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Fossil Weathering and Preparation Mimic Soft Tissues


and white pixels corresponding to spots with heavy elements (that is, high atomic number Z). Te EDX allowed, at low volt- ages, optimized acquisition of spectra revealing the distribu- tion of light elements (that is, carbon). At high voltages, heavy elements, such as iron, were detected. In addition, X-ray fluo- rescence (XRF) elemental maps of these two specimens were obtained using a Bruker M4 Tornado micro-XRF instrument operating under vacuum at 50 kV and 600 µA. As both specimens were not completely flattened it was


difficult to assess if negative elemental results were due to the actual absence of elements in the analyzed spot, or if the nega- tive results were due to a topographic effect that inhibited the beam from reaching the spot. For this reason general elemen- tal maps of rhodium Rh, the source of the XRF machine, were made. When Rh signal was absent it indicated that topographi- cal effects prevented the beam from reaching the spot. Pres- ence of Rh implied that the beam did reach the surface and the absence of an element was due to actual absence of this element from the chemical composition of the analyzed area.


Results Te eocrinoid specimen (Figure 1A) is


entirely preserved in iron (Fe), which is shown by the green fluorescence in Figure 1C and the SEM EDX map in Figure 1E. Tis contrasts with the matrix, which is rich in silicon (Si) as shown by the red fluorescence in Figure 1B and the SEM EDX map in Figure 1E. Some Fe- rich small and repetitive structures are present within the brachioles (skeletonized arm-like feeding appendages) ( Figure 1D). Most of the Fe-rich areas are preserved in small iron crys- tals that do not have any specific shape (that is, star-like minerals that are not euhedral nor framboidal; Figures 2A–D). Te somasteroid specimen ( Figure 3A) is


preserved as an imprint in the rock, and it shows the same elemental signature as the matrix, which is


rich in Si (Figure 3B). However,


some areas of the fossil appear to be depleted in Si (Figure 3B). Rh distribution shows that some of these anomalies are due to the actual absence of Si (that is, when Rh is present Rh+; Figure 3C), while the absence of Si in other regions is simply due to a hidden zone that was not analyzed by the beam (that is, Rh-; Figure 3C). Te Rh+ zones are rich in carbon that is mostly detected when analyzing the specimen at 5 kV (Figure 3D). Te carbon shows a repeti- tive pattern of small identical structures along the arms of the somasteroid (Figures 4A–B). However, carbon is also present elsewhere in the specimen (Figures 4C–F).


Interpretation and Discussion Te structures found in both the eocri-


noid (Figure 1D) and somasteroid (Figures 4B–C) specimens resemble tube feet of water vascular system, as, for example, those


26 the Figure 1: Analyzed specimen of eocrinoid echinoderm, late Tremadocian, Fezouata Shale, Bou


Izargane, Zagora area, Morocco; ML20-269159. (A) Photograph of the specimen; (B) X-ray fluores- cence image of inset shown in Figure 1A. Red indicates a high concentration of silicon distribution; (C) X-ray fluorescence of iron distribution (green) from the inset region of Figure 1A; (D) tube feet-like structures of the putative water vascular system showing iron distribution; (E) SEM-EDX elemental distribution in the matrix and the fossil.


www.microscopy-today.com • 2020 January


evidenced in stylophoran echinoderms from the Fezouata Shale [12]. However, both the elemental and mineralogical sig- natures are different in these three occurrences. Soſt parts in exceptionally preserved fossils from the Fez-


ouata Shale are preserved in both euhedral and framboidal pyrite [23]. Tese mineralogical morphologies are indicative of an early authigenic pyritization that occurred under anoxic conditions at time of burial [24], replicating soſt tissues that are the most prone to decay [25]. Aſter that, pyrite was transformed due to recent weathering to iron oxides [12,23]. Tis situation was observed in the soſt parts reported in the stylophoran material from Bou Izargane in Morocco [12]. In contrast, the near absence of both euhedral and fram-


boid minerals in the eocrinoid specimen indicates that iron- rich minerals that are found in this fossil are not the result of the combined activity of authigenic mineralization and recent weathering. In this fossil, the iron oxides are simply related to the activity of recent weathering and, thus, they do not replicate any original anatomy. Te distribution of these Fe-rich minerals


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