New insights into the paleoecology of brachiopod-arthropod-microbial assemblages from Upper Triassic cryptic submarine cavities

A unique record of an Upper Triassic brachiopod (Halorella)-arthropod-microbial (Frutexites) assemblage is documented from iron-rich carbonates filling the Neptunian dikes and a sill developed in Carnian-lower Rhaetian Dachstein Limestone host rock from Leurdeasa Hill in the Apuseni Mountains, Romania. The host rock at Leurdeasa Hill through which the dikes and sills cross-cut is dated as Carnian by the conodont assemblage. The paleoecology of this brachiopod-arthropod-microbial assemblage along with the paleoenvironmental context and genesis of iron-rich carbonates filling the Upper Triassic (upper Norian-lower Rhaetian) Neptunian dikes and sills is examined through integrated analyses including paleontology, taphonomy, microfacies, mineralogy and stable isotopes of carbon and oxygen, as well as patterns of rare-earth elements. Frutexites microstructures and their possible microbial origin are documented for the first time in Upper Triassic (upper Norian-lower Rhaetian) Neptunian dikes and sills. Given the broad paleogeographic distribution of the brachiopod Halorella during the Late Triassic and the scarcity of marine paleoenvironments from where this genus has been reported, we conclude that the proliferation of significant populations of Halorella alongside with arthropods and microbial consortia were restricted to submarine cryptic paleoenvironments characterized by peculiar conditions, namely, warm waters rich in nutrients in dark habitats, with variable oxic/suboxic conditions, limited detrital sediment input and hydrogenetic fluxes.