Stable isotope composition and rare earth element content of vertebrate remains from the Late Cretaceous of northern Spain (Laٌo): did the environmental record survive?

Oxygen and carbon isotope measurements have been performed on phosphatic remains from faunal associations (dinosaurs, crocodiles, turtles and fish) of the Late Cretaceous continental and marine sediments from northern Spain (Basque Country). The environmental meaning of the oxygen isotope record in fossil reptiles is discussed on the basis of known paleoecology, modern fauna analogs, and apatite chemistry (rare earth elements (REE), CO32− and PO43− contents). Fossil remains in sandstones and argillites from two localities (Urria and Cuezva) have low (down to 16‰) and scattered δ18O(PO4,CO3) and δ13C values with REE patterns characterized by strong middle REE enrichments. These geochemical compositions of the biogenic apatites have most likely been acquired during a multi-stage late diagenesis involving complete recrystallization of the apatite in the presence of aqueous fluids. REE contents of fossils preserved in the clayey sands from the site of Laño suggest that samples have only been affected by early diagenesis. Despite the diversity of analyzed taxa and the close association of marine (sharks and rays) and freshwater species (crocodiles and turtles) evolving in a littoral environment, the narrow range of phosphate δ18O values is interpreted as reflecting the temperature and isotopic composition of groundwaters that prevailed in the burial environment. The potential information relative to the physiology and thermal regime of the vertebrates has been erased during the process of early diagenesis. If all oxygen in the phosphate has exchanged with groundwaters, the oxygen isotope composition of vertebrate remains suggests that a warm climate (T=20–25°C) prevailed in northern Spain during the Late Cretaceous.