Deciphering Exceptional Preservation of Fossils Through Trace Elemental Imaging
Pierre Gueriau 1, 2 * and Loïc Bertrand 1, 2 1 Synchrotron SOLEIL , BP 48 Saint-Aubin , 91192 Gif-sur-Yvette Cedex , France 2 IPANEMA , USR 3461 – CNRS , MCC – BP 48 Saint-Aubin , 91192 Gif-sur-Yvette Cedex , France
*
pierre.gueriau@
synchrotron-soleil.fr
Introduction T e fossil record consists essentially of biomineralized remains of shelly invertebrates and skeletons of vertebrates, but rare “soſt -bodied” fossils have also been preserved over time. Soſt -bodied organisms normally degrade too fast to become fossilized, but such fossils have been conserved in an exceptional preservation state over millions of years in specifi c deposits called Konservat-Lagerstätten . Exceptional preservation may include the preservation of complete organisms, hard-part mineralogy, detailed morphology of soſt tissues at cellular or subcellular levels, and organic molecules or fragments. Decay-prone tissues can be preserved in the fossil record either in an altered organic form or as replicated authigenic minerals, commonly calcium phosphates through phosphatization [ 1 ]. For such soſt tissue preservation to occur, processes normally involved during degradation had to be dramatically slowed or arrested soon aſt er death [ 2 – 3 ]. Despite much eff ort in the past decades, processes governing exceptional preservation processes remain poorly understood.
Detailed knowledge of the chemical composition of fossils, including at trace levels, is likely to provide crucial information on the chemical and structural changes that may aff ect a specimen aſt er burial. T is has led to interdisciplinary eff orts involving paleontologists, geochemists, and physicists who use and adapt cutting-edge analytical and microscopic techniques on fossils, for example: new sequencing approaches that triggered the hunt for ancient DNA in fossil humans or in a woolly mammoth found in the Scandinavian permafrost; computed X-ray tomography that is now commonly used to study the internal structure of fossils; and extraction of information about them from amber. In addition, recent synchrotron-based microscopic techniques have produced several unexpected discoveries regarding the preservation of ancient specimens (see [ 4 , 5 ] for detailed reviews).
Among the new analytical tools available to paleontologists, synchro- tron-based X-ray fl uorescence (XRF) has proved a particularly valuable tool. While a century has passed since Henry Moseley invented the technique of X-ray spectrometry for element identifi cation, the fi rst setups allowing XRF mapping were developed in the 1960s [ 6 ]. Since then, XRF mapping has vastly benefi ted
20
Figure 1 : XRF principle. (a–b) Schematic representation of the photo-excitation process (a) and subsequent allowed XRF transitions (b), with the main emission lines in bold (after [ 4 ]). (c) Schematic experimental setup at the DIFFABS beamline, SOLEIL synchrotron, France.
Figure 2 : Optical photograph (a) and reconstruction (b) of the well-preserved clupeomorph fi sh Diplomystus sp. (Poi-SGM 10) from the Djebel Oum Tkout Lagerstätte (Upper Cretaceous, Kem Kem Beds, Morocco) exhibiting a high level of preservation, including fi nely mineralized muscles as shown by SEM imaging (c). The red box in (a) indicates where the SEM image in (c) was taken. Image courtesy of and modifi ed with permission from DB Dutheil [ 16 ].
doi: 10.1017/S1551929515000024
www.microscopy-today.com • 2015 May
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