Oxygen isotope compositions of Late Jurassic vertebrate remains from lithographic limestones of western Europe: implications for the ecology of fish, turtles, and crocodilians
Résumé
Oxygen isotopes can be used to reconstruct the palaeoenvironmental conditions in which vertebrates lived, and thereby give an indication of their ecology. This has been done for faunas from the famous Late Jurassic lithographic limestones of Western urope. Oxygen isotope compositions of phosphate from apatite were measured in aquatic or semiaquatic vertebrate remains fish, turtles, and crocodilians) deposited in these protected coastal marine environments. The unknown water composition in the oxygen isotope fractionation equations between phosphate and water was solved for turtles by assuming that modern relationship between environmental water and turtle bone isotope compositions applies to the past. The results show that: (1) coastal marine waters were thermally homogenous at the regional scale of Western Europe; (2) oxygen isotope data discriminate between coastal marine and freshwater to brackish water inhabitants; (3) the plesiochelyid turtles were coastal marine inhabitants—their δ18O values allow the calculation of the oxygen isotope compositions of marine waters, thus refining the estimate of water temperatures by combining δ18O values from fish; and (4) the marine isotopic signature and the cranial anatomy of plesiochelyid turtles lead us to propose that they had a salt-excreting system (i.e., the fundamental physiological feature that controls osmoregulation in the marine environment). These Late Jurassic turtles are proposed to be the first know marine turtles, several tens of million years before the chelonioids or ‘sea turtles.'