaeontological, sedimentological, and geochemical (δ 13 C, δ 18 O and element/Ca ratios) analyses of the shells, micrite and shell-infilling calcite cement was carried out in order to determine the habitat. Our results suggest that karstic cavities, formed during lowstand on the supratidal platform, were filled by seawater through ingression when the sea-level started to rise and were subsequently colonised by H. amphitoma . Stable isotope and element/Ca ratio data suggest a good preservation of the shells. Furthermore, supporting the sedimentological observations, the data imply a semi-closed environment with variable seawater salinity, carbonate chemistry and oxygen availability. This newly identified habitat provides further support for the exceptional adaptational flexibility of Halorella .