OXYGEN AND SULFUR STABLE ISOTOPE ANALYSIS OF LATE DEVONIAN VERTEBRATES TRACE THE SALINITY OF THEIR AQUATIC ENVIRONMENTS

The Middle to Late Devonian Period (390 – 360 Ma) saw the rise of terrestrial ecosystems and the diversification of vertebrates including the appearance of early tetrapods. Coastal paleoenvironments with variable salinities including estuaries and tidally-influenced rivers have been identified as habitats that were crucial to vertebrate evolution during this time period. Upper Devonian (365 - 360 Ma) strata of the Appalachian Basin in Pennsylvania, USA, preserve a diversity of fishes and tetrapods in coastal marine to fluvial depositional environments making this region ideal for investigating the ecology and evolution of Late Devonian vertebrates. A key unresolved issue has been reconstructing the specific aquatic habitats that hosted various vertebrates during this time period. Specifically, the salinity of environments spanning shallow marine to freshwater is difficult to discern from sedimentological and paleontological analyses alone. Previous work has shown that stable isotope compositions of oxygen and sulfur (δ¹⁸O, δ³⁴S) in fossilized bioapatite record the environmental conditions that prevailed during an animal’s life. Bioapatite of “placoderms” (Bothriolepis sp. and pachyosteomorphs) and lobe-finned fishes (Holoptychius sp. and tristichopterids) were sampled from four different outcrops that span shallow marine to fluvial types of depositional environments. Here, we show that stable oxygen and sulfur isotope compositions in fossil bioapatite from fluvial and transitional strata of the Catskill Formation and marine strata of the Lock Haven Formation closely align with the interpreted depositional settings for each locality from where fossil samples originated. Our results imply that combined oxygen and sulfur isotope analysis allows finely tracing the salinity of vertebrate habitats deep in the past.