RARE EARTH ELELMENTS (REE) IN PALEOZOIC VERTEBRATE MICROREMAINS : PALEOECOLOGY AND PALEOENVIRONMENTAL SIGNATURES

Rare earth element (REE) compositions from fossil vertebrate bioapatite are known to be diagnostic of fossil provenance and reworking, and have been used as proxies for paleoseawater chemistry and redox conditions helping to accomplish paleoenvironmental and paleogeographic interpretations.

We have studied in-situ rare earth element compositions by Laser Ablation ICP-MS, in a range of Paleozoic vertebrate micromeric dermoskeletal elements (acanthodians, chondrichthyans and thelodonts) from the Lower Silurian of Central Asia, Upper Silurian of Baltic Basin as well as the Lower Devonian of Spitsbergen. Using absolute and shale-normalized REE concentrations, we have demonstrated several approaches to describe different depositional and diagenetic environments, to discriminate stratigraphical or long-distance reworking in early vertebrate microfossil assemblages.

The results suggest that the REE patterns in fossil bioapatite do not show any recognizable specific taxonomic behavior, but have rather well expressed differences of REE compositions in relation to the internal histology and biomineral structure of the dental scales, the taphonomic conditions, and early diagenetic history. The potential of Europium (Eu) and Cerium (Ce) anomalies to reveal taphonomic and palaeoenvironmental regimes have been assessed through the range of taxa with discrete histologies and microstructure. The shale normalized compilation of Lanthanum (La) and Ytterbium (Yb), and La and Samarium (Sm) ratios proved to be indicative of the REE adsobtion and/or substitution. The REE concentrations appear to have visible differences between separate dental tissues, particularly between enameloid and dentine of thelodonts, emphasising the importance of the in-situ measurements in geochemical studies of fossil vertebrate microremains.