Carbon isotope ratios in the apatite-protein composites of conodont elements—palaeobiological proxy

Abstract

Conodonts were the Palaeozoic and Triassic extinct group of the marine animals possessing discussed affinities. The only mineralized parts of conodonts are tooth-like elements composing feeding apparatus. The conodont elements are consisted of complex fluorapatite-protein nanocomposites. The mineral component of conodont elements is represented by apatite-(CaF) with minor content of CO3 ions, Na, and Sr. Organic matter is composed of protein(s), consisting less than 4% of a conodont element. The study material comprises conodont elements of the middle Frasnian genera Youngquistognathus and Mehlina, and latest Famennian-Tournaisian species: Polygnathus parapetus Druce, Hindeodus crassidentatus (Branson et Mehl), and Ligonodina sp. Conodont elements were used for analysis of composition of organic matter and apatite matrix, and organic carbon isotope values. Studied conodont elements are characterized by quite high Sr/Ca values in the albid tissue ranging from 0.002 up to 0.016 that is specific for marine animals of low trophic level. Low δ13Corg values in conodont elements ranging in wide interval from −30.4 up to −22.5‰ also suggest low trophic level of conodonts. Thus, data obtained advocate that conodonts were marine consumers of low trophic level feeding on phytoplankton or/and zooplankton. This conclusion promises possibilities to revise functional morphology of conodont elements and apparatuses, and elucidates significant biochemical differences between conodonts and marine vertebrates.