CONODONT DIVERSITY ACROSS THE ONSHORE-OFFSHORE GRADIENT: COZY IN BRINE

Hypersaline marginal-marine settings are commonly assumed to be characterized by low-diversity, high-dominance faunas. They are consequently unattractive for most paleontologists and undersampled. However, in early Paleozoic marine ecosystems, in which conodonts were the most diverse nektonic organisms, beta diversity increased towards the lagoonal end of the onshore-offshore gradient. We document two case studies from hypersaline settings in Estonia and Ukraine, in which specialist conodont taxa and thelodonts are found together with syndepositional evaporite minerals. To examine the role of the environment in shaping conodont assemblages, we have compiled a database of middle Silurian conodont occurrences tied to their position along this gradient. This compilation reveals that a number of taxa were specialized to live in marginal-marine settings, with sedimentological evidence of evaporite formation and repeated emergence. Conodont assemblages from these settings are disparate from their open-marine counterparts, allowing us to exclude post-mortem transport. Evidence of in situ conodont occurrence along with evaporitic minerals indicates their efficient osmoregulation. Our findings reveal an unappreciated diversity in Silurian marginal-marine environments, largely concealed by sampling bias against this type of setting.