Palaeoenvironmental and palaeoecological reconstructions based on oxygen, carbon and sulfur isotopes of Early Permian shark spines from the French Massif central

Abstract

The palaeoecology of the Xenacanthiformes (Chondrichthyes) is still a matter of discussion. Historically considered as freshwater organisms, hypotheses of euryhaline ecology including migratory behaviour were recently proposed based on the histology of their dorsal spines. However, recent studies still argued for a full freshwater ecology based on the geochemical compositions of their teeth. Their ecology is particularly interesting to investigate for those coming from the environmentally ambiguous Early Permian intra-mountainous localities of Buxières-les-Mines (Bourbon-l'Archambault Basin, Allier) and the Muse oil-shale bed (OSB) (Autun Basin, Saône-et-Loire) from the French Massif central. These two localities were interpreted as freshwater settings but doubts on marine influences exist. To assess their palaeoenvironment and the palaeoecology settings based on xenacanthiforms occurrences, we have analysed the stable isotope compositions (δ18O, δ13C and δ34S) of the bioapatite of their vertebrate faunas. From these stable isotope compositions, we interpret that Buxières-les-Mines was a large and deep freshwater lake, equivalent to the modern Indonesian Lake Matano, whereas the Muse OSB is interpreted as a shallow tropical lake with drying events, equivalent to the Lake Punta Laguna in Amazonia or the Lake Tanganyika in Africa. The oxygen isotope compositions of the xenacanth dorsal spines of Buxières-les-Mines are relatively constant from the apex to the base. Such uniform isotopic compositions recorded during the life of xenacanths support either their sedentary lifestyle or stenohalinity. The apparent ability of small xenacanthiforms to endure drying up, as shown by the variations in isotope composition of the Muse OSB specimens, could explain their resilience during the Early Permian whereas Carboniferous xenacanthiforms occurring in large lake and river systems almost disappeared.