First isotopic and multidisciplinary evidence for nonmarine coelacanths and pycnodontiform fishes: palaeoenvironmental implications

Abstract

The Recent coelacanth Latimeria chalumnae, one of the best known `living fossils', dwells in deep marine water. Fossil coelacanths have been globally related to marine environments, specially after the Triassic, an association that has led to a general belief that they have always been marine. Previous reports of fossil coelacanths in continental deposits have been largely neglected. Prior to this report, uncontested Cretaceous freshwater coelacanths were unknown. In turn, the pycnodontiform fishes have always been considered exclusively marine, apart from a few controversial exceptions. Here we present the first multidisciplinary evidence for nonmarine coelacanths and pycnodonts. Our conclusions are based upon palaeogeographic, sedimentologic, taphonomic, and palaeoecologic criteria, strongly supported by strontium (87Sr–86Sr) and stable carbon and oxygen isotopic studies. The coelacanth, provisionally attributed to the genus `Holophagus', and the pycnodontiforms Eomesodon sp. and Macromesodon aff. bernissartensis were unearthed at the Early Cretaceous locality of Las Hoyas (Cuenca, Spain), where they grew in and inhabited a freshwater environment without marine influence. Fossil coelacanths and pycnodonts cannot, therefore, be used as unambiguous indicators of a marine environment. Caution is needed when using a single or a few taxa as palaeoenvironmental indicators, especially fish; in this sense, communities are much more reliable. Arguments based on actualism or taxonomic uniformitarianism, morphologic convergence and functional morphology are truly significant only within the framework of a sound multidisciplinary approach to the study of the palaeoenvironment.