The bone artefact described in this paper (specimen inventory number DES1-12 N502-659) is currently housed at the Laboratory of the National Institute of Archaeological Science and Heritage, Rabat (Morocco). The permit to conduct archaeological excavations at Dar es- Soltan 1 cave was granted by the Moroccan Ministry of Culture (Authorisation numbers 676 and 677, March 14th 2012). The bone specimen is publicly deposited at the National Institute of Archaeological Science and Heritage, Rabat (Morocco) and accessible to the scientific team members and to any authorizied researcher.

The bone tool was found in a ‘sealed’ context, separated from the overlying LSA, Neolithic and Protohistoric (Group 5) horizons by an intervening MSA deposit (Group 4), which is more than 1.50 m thick. Furthermore, although 42 bone tools were discovered in the Neolithic layer B and ‘Protohistoric’ layers (layers B and A, respectively) none of these bone tools (which include a comb, two hooks, a hide-working tool, three worked teeth, including a human molar, one sawn bone, a spear-like bone, one side scraper, two perforated needles, two pins, and 28 other bone points), share any similarities with the bone tool from G3.v.

(A) Drawing of the tool with sequence of post-depositional breaks indicated [ 1 – 3 ]; (B) Photograph of the cortical side, sharp edge, trabecular side, smooth edge, tip and base of the tool. (C) SEM images detailing (a) scrape marks and polish along the smooth edge on the cortical side of the tool and (b) deep scrape marks (re-sharpening) along the sharp edge on the cortical side; (c) flaking near the tip of the tool, and (d) detail of the flat break surface near the base of the tool; (e) wear pattern near the sharp edge on the trabecular side of the tool, and (f) deep scrape-marks near the smooth edge on the trabecular side; (g) flaking close to the tip, and (h) polishing of the smooth edge of the tool.

The bone artefact was found lying in the deposit at a shallow, oblique angle such that the tip appeared first, protruding slightly through the surface. When uncovered the bone seemed to be intact, but the tip separated from the main portion of the tool during excavation. Concreted sediment embedded within the broken edges indicates that the break occurred during burial, probably as a result of trampling and sediment compression ( Fig 2 ). The MSA lithic artefacts associated with the bone tool in square N502 include typical MSA products such as a pedunculate piece and preferential radial Levallois cores ( S2 Fig ) which, with other components, are common at similar levels elsewhere in the cave ( S3 Fig ).

Only a broad level of correlation can be established between the various exposures of Group 3: units r-o (N502) correlate with G3.3 (in L500-499 for example), whilst units h-a (N502) correlate with G3.9–11. The general sedimentary sequence in the Group 3 material in N502 shows two very clear trends. First, human influence was very strong, both in terms of material input (lithics, bone, shell, charcoal, ash) and mass disturbance, at the base but waned steadily upwards, even if it never completely disappeared. Second, the area was comparatively dry at the start but became increasingly wet, with ponding and eventually current flow upwards (with much sorting of fine ash and charcoal inwards, to the east). The presence of water in the cave would therefore have acted as a limitation to the floor space available (driving actual ‘occupation’ up and westwards/outwards through time) but also as an attractor in a generally sandy landscape, where a water source may well have been useful.

The bone artefact described in this paper was found in unit G3.v in square N502 ( Fig 1B–1D ). Most of Group 3 in other parts of the cave (as reported in [ 2 ]) is almost wholly phosphatic (no HCl-reaction) and very strongly bioturbated at various scales; this has resulted in an extremely ‘fuzzy’ stratigraphy without faunal remains, that is contextually unsatisfactory, despite the common durable archaeological finds. However, in N502, both thin carbonate ash deposits and phosphatisation of the latter have contributed to bone/shell preservation. The cementation has also helped to inhibit bioturbation as did, no doubt, the relatively damp to wet condition developing in these sediments. The large capping roof-fall blocks have also inhibited later burrow penetration and have slowly added ground carbonate. Yet another factor may be that, in much of the southern side of the cave, there are deep alcoves, suitable for nesting birds and roosting bats, which would have contributed guano (aggressively acidic); in contrast, the cave walls around N502 are quite vertical, leading up to a major roof opening, perhaps being less attractive for nesting/roosting. The outcome is that bone/shell preservation in N502 is much better and the stratigraphic integrity and detail very much improved. Preliminary analysis of the faunal remains has identified auroch (Bos primigenius) among the large mammal material, together with smaller mammals (including Hystrix cristata and unidentified carnivores), reptiles (Testudo sp.), microfauna ( S1 Table ) and marine molluscs.

The new research program at Dar es-Soltan 1 cave was coordinate by the Moroccan Institut National des Sciences de l’Archéologie et du Patrimoine and the University of Oxford (UK), with permission from the Ministry of Culture (Kingdom of Morocco). We undertook new excavations in the cave in 2005 [ 19 ], in 2008, to collect samples for tephra analysis and to excavate a Neolithic burial in G5 and, finally, in 2012, when we excavated additional areas alongside previous excavation trenches ( Fig 1B ). The bone tool was found during this last season of excavation. A composite sedimentary log was recorded in various vertical segments spaced, according to clarity of exposure, along longitudinal sections left by Ruhlmann. The overall sequence has been divided into five ‘Groups’, considered to be equivalent to the grade of a geological ‘member’ and valid for all parts of the cave ( Fig 1 and S1 Fig ) [ 19 ].

(A) Location of the Dar es-Soltan 1 and El Mnasra caves on the Atlantic coast of Morocco. Spatial (B) and stratigraphic (D) context of the bone tool (C) within Dar es-Soltan 1 cave. Shaded squares excavated in 2012. (Relief map modified from Wikimedia Creative Commons).

Dar es-Soltan 1 cave (33°58’44”N, 6°53’51”W) lies about 10 km south-west of Rabat and 260 m inland from the present Atlantic coast ( Fig 1 ). The cave entrance faces west towards the ocean and is at an altitude of 13.52 m. The cave measures about 21 m in depth and is about 10 m at its widest point. Previous excavations by Dr. Armand Ruhlmann in 1937 and 1938 confirmed the presence of Aterian (MSA) and Iberomaurusian/Late Stone Age (LSA) layers that are rich in artefactual and organic remains. Human skeletal remains were also reported from the uppermost layers [ 1 ].

This model suggests that the deposition of unit G3-v occurred at approximately 90.41 ± 3.41 ka (OSL 43; S4 Fig and S2 Table ). The estimate overlaps at 1 sigma uncertainty with the un-modelled OSL age of 98.3 ± 9.3 ka, but makes allowance for the slight stratigraphic age reversal between OSL 42 and OSL 43 ( S4 Fig and S3 Table ). The main effect of this model, however, has been to increase the precision of the OSL ages for the new samples. There is excellent agreement between the modelled ages for Sequence 1 obtained via the updated model and the results previously published [ 19 ]. Ages are nearly identical, barring slight discrepancies of less than 7 ka for samples OSL 5a/b and OSL 11. The new data indicates that Group 1 was deposited prior to ~120 ka, with subsequent group boundaries occurring at approximately 118ka, 97 ka, 73 ka, and 43 ka ( S3 Table ).

Two sequences of OSL determinations were used to model the age of deposition of G3-v. Previously published OSL ages (Sequence 1) [ 19 ] and a new series of OSL determinations (Sequence 2) based on samples collected in 2012 from square N502 [ 20 ] were incorporated into an updated Bayesian model using OxCalv4.2. Due to the different sampling strategies used during different campaigns, the two datasets may initially be thought of as two independent stratigraphic sequences. Whilst all OSL samples within each sequence can be ordered in relation to every other sample within the same sequence, the only robust cross-correlations between the sequences are the sedimentological boundaries between Groups 2 and 3, and Groups 3 and 4. Therefore, the overall model incorporates the two stratigraphic sequences, linked only by cross-referenced probabilities calculated for these particular boundaries ( S4 Fig and S2 Table ). Other Group boundaries were extracted from a previous work [ 19 ] and data sequence via the ‘Probability’ command. Replicate samples OSL 5a and 5b, and OSL 48a and 48b were entered as combined likelihoods.

The bone tool

Detailed observations of the bone tool from Dar es-Soltan 1 were conducted using standard taphonomic analyses. The state of preservation was evaluated according to bone fragmentation and soundness of cortical surface using established criteria [21–25]. Humanly-induced modifications were classified as slicing cut marks and scrape marks made during the production and resharpening of the tool edge, flaking and polishing from use [26–31]. The artefact was initially examined using a hand lens and binocular microscope. Detailed analyses of surface modifications were enhanced with observations at higher magnification using a scanning electron microscope (SEM [32, 33]). The SEM (LEO1455VP) was operated in variable pressure mode (chamber pressure 15 Pa), enabling back-scattered electron (BSE) images to be obtained without the application of a conducting layer on the specimen. SEM images were taken in order to conduct detailed analyses of surface modifications and use-wear patterns. Micro-computed tomography (micro-CT) was performed to record the surface topography, to gauge the extent of surface modifications and to resolve features in areas where the surface was obscured by concretions. The specimen was scanned using a HMX-ST CT 225 System (Metris X-Tek, Tring, UK). This scanner combines a cone beam projection system [34] with a four megapixel Perkin Elmer XRD 1621 AN3 HS detector panel; different settings were used to optimize contrast and minimize beam hardening. The long axis of the bone tool was oriented vertically with respect to the beam, thus ensuring maximum resolution whilst minimizing streak artefacts [35]. The micro-CT data were manipulated using CT-PRO software version 2.0 (Metris X-Tek) and rendered using VG Studio MAX 2.1 (Volume Graphics, Heidelberg, Germany).

The Dar es-Soltan 1 bone tool was produced on an oblong ‘plaque’ of bone that is gently curved both longitudinally and transversely (Fig 2). The dimensions, cortical thickness and overall morphology of the trabeculae suggest it is formed from a portion of rib shaft, consistent in size with a large mammal, probably from a bovid (Fig 3A).

PPT PowerPoint slide

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larger image TIFF original image Download: Fig 3. Reconstruction and micro-CT sections of the bone tool from Dar es-Soltan 1. (A) Reconstruction of the portion of a large mammal rib used in the manufacture of the bone implement from Dar es-Soltan 1. (B) Micro-CT scan sections close to the base (B1), the middle (B2) and the tip (B3) of the tool (white outlines show the reconstructed cross-section of the rib). (C) Sequence of 19 micro-CT scan sections superimposed on an outline of the bone tool with sediment digitally removed (C2). https://doi.org/10.1371/journal.pone.0202021.g003

The reconstructed length of the bone tool is 122 mm: the length of the main piece is 111 mm and the smaller piece is 12 mm long. We refer to the smaller piece as the ‘tip’ (distal end), and the other end is defined as the ‘base’ (proximal end) of the implement. The width of the tool varies from 18 mm to 27.5 mm along its length, resulting in a ‘fish-shaped’ outline with tapered edges converging towards the rounded tip (Fig 2). The maximum thickness of the piece at the base is 4.7 mm; scraping has reduced the thickness of the cortical bone to ~1.2 mm at 5 mm from the tip.

The tool presents at least three flexion breaks and longitudinal internal cracks (Fig 2A). The first flexion break (1 in Fig 2A) occurred transversely across the specimen at 17 to 18.5 mm from the tip end. This resulted in two pieces now held together by sediment; the break caused a slight deflection of the longitudinal profile. The shorter fragment then broke longitudinally (2 in Fig 2A) and the narrower part was also bent, causing further distortion. Finally, a second transverse break (3 in Fig 2A) separated the tip from the middle section. The base is broken transversely probably during manufacture and shaping of the tool. This sequence of the breaks and pattern of distortion would suggest that the tool was discarded in one piece and that the three breaks towards the tip only occurred after deposition. No cut marks that can be unambiguously related to defleshing or skinning were observed on the rib fragment; however any such marks could have been erased by repeated scraping of the bone surface associated with the shaping of the tool (see below). Similarly, no traces from carnivore chewing or weathering were observed.

Part of the tool is obscured by sediment still cemented onto the central area on both sides of the specimen (Fig 2). This concretion matches the carbonates found in same sedimentary horizon. After testing the feasibility of removing the adhering sediment from a small area at the proximal end, it was decided that further mechanical preparation would result in damage to the surface (Figs 2B and 4O), and therefore the remaining sediment was left untouched.

The manufacture of the bone tool followed a succession of precise actions. The rib was first modified to reduce its length and bisected longitudinally along the cranial and caudal edges to produce a shorter and narrower ‘blank’. Adherent sediment along with subsequent scarping and use wear (Fig 2) make it difficult to determine whether the breaks took place when the bone was fresh or dry, or whether cutting, chipping or abrasion were used to shape the blank. However, deliberate shaping of the rib is clearer on the sides of the implement, where the cranial and caudal edges were modified to create more or less straight sides (Fig 3, micro-CT scans 2 to 9). The removal of the cranial and caudal margins of the rib would have been undertaken to facilitate longitudinal splitting of the rib into two halves. The longitudinal division of the rib into two halves results in two surfaces: one cortical (we will refer to this as the ‘cortical side’), and the other mainly composed of trabecular bone (‘trabecular side'; Fig 2). One edge is much sharper due to further thinning of the cortical and trabecular surface by scraping and flaking towards the tip (‘sharp edge’), whereas the other edge is more rounded with polishing that obliterates the scraping marks along much of its length (‘smooth edge’).

Trabecular side. On the trabecular side, the scrape marks are found primarily towards the tip of the tool and appear to stop around 35 mm from the tip (Fig 5I and 5J). The scrape marks are generally parallel to each other and more intense scraping occurred towards the tip. Here, scraping has removed trabecular bone and exposed denser cortical bone beneath (Fig 5A–5I). These modifications suggest that scraping was an integral component in the thinning and shaping of the specimen (also observed at other MSA sites) [38, 39]. Use-wear in the form of fine overlapping striations perpendicular and oblique to the main axis of the tool, is visible on the trabecular surface close to the sharp edge of the tip of the implement (Fig 5A–5G). Use-wear is more intense close to the tip of the tool and partially masks the earlier scrape marks. PPT PowerPoint slide

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larger image TIFF original image Download: Fig 5. Trabecular side of the bone tool from Dar es-Soltan 1. Photo and details of bone surface modifications. (SEM images, a to l). https://doi.org/10.1371/journal.pone.0202021.g005