Devonian sea surface temperature and paleoecology changes constrained by in situ oxygen isotopes of fish fossils

Significant changes occurred in both terrestrial and marine environments in the Devonian, during which complex terrestrial life radiated rapidly from the ocean. However, the reconstruction of the seawater temperatures for this period is still noticeably insufficient, largely due to a lack of appropriate geological archives especially in freshwater and near-shore marine deposits. In this study, we carried out a detailed characterization of microstructure, elements and isotopes of Early and Late Devonian fish fossils from Yunnan and Ningxia, China, respectively. The results show that fish enamels were relatively more resistant to diagenetic alteration than fossil dentine and bones, and can serve as valuable archives for Paleozoic temperature and ecology reconstruction. In situ oxygen isotopes of the enamel apatite (delta 18O(p)) of Early Devonian fossil fishes (sarcopterygians and antiarchs) vary from 16%o to 18%o, in good agreement with those of contemporaneous conodonts. The corresponding seawater temperatures are calculated to be >30degree celsius, therefore lending support to a greenhouse climate of the Early Devonian. In contrast, Late Devonian fish enamels tend to have lower delta 18Op than those of the contemporaneous conodonts. The difference in delta 18O(p) is attributable to the change in the living environment during the transition of the fish-to-tetrapod.