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, 10 (9), e0137162
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Limpet Shells From the Aterian Level 8 of El Harhoura 2 Cave (Témara, Morocco): Preservation State of Crossed-Foliated Layers

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Limpet Shells From the Aterian Level 8 of El Harhoura 2 Cave (Témara, Morocco): Preservation State of Crossed-Foliated Layers

Julius Nouet et al. PLoS One.

Abstract

The exploitation of mollusks by the first anatomically modern humans is a central question for archaeologists. This paper focuses on level 8 (dated around ∼ 100 ka BP) of El Harhoura 2 Cave, located along the coastline in the Rabat-Témara region (Morocco). The large quantity of Patella sp. shells found in this level highlights questions regarding their origin and preservation. This study presents an estimation of the preservation status of these shells. We focus here on the diagenetic evolution of both the microstructural patterns and organic components of crossed-foliated shell layers, in order to assess the viability of further investigations based on shell layer minor elements, isotopic or biochemical compositions. The results show that the shells seem to be well conserved, with microstructural patterns preserved down to sub-micrometric scales, and that some organic components are still present in situ. But faint taphonomic degradations affecting both mineral and organic components are nonetheless evidenced, such as the disappearance of organic envelopes surrounding crossed-foliated lamellae, combined with a partial recrystallization of the lamellae. Our results provide a solid case-study of the early stages of the diagenetic evolution of crossed-foliated shell layers. Moreover, they highlight the fact that extreme caution must be taken before using fossil shells for palaeoenvironmental or geochronological reconstructions. Without thorough investigation, the alteration patterns illustrated here would easily have gone unnoticed. However, these degradations are liable to bias any proxy based on the elemental, isotopic or biochemical composition of the shells. This study also provides significant data concerning human subsistence behavior: the presence of notches and the good preservation state of limpet shells (no dissolution/recrystallization, no bioerosion and no abrasion/fragmentation aspects) would attest that limpets were gathered alive with tools by Middle Palaeolithic (Aterian) populations in North Africa for consumption.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Map of North Morocco and localization of the caves of archaeological interest on the Atlantic coast of Rabat-Témara.
Fig 2
Fig 2. El Harhoura 2 cave.
a) Stratigraphic section of the cave, showing the front test pit and back excavation. Arrow marks level of interest 8. Level 1, Neolithic; level 2, Upper Paleolithic; levels 3—11, Middle Paleolithic. b) Picture of the front drill, with highlighted delimitation of level 8. c) Plot of the surface of the section of level 8 excavated during the reconnaissance drill, with the localizations of Patella sp. specimens. d) East-west profile of the drill at level 8 depth showing the concentration of Patella sp. specimens, complete or fragmented bones, and lithic materials.
Fig 3
Fig 3. Morphologies of specimens of interest.
a-d) Modern Patella vulgata from the Brittany sea shore. Blue bar: section of m+3 and m+2 layers. Arrow shows the mark left by the knife used to collect the specimen. e-h) Unbroken Patella sp. from El Harhoura 2 cave (level 8). my: myostracum. hy: hypostracum. Blue bar: section of m+3 and m+2 layers.i-m) Patella sp. from El Harhoura 2 cave (level 8) with a large broken edge (marked by arrow). n-r) Patella sp. from El Harhoura 2 cave (level 8) showing a small imprint on its edge (marked by arrow).
Fig 4
Fig 4. Microstructural organization of modern Patella vulgata calcite crossed-foliated outer layers.
a) PLM view (polarized and analyzed light) of a radial thin section. Outer surface is at bottom. b) PLM view (polarized light) showing the growth increments (green arrows) between two consecutive 1st order lamellae (I). c) PLM view (polarized and analyzed light) of three consecutive 1st order lamellae (I), displaying the preserved alternate orientation one lamella on two. 2nd order lamellae are visible (black arrows), as well as the individual, faintly disoriented 3rdorder rods (white arrow). d-g) Electron micropobe maps. d) Backscattered image of the scanned area, showing the alternate 1st order lamellae. e) Distribution of Mg content, layered following growth layers. f) Distribution of Sr content, displaying very faintly marked growth layers. g) Distribution of S content, faintly marking the crossed-foliated structure, strongly marking the growth layering. h-k) SEM images of a radial, unetched, freshly broken section. h) Several consecutive 1st order lamellae (I) (SEM). i) Limit between two 1st order lamellae (I), showing the change of orientation of its constituting 3rdorder rods (white arrow) or slats (FEG-SEM). j) 2nd order lamellae, composed of superimposed rows of 3rdorder units (SEM). k) Surface view of three consecutive 3rdorder slats within a second order row, separated by faint, punctuated limits (white arrows) and showing an inner texture (FEG-SEM). l-m) FEG-SEM images of a radial freshly broken section etched by OsO4 vapor, revealing organic membranes that separates each 3rdorder rod (white arrows). n-q) AFM scans. n) Phase image of the contact between two 1st order lamellae. Green arrows mark a growth increment. o-p) Height and phase images of 2nd order lamellae within a 1st order lamella. q) Phase image of several 2nd order lamellae, separated by a seemingly continuous membrane (blue arrow). Some ovoid sub-units (white arrow) can be seen, constituting the lamellae.
Fig 5
Fig 5. Microstructural organization of fossil Patella sp. calcite crossed-foliated outer layers.
a) PLM view (polarized and analyzed light) of a radial thin section. Outer surface is on bottom. b) PLM view (polarized light) of several consecutive 1st order lamellae (I). c) PLM view (polarized and analyzed light) of the same area, displaying the preserved alternate orientation one 1st lamella on two. Black arrow marks a region of possible recrystallization. d-g) Electron micropobe maps. d) Backscattered image of the scanned area, showing the alternate 1st order lamellae. e) Distribution of Mg content, strongly marking the growth layering. f) Distribution of Sr content, faintly marking the growth layering. g) Distribution of S content, enriched along growth layers but also within the canaliculi left by microboring organisms (white arrow). h-k) FEG-SEM images of a radial, unetched, freshly broken section. h) 1st order lamellae (I). i) Limit between two 1st order lamellae (I), showing the change of orientation of its constituting 3rdorder slats. j) Surface view of 3rd order slats forming one second order row. k) Surface view of one 3rdorder units, displaying angular-shaped sub-units. l) SEM image of abnormally thick 2nd order units, probably composed of several fused lamellae. m-o) AFM scans. m)Height image within a 1st order lamella. n) Phase images of its constituting 3nd order rods. o) Phase image of the 3nd order rods, showing that they are composed of ovoid sub-units (white arrow).
Fig 6
Fig 6. Epifluorescence and laser scanning microscopies of modern Patella vulgata calcite crossed-foliated outer layer.
a) Canaliculi left by microboring organisms (white arrow) on the outer side of the shell (UV excitation). b) Inner border of the m+2 layer (UV excitation). c) Acridine orange-stained sample (blue excitation). White arrow marks micro-borer canaliculi on the outer side. d) Same as, focused in the m+3 layer. e) Zoom between two 1st order lamellae (I), white arrows mark a growth increment. e) Confocal image of natural fluorescence of the sample under 488 nm excitation. White arrows mark a growth layer. f) Raman spectra extracted from following scan. g)Confocal Raman map showing the distribution of the ratio of calcite peaks Lc (librational mode, 282 cm−1) / ν 1 (symmetric stretch, internal mode, 1085 cm−1), which highlights changes of crystallographic orientations of calcite between 1st order lamellae (I). h) Map of the distribution of 1134 cm−1 band of polyenes. i) Map of the distribution of 1525 cm−1 band of polyenes. j) Composite map of i) (in blue) and j) (in red) showing anti- and co-localization of polyenic molecules. (k) Map showing the intensity distribution of the background intensity (between 2400 cm−1 and 2500 cm−1) related to the fluorescence of the sample.
Fig 7
Fig 7. Epifluorescence and laser scanning microscopy of fossil Patella sp. calcite crossed-foliated outer layer.
a) Canaliculi left by microboring organisms (white arrow) on the outer side of the shell (UV excitation). b) Inner border of the outer layer (UV excitation). c-d) Faint (c) and strong (d) changes of natural fluorescence hue of the shell at the contact with the sediment. e) Acridine orange-stained sample (blue excitation). white arrows mark some micro-borer canaliculi on the outer side. f) Confocal image of natural fluorescence of the sample under 488 nm excitation. White arrows mark a growth layer. g) Raman spectra extracted from previous scan. h)Confocal Raman map showing the distribution of the ratio of calcite peaks Lc (librational mode, 282 cm−1) / ν 1 (symmetric stretch, internal mode, 1085 cm−1), which highlights changes of crystallographic orientations of calcite between 1st order lamellae (I). i) Map of the distribution at the wavenumber that should correspond to 1134 cm−1 band of polyenes. j) Map showing the intensity distribution of the background intensity (between 2400 cm−1 and 2500 cm−1) related to the fluorescence of the sample.

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Grant support

This research received support from CNRS through the interdisciplinary action AIR-Archaeometry and from the SYNTHESIS Project http://www.synthesis.info/ which is financed by European Community Research Infrastructure Action under FP7 Integrating Activities Programme (the authors especially thank J. Spratt, A. Ball and L. Howard from the mineralogy department of NHM, London for their support). The authors would like to thank the Institut National des Sciences de l’Archéologie et du Patrimoine (dir. A. Akerraz), the Mission archéologique El Harhoura-Témara, funded by the Commission consultative des recherches archéologiques à l’étranger of Ministère des Affaires Etrangères et Européennes of France and the Ministry of Culture of Morocco. The authors also would like to thank the French Agence Nationale de la Recherche and the ANR-09-PEXT-004 MOHMIE project (dir. C. Denys) for the funding provided in the field work. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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