RELATING RARE EARTH ELEMENT UPTAKE TO ORIGINAL SOFT TISSUE PRESERVATION IN CRETACEOUS AND MIOCENE FOSSIL BONES

Original soft tissues and biomolecules, such as bone cells, blood vessels, and proteins, have recently been recovered from fossils of dinosaurs and other extinct vertebrates. However, it is unclear how the geology and geochemistry of burial environments and soft tissue preservation are related. To explore this topic, laser ablation inductively coupled plasma mass spectrometry was used to characterize the trace element composition of nine fossil bones, all of which were previously shown to preserve soft tissues and/or protein sequences. These included a Tyrannosaurus femur from a Cretaceous fluvial sandstone, a Dreadnoughtus humerus from a Cretaceous floodplain mudstone, and crocodile and turtle bones from Cretaceous and Miocene shallow marine deposits. The average ƩREE of the samples was 956 ppm, with the Tyrannosaurus femur (2,720 ppm) and Miocene turtle shell (39 ppm) exhibiting the maximum and minimum ƩREE, respectively. Samples from fluvial deposits exhibited higher REE concentrations than the marine samples. All samples exhibited neutral to positive Ce/Ce** anomalies indicative of uptake under oxidizing conditions. Most of the Cretaceous fossils exhibited La and Yb depth profiles indicative of uptake by simple diffusion. A crocodile tibia yielded the lowest ƩREE of any Cretaceous sample, highest Ce/Ce** anomaly, and irregular La and Yb depth profiles. The Tyrannosaurus femur showed a shift to HREE enrichment after the first 5 mm of bone due to apparent fractionation during uptake. The Miocene sample exhibited more U (948 ppm) than all the other samples (avg. 58 ppm). Despite differences in REE concentrations of the marine fossils compared to those of the fossils from fluvial deposits, all of these bones yield original soft tissues upon demineralization. When combined with other recent findings, variation in REE composition is indicative of the possibility of soft tissues being recovered from chemically diverse environments. In testing future samples, examining diagenesis of REE in individual fossil bones may potentially identify trends in REE uptake indicative of soft tissue preservation. Identifying conducive chemical regimes will also help resolve the molecular mechanisms by which biomolecules can be preserved through geologic time.