Inferring the environmental conditions present during early tetrapod terrestrialization from skeletal [delta]18O values

Abstract

To test the extent of marine influence in the Joggins Formation, Nova Scotia, Canada and the possible habitat segregation and the net moisture balance at the Red Hill roadcut exposure of the Duncannon Member of the Catskill Formation in Pennsylvania, phosphatic fossils from both locations were analyzed to determine local and/ or regional changes in the oxygen isotopic composition of local surface waters. Marine to brackish taxa are present within the Joggins Formation, but disagreement exists over the degree of marine influence during deposition. In this study, the [delta]18Owater values calculated for sections of the Joggins Formation compared with typical values for Carboniferous marine water don't reflect fully marine conditions but are consistent with a brackish setting in which seawater mixed with freshwater in the Cumberland Basin during the deposition of the Joggins Formation. Evidence of net evaporative conditions present in the sedimentological and paleontological record at the Red Hill floodplain suggest [delta]18O of water would be high and variable. Variability in [delta]18O across the floodplain show no general trends toward net evaporative conditions. However, variability within individuals does reflect variation in aquatic conditions, not artifacts related to sample types. Observations presented in this study provide independent constrains on two windows into the transition from fish to fully terrestrial vertebrates.