Trace Elemental Imaging of Rare Earth Elements Discriminates Tissues at Microscale in Flat Fossils

Abstract

The interpretation of flattened fossils remains a major challenge due to compression of their complex anatomies during fossilization, making critical anatomical features invisible or hardly discernible. Key features are often hidden under greatly preserved decay prone tissues, or an unpreparable sedimentary matrix. A method offering access to such anatomical features is of paramount interest to resolve taxonomic affinities and to study fossils after a least possible invasive preparation. Unfortunately, the widely-used X-ray micro-computed tomography, for visualizing hidden or internal structures of a broad range of fossils, is generally inapplicable to flattened specimens, due to the very high differential absorbance in distinct directions. Here we show that synchrotron X-ray fluorescence spectral raster-scanning coupled to spectral decomposition or a much faster Kullback-Leibler divergence based statistical analysis provides microscale visualization of tissues. We imaged exceptionally well-preserved fossils from the Late Cretaceous without needing any prior delicate preparation. The contrasting elemental distributions greatly improved the discrimination of skeletal elements material from both the sedimentary matrix and fossilized soft tissues. Aside content in alkaline earth elements and phosphorus, a critical parameter for tissue discrimination is the distinct amounts of rare earth elements. Local quantification of rare earths may open new avenues for fossil description but also in paleoenvironmental and taphonomical studies.

Informative Potential of Multiscale Observations in Archaeological Biominerals Down to Nanoscale

“…Nowadays, two and sometimes three-dimensional (2D and 3D) elemental, molecular, structural and isotopic imaging of entire objects has becomes possible owing to recent technological developments off ocused analytical probes used in scanning mode and detectors (e.g. Lebon et al 2011a;Reiche et al 2011aReiche et al , 2003Reiche et al , 2010Reiche and Chalmin 2008;Anne et al 2014;Bergmann et al 2010;Gueriau et al 2014;Gourrier et al 2007bGourrier et al , 2010.…”

Deciphering Exceptional Preservation of Fossils Using Trace Elemental Imaging

“…We deeply investigated on trace elements, particularly strontium, yttrium and rare earth elements (REEs), known to be present in significant quantities in fossil bones, teeth and sedimentary apatites [4], and used in paleontological, paleoenvironmental and taphonomic research as their distribution and fractionation depend on fossilization and diagenesis processes [4][5]. The contrasting elemental distributions greatly improved the discrimination of hard tissues (bones, carapaces or cuticles) from both the sedimentary matrix and the fossilized soft tissues on the basis of variations in elemental relative concentrations (figure 1) [6].Although the compression of their anatomical features during fossilization makes harder the interpretation of well-preserved flattened fossils from sedimentary matrix, it is of paramount interest for taxonomic, phylogenetic, taphonomic and environmental studies of exceptional preservation deposits that are unique witnesses of the ancient traces of life on Earth. Discriminating between skeletal and soft tissues therefore appears very promising as it allows visualizing anatomical features that are hidden under well-preserved decay prone tissues or an unpreparable sedimentary matrix (figure 1, see particularly the second antennae and the legs in the fossil shrimp, and the entire skull together with vertebrae in the fossil fish) [6].…”

“…The contrasting elemental distributions greatly improved the discrimination of hard tissues (bones, carapaces or cuticles) from both the sedimentary matrix and the fossilized soft tissues on the basis of variations in elemental relative concentrations (figure 1) [6].…”

“…We deeply investigated on trace elements, particularly strontium, yttrium and rare earth elements (REEs), known to be present in significant quantities in fossil bones, teeth and sedimentary apatites [4], and used in paleontological, paleoenvironmental and taphonomic research as their distribution and fractionation depend on fossilization and diagenesis processes [4][5]. The contrasting elemental distributions greatly improved the discrimination of hard tissues (bones, carapaces or cuticles) from both the sedimentary matrix and the fossilized soft tissues on the basis of variations in elemental relative concentrations (figure 1) [6].…”

“…Discriminating between skeletal and soft tissues therefore appears very promising as it allows visualizing anatomical features that are hidden under well-preserved decay prone tissues or an unpreparable sedimentary matrix (figure 1, see particularly the second antennae and the legs in the fossil shrimp, and the entire skull together with vertebrae in the fossil fish) [6].…”