Lila Geis
email: lila.geis@u-bordeaux.fr

Francesco D'errico
email: francesco.derrico@u-bordeaux.fr

Fiona M Jordan
email: fiona.jordan@bristol.ac.uk

Michel Brenet
email: michel.brenet@inrap.fr

Alain Queffelec
email: alain.queffelec@u-bordeaux.fr

Multiproxy analysis of Upper Palaeolithic lustrous gravels

Upper Palaeolithic sites in southwestern France attributed to the Upper Gravettian and the Solutrean yielded sub spherical gravels with a highly shiny appearance that have intrigued researchers since the 1930s. In this work, we analyze specimens from five sites, including the recently excavated Solutrean site of Landry, to establish whether their presence in archaeological layers and peculiar aspect are due to natural processes or human agency. We study the spatial distribution of gravels at Landry and submit archaeological gravels from the five sites, natural formations, Landry sediment sieving, and polishing experiments with a rotary tumbling machine to morphometric, colorimetric and textural analyses. Our results suggest that the lustrous gravels found at the five sites result from deliberate selection and their shiny appearance is the probable consequence of prolonged contact with a soft material such as animal skin. Ethnographic accounts indicate that these gravels may have been used for magicoreligious ritual purposes (charms, sorcery, divination etc.), in games, as elements of musical instruments, and as items serving other social and personal purposes. We argue that these objects reflect a cultural innovation emerged during the Gravettian and continued into the Solutrean.

Introduction

Upper Palaeolithic sites in southwestern France attributed to the Upper Gravettian and the Solutrean have yielded sub spherical gravels with a highly shiny appearance that have attracted the attention of Palaeolithic archaeologists since the 1930s [START_REF] Baumann | Le gisement préhistorique du Fourneau du Diable (Bourdeilles, Dordogne). Rapport de fin d'opération annuelle, Gironde, UMR 5199-PACEA[END_REF][START_REF] Detrain | aux Eyzies-de-Tayac, Dordogne. Demande de Programme Collectif de Recherche[END_REF]Detrain et al., 1992[START_REF] Detrain | Fouilles de sauvetage du Musée National de Préhistoire des Eyzies[END_REF][START_REF] Moncel | Émergence de la notion de précieux: objets insolites et extraordinaires au Paléolithique[END_REF][START_REF] Peyrony | Rapport de D. Peyrony. Chargé de mission aux Eyzies-de-Tayac[END_REF][START_REF] Peyrony | Station préhistorique du Pech de la Boissière (Dordogne)[END_REF]. [START_REF] Peyrony | Rapport de D. Peyrony. Chargé de mission aux Eyzies-de-Tayac[END_REF] thought that they were attached on a leather tag with a resin and arranged next to each other to be used as ornaments (Fig 1). Other researchers have limited themselves to reporting their discovery without attempting to determine whether their presence in archaeological layers and their shine were due to natural processes or human activity.

Answering these questions has been made difficult by the fact that these gravels were uncovered during excavations for which we lack relevant contextual data. We do not know, for example, whether gravels were systematically collected nor do we have precise information on their spatial distribution and eventual association with other archaeological remains. The discovery of about a hundred lustrous gravels on the recently excavated Solutrean site of Landry (Dordogne, France) (rescue excavation by Inrap in 2011, Brenet dir.), and the application of new analytical methods make it possible to approach the question of the origin and eventual anthropogenic modification of these objects in new ways.

In order to address this topic we combine in our study four approaches: 1) spatial analysis of lustrous gravels and their relationship with other archaeological remains unearthed at Landry, 2) morphometric and colorimetric analysis of gravels discovered at this site, four other Upper Palaeolithic sites from Dordogne, and gravels from geological deposits accumulated by known sedimentary processes, 3) surface texture analysis of lustrous and non-lustrous gravels from archaeological sites and natural accumulations as well as gravels experimentally polished to mimic natural and anthropogenic actions, and 4) a survey of ethnographically-attested uses of gravels and small pebbles. The objective of the spatial analysis was to establish whether the gravels' distribution was random or not, and connected with that of other archaeological remains. The morphometry and colorimetry of the lustrous and non-lustrous gravels aimed at verifying whether the former represents a selection of the gravels naturally available on the site.

The textural analysis of archaeological, natural and experimentally polished gravel was intended to document and quantitatively compare their wear patterns to establish whether shine on archaeological objects could be found in nature, and which experimental polishes generated by a tumbler and a metallographic polisher better mimic archaeological wear.

The last decade has seen a substantial development of studies applying confocal microscopy and roughness parameters to archaeological artifacts with the objective of understanding the function of tools, the intensity of use, and taphonomic processes affecting the artefact after its production and use (Calandra, Pedergnana, et al., 2019;[START_REF] Calandra | Back to the edge: relative coordinate system for use-wear analysis[END_REF][START_REF] Evans | Laser scanning confocal microscopy: a potential technique for the study of lithic microwear[END_REF][START_REF] Galland | Quantifying lithic surface alterations using confocal microscopy and its relevance for exploring the Châtelperronian at La Roche-à-Pierrot (Saint-Césaire, France)[END_REF][START_REF] Henshilwood | An abstract drawing from the 73,000-year-old levels at Blombos Cave, South Africa[END_REF][START_REF] Li | Engraved bones from the archaic hominin site of Lingjing, Henan Province[END_REF][START_REF] Marreiros | Rethinking Use-Wear Analysis and Experimentation as Applied to the Study of Past Hominin Tool Use[END_REF][START_REF] Martisius | A method for the taphonomic assessment of bone tools using 3D surface texture analysis of bone microtopography[END_REF][START_REF] Procopiou | The role of tactile perception during stone-polishing in Aegean prehistory (5th-4th millennium B.C.)[END_REF][START_REF] Rosso | Patterns of change and continuity in ochre use during the late Middle Stone Age of the Horn of Africa: The Porc-Epic Cave record[END_REF]. The present study is the first in which this approach is combined with others to establish whether the presence of objects in archaeological layers is the consequence of human action and to infer possible function just from wear. Implementation of this holistic approach leads us to conclude that the lustrous gravels found at the five sites result from deliberate selection and their shiny appearance is the probable consequence of prolonged contact with animal skin. The characteristic shine may have been produced deliberately or be the consequence of transport or use of the gravels. A survey of historical and ethnographic sources allows us to explore, for these three cases, what functions these gravels may have had and to suggest a general strategy for how to approach the study of archaeological objects presenting minimal modifications but potentially reflecting significant cultural innovations.

Archaeological context

Lustrous gravels are reported from four sites of the South-West of France: Fourneau du Diable, Casserole, Pech de la Boissière and Laugerie Haute (Fig 1 andTable 1). The recent excavation at a fifth site, Landry, has for the first time yielded these objects in a stratigraphically secure and culturally well assigned context. 1863,1892,1895,[1921][1922][1923][1924][1925][1926][1927][1928][1929][1930][1931][1932][1933][1934][1935]1950 West sector 18

Landry

Excavated by one of us (MB) in 2011, Landry is an open-air site located close to Boulazac, Dordogne, France (Table 1). The site features several levels of aeolian silts overlying a low alluvial terrace of the Isle River (Fig 2). The Solutrean occupation layer is interstratified in the aeolian silts. Although periglacial processes, such as solifluxion, have slightly affected the uppermost layers of the site, geoarchaeological analyses [START_REF] Brenet | The Late Solutrean open-air site of Landry (Aquitaine, France). A preliminary spatio-temporal analysis[END_REF] and refitting of stone artifacts show the well-preserved spatial distribution of archaeological finds has not been significantly disturbed (Fig 3). Furthermore, due to the well-drained soils, bacteria in an oxidizing environment have completely degraded organic matter such as bones. The remarkable diversity of the lithic raw materials used (local Senonian flint, and allochtonous flints, quartz, granite, and dolerite) and the discovery of schist slabs engraved with geometric patterns make this site unique in the region [START_REF] Feruglio | Le site solutréen supérieur de plein-air de Landry (Boulazac, Dordogne, France), un campement de chasseurs révélant de l'art mobilier[END_REF]. place during the same occupation [START_REF] Brenet | The Late Solutrean open-air site of Landry (Aquitaine, France). A preliminary spatio-temporal analysis[END_REF]. This is particularly well evidenced by the analysis of sector 3. Combining data on the different nature, spatial distribution and use wear of artifacts, it is proposed that this concentration retains evidence of activities related to returning from hunting, flint tool knapping and shaping, butchery, skin preparation, hard organic material working, slab engraving and ochre processing [START_REF] Bachellerie | Le site solutréen supérieur de Landry à Boulazac : Analyse technoéconomique de la production lithique (silex) du secteur 3[END_REF][START_REF] Mesa | Interaction outillage en silex/macro-outils passifs en dolérite sur le site Solutréen de Landry[END_REF]. Thermoluminescence dating of burned flints has provided an average age of 21.5 ± 1.25 ka for the site [START_REF] Brenet | fouille d'un campement paléolithique solutréen[END_REF], which is consistent with an attribution taking into account the margin of error, based on diagnostic stone tools found in the five concentrations, to the Late Solutrean. As many as 106 lustrous gravels were retrieved in the sieving from sector 3 and, to a lesser degree, from sector 2 (Figs 1 and 3). Those gravels are kept in the Inrap research laboratory of Campagne, Dordogne, France. All sediment from the excavated surface was sieved with a 2, 5 and 10mm mesh. The spatial origin of the gravels is known with a precision of a quarter of square meter.

Fourneau du Diable

This site is located on the right bank of the Dronne River, near Brantôme, Dordogne, France (Fig 2A). It consists of a small shelter, delimited to the north by a cliff, overhanging two superimposed terraces. The Quaternary sediments covering these terraces (Table 1) contains archaeological layers attributed to the Gravettian, the Solutrean and the Magdalenian [START_REF] Baumann | Le gisement préhistorique du Fourneau du Diable (Bourdeilles, Dordogne). Rapport de fin d'opération annuelle, Gironde, UMR 5199-PACEA[END_REF]. These layers were covered by Holocene sediments and the fall of hanging blocks. Peyrony reports the discovery of lustrous gravels in the Gravettian or the Solutrean layers from the upper terrace (D. [START_REF] Peyrony | Rapport de D. Peyrony. Chargé de mission aux Eyzies-de-Tayac[END_REF].

According to this author, 136 spherical gravels, aligned and positioned closed to each other, were arranged in nine rows of 15 forming a rectangle (Fig 4 [START_REF] Peyrony | Station préhistorique du Pech de la Boissière (Dordogne)[END_REF]. The archaeological deposit has suffered rock falls and runoff from the upper plateau.

Of the 30 lustrous gravels discovered in the Solutrean layers by Peyrony, only nine are kept in the Musée National de Préhistoire, Les-Eyzies-de-Tayac (Fig 1 andTable 1).

Casserole

One of the shelters overlooking the Musée National de Préhistoire, Les Eyzies-de-Tayac (Fig 2A), this comprises a stratigraphic sequence containing 13 archaeological layers attributed to the Gravettian, the Badegoulian, the Proto-Solutrean, the Solutrean and the Magdalenian (Table 1).

Although geoarchaeological analysis shows that boulder falls, trampling, percolation, gelifraction and bioturbation have, albeit at small scale, contributed to the site formation process, the excavation conducted between 1991 and 1993 (Detrain et al., 1992[START_REF] Detrain | Fouilles de sauvetage du Musée National de Préhistoire des Eyzies[END_REF], showed a low degree of disturbance of the Solutrean layers and identified a concentration of remains on a small surface interpreted as an area of intense human activity [START_REF] Detrain | Fouilles de sauvetage du Musée National de Préhistoire des Eyzies[END_REF]. The Solutrean layers are dated by 14 C AMS on burned bones to 20.5 -23.3 cal BP [START_REF] Detrain | aux Eyzies-de-Tayac, Dordogne. Demande de Programme Collectif de Recherche[END_REF][START_REF] Ducasse | The abri Casserole (Dordogne, France): Reassessing the 14C chronology of a key Upper Paleolithic sequence in Southwestern France[END_REF]. Fourteen lustrous gravels were retrieved in this layer (Fig 1 andTable 1). They are kept in the Musée National de Préhistoire at Les-Eyzies-de-Tayac.

Laugerie Haute

Located at Les Eyzies-de-Tayac, on the right bank of the Vézère valley (Fig 2A ), and firstly excavated in 1863 by Lartet and Christy, this iconic rock shelter has been since then the subject of numerous digs (Table 1). It includes two distinct sectors, Laugerie Haute East and Laugerie Haute West.

The excavations yielded a long stratigraphic sequence encompassing Solutrean [START_REF] Bachellerie | Recognizing heat-treated chert: A comparison between infrared spectroscopy and fracture surface analysis[END_REF], in particular Proto Solutrean, Lower Solutrean, and Late Solutrean layers [START_REF] Bosselin | Une révision du Solutréen de Laugerie-Haute et le problème des transitions Gravettein-Solutréen et Solutréen-Badegoulien en AquitaineBosselin B, Djindjian F. Une révision du Solutréen de Laugerie-Haute et le problème des transitions Gravettein-Solutréen et Solutréen-Badegoulien en Aquitaine[END_REF][START_REF] Demars | Le Solutréen de Laugerie-Haute (Dordogne). Économie du silex[END_REF][START_REF] Smith | Taylor SI. Rainbow in the rock: The people of rural Greece[END_REF]. Radiocarbon dating indicates these layers were accumulated between 28 and 19 ka cal BP. Late Solutrean layers of Laugerie Haute West are dated by 14C AMS from 19 600 to 23 500 ka cal BP [START_REF] Texier | Sédimentogenèse des sites préhistoriques du Périgord : Excursion des 23-24 avril 2004[END_REF][START_REF] Tisnérat-Laborde | AMS Radiocarbon Dating of Bones at LSCE[END_REF]. Successive excavations conducted the last century by Peyrony and Bordes [START_REF] Bosselin | Une révision du Solutréen de Laugerie-Haute et le problème des transitions Gravettein-Solutréen et Solutréen-Badegoulien en AquitaineBosselin B, Djindjian F. Une révision du Solutréen de Laugerie-Haute et le problème des transitions Gravettein-Solutréen et Solutréen-Badegoulien en Aquitaine[END_REF][START_REF] Demars | Le Solutréen de Laugerie-Haute (Dordogne). Économie du silex[END_REF][START_REF] Smith | Taylor SI. Rainbow in the rock: The people of rural Greece[END_REF]

Methods

Table 2 below describes the various natural and geological factors that can modify the gravel surfaces. We thus rely on this inventory to better characterize the surfaces of archaeological gravels. [START_REF] Abrahams | Geophagy (soil consumption) and iron supplementation in Uganda[END_REF][START_REF] Bertran | Facies and microfacies of slope deposits[END_REF][START_REF] Bishop | The geological setting of ventifacts and wind-sculpted rocks at Mason Bay, Stewart Island, and their implications for late Quaternary paleoclimates[END_REF][START_REF] Bourguignon | Préhistoire et approche expérimentale[END_REF][START_REF] Cailleux | Initiation à l'étude des sables et des galets[END_REF][START_REF] Cane | Bustard gastroliths in the archaeological record[END_REF]Clark, 2006;[START_REF] Esque | Ingestion of bones, stones, and soil by desert tortoises[END_REF][START_REF] Fiedler | Early Man News 9/10/11 Part I[END_REF][START_REF] Galland | Utilisation du microscope confocal pour la caractérisation des états de surface d'artefacts lithiques : mise en place d'un protocole analytique ; application au Châtelperronien de la Roche à Pierrot (Saint-Césaire[END_REF][START_REF] Galland | Quantifying lithic surface alterations using confocal microscopy and its relevance for exploring the Châtelperronian at La Roche-à-Pierrot (Saint-Césaire, France)[END_REF][START_REF] Hladik | Géophagie et nutrition minérale chez les Primates sauvages[END_REF][START_REF] Judson | Microstriations on polished pebbles[END_REF][START_REF] Klaus | The environmental significance of ventifacts: a critical review[END_REF][START_REF] Klaus | The environmental significance of ventifacts: a critical review[END_REF][START_REF] Ribault | Techniques et méthodes sédimentologiques[END_REF][START_REF] Lenoble | Le rôle du ruissellement dans la formation des sites préhistoriques : approche expérimentale[END_REF][START_REF] Schlyter | Ventifacts as palaeo-wind indicators in southern Scandinavia[END_REF][START_REF] Tryon | Exotic minerals or ostrich gastroliths? An alternative explanation for some early evidence of hominin non-utilitarian behavior at Wonderwerk Cave, South Africa[END_REF][START_REF] Wings | Identification, distribution, and function of gastroliths in dinosaurs and extant birds with emphasis on ostriches (Struthio camelus)[END_REF] Table 3 provides information on the studied material and the analytical techniques applied.

This includes lustrous and non-lustrous gravels recovered at Landry, lustrous gravels from Fourneau du Diable, Casserole, Pech de la Boissière, Laugerie Haute, gravels from four natural deposits, gravels polished experimentally (see below), and the number of gravels presenting micro-concretions. Each archaeological gravel displays a similar degree of luster all over its surface. Permission was obtained to temporarily store and analyze the archaeological material at the PACEA laboratory, University of Bordeaux, Pessac, France. Lustrous gravels from Landry were retrieved in the sieving. Their white and shiny appearance facilitated their identification and comprehensive collection. Non-lustrous gravels come from Landry upper layers and were collected in the sieving from sectors bordering the area that yielded the lustrous gravels ( Fig Regarding the spatial analysis of the Landry gravels, we have included all the lustrous gravels in the analysis. Data on the spatial distributions of the gravels at the other sites were lacking, so we did not perform any spatial analysis.

Distribution and spatial association of Landry's lustrous gravels

It has been proposed that distributions of items at an archaeological site fall in three categories: concentration, uniform distribution, or random distribution [START_REF] Bertran | Le site aurignacien de plein-air de Combemenue à Brignac-la-Plaine (Corrèze) : apport de la géochéologie et de l'étude de l'industrie lithique à la compréhension des processus taphonomiques[END_REF][START_REF] Djindjian | Théories, méthodes et reconstitutions[END_REF]. The occurrences of the items in a grid or their spatial coordinates can be used to establish to which of these three cases the distribution most closely approximates [START_REF] Djindjian | Méthodes pour l'Archéologie[END_REF]. We used gravels' occurrences in the 50x50 cm Landry excavation grid to calculate the dispersion index. The corrplot package [START_REF] Wei | Package "corrplot": visualization of a correlation matrix[END_REF] was used to plot the spatial distribution of the gravels. The Poisson function law was applied to simulate numerous random distributions of the same number of lustrous gravels retrieved at the site (n= 106). The dispersion index was calculated and compared to simulated uniform distributions. A varianceto-mean ratio indicates whether the concentration of items is random or not.

We explored the degree of spatial association between the lustrous gravels and major knapped stone tool classes, as well as between the former and artefacts made of metamorphic rocks, via a permutation test whose script was created by F. Santos [START_REF] Merrill | A new method using graph and lattice theory to discover spatially cohesive sets of artifacts and areas of organized activity in archaeological sites[END_REF].

This test allows us to statistically verify the non-random association between two categories of artifacts by measuring their average Euclidian distance. This distance is then compared to values generated by a large number of runs in which the category of each object is randomly assigned, while the positions of all artefacts remain unchanged [START_REF] Berry | A permutation technique for the spatial analysis of the distribution of artifacts into classes[END_REF]. Since they were not spatially plotted during the excavation, we assigned Landry gravels random coordinates within their original 50 x 50 cm square. The results of this test give association indices for each pair of remains categories compared (gravel/flint, gravel/metamorphic rock).

Experimental polishing

Three experiments were implemented to produce lustrous gravels. In the first two experiments we used two models (3A and 33B/3-1.5) of rotary tumbler machines produced by Lortone Inc. (Mukilteo, WA, USA), commonly used to smooth and polish semiprecious stones.

In the first two experiments, aimed at testing the effect of anthropogenic modifications, we used two animal hides. In the first experiment the inner surface of the barrel was covered with tanned cow leather (Fig 6A), in the second with a leather treated with a mixture of finely ground ochre powder and duck fat (Fig 6B). The choice of skin is justified by the fact that the available data suggest animal skin was widely used in the Gravettian and the Solutrean to produce articles of leather. As well, the small size of the gravels makes it unlikely that they could have been transported in nets or similar containers. Ten non-lustrous gravels from Landry were put in the tumbler barrel. The tumbler was set at 60 revolutions per minute and run for 504 hours. The gravels were cleaned under running water and acetone at the end of both experiments. The third experiment aimed at reproducing abrasion attested in periglacial environments, caused by sediments in contact with gravels mobilized by sedimentary processes [START_REF] Caspar | Poli de bois ou poli de glace au Paléolithique inférieur et moyen? Problèmes de convergence taphonomique et fonctionnelle[END_REF][START_REF] Iverson | Laboratory simulations of glacial abrasion: comparison with theory[END_REF][START_REF] Mathews | Simulated glacial abrasion[END_REF]. A board covered with silt from the site of Landry, moistened with water, was attached to a metallographic polisher (ESC300 GTL model, produced by ESCIL, Chassieu, France) (Fig 6C). The gravels were manually held in contact with the siltcovered surface for 7 hours. They were then attached to a metal rod and maintained in contact with the silt for an additional 5 hours. Only one side per gravel was polished following this protocol. The polisher was operated at 110 rpm. The experimental gravels were then subjected to the same morphometric and roughness analyses applied to the other gravels analyzed (see below).

Microscopic, morphometric and colorimetric analysis

Archaeological, experimental and natural gravels were photographed with a Sony A6000 equipped with a Sony E30-mm F3.5 macro lens. Selected gravels were molded using Coltène ® President regular body high resolution dental impression material and cast in resin (Epoxy Araldite 2020, Escil, Chassieu, France). Replicas were observed and photographed in transmitted light with a Leica Z6 APOA microscope equipped with a multifocus module. The Particle Analysis plug-in of the image processing software ImageJ [START_REF] Schneider | NIH Image to ImageJ: 25 years of image analysis[END_REF] was applied to digital images of archaeological lustrous and non-lustrous gravels as well as gravels collected at natural sites to measure length, width, area, perimeter, and calculate circularity (4π*area/perimeter^2) and rounding indexes (4*area/(π*major axis^2). Thickness was measured manually using a digital caliper. Colorimetric values, expressed in RGB, were extracted by averaging the color of each gravel thanks to the Color Inspector 3D plug-in, and converted into L*a*b* with the grDevices package (Team, 2009). Inter-group comparison of the distribution of those quantitative data was performed through the Kruskal-Wallis nonparametric statistical test with the pgirmess package [START_REF] Giraudoux | Package 'pgirmess.' Spatial Analysis and Data Mining for Field Ecologists[END_REF].

Surface texture analysis

Confocal microscopy applied to archaeological artifacts is able to distinguish wear signatures of natural and anthropogenic agents [START_REF] Evans | Laser scanning confocal microscopy: a potential technique for the study of lithic microwear[END_REF][START_REF] Galland | Quantifying lithic surface alterations using confocal microscopy and its relevance for exploring the Châtelperronian at La Roche-à-Pierrot (Saint-Césaire, France)[END_REF][START_REF] Ibáñez | Cereal cultivation and domestication as shown by microtexture analysis of sickle gloss through confocal microscopy[END_REF][START_REF] Rosso | Patterns of change and continuity in ochre use during the late Middle Stone Age of the Horn of Africa: The Porc-Epic Cave record[END_REF]. Archaeological lustrous and non-lustrous gravels from Landry and lustrous gravels from Fourneau du Diable, Casserole, Pech de la Boissière, Laugerie

Haute, from natural deposits and gravels experimentally polished (Table 3) were analysed with a Sensofar S-Neox confocal microscope equipped with a long-working-distance 50x objective (numerical aperture = 0.45), allowing a spatial sampling of 0.26µm. We measured on opposite aspects of each gravel two areas of 351x264µm with a vertical repeatability of 3 nm and 42° of maximum acquisition slope. Measures were taken on areas free of micro-concretions. The data obtained were processed by the SensoMAP Premium 7.4 software. Post-acquisition data treatment followed a procedure similar to that described by Martisius and colleagues [START_REF] Martisius | Time wears on: Assessing how bone wears using 3D surface texture analysis[END_REF]. Using built-in operators, this included levelling the surface with the least squares method, removing isolated outliers and noise, filling in non-measured points by interpolating from neighbours, and removing the surface form using a polynomial of third order.

A Gaussian filter of 80µm was applied as a cut-off to remove waviness from roughness, which appeared to separate roughness values of glossy from non-glossy gravels better than the 25µm filter. Three ISO 25178 parameters were calculated (Table 4): the root mean square height (Sq), the degree of bias of the shape of the roughness (Ssk), and the developed surface ratio (Sdr). Previous studies have shown the pertinence of these parameters to characterize different degrees of polish on archaeological artefacts (d'Errico et al., 2022;[START_REF] Galland | Utilisation du microscope confocal pour la caractérisation des états de surface d'artefacts lithiques : mise en place d'un protocole analytique ; application au Châtelperronien de la Roche à Pierrot (Saint-Césaire[END_REF][START_REF] Galland | Quantifying lithic surface alterations using confocal microscopy and its relevance for exploring the Châtelperronian at La Roche-à-Pierrot (Saint-Césaire, France)[END_REF]. In addition, they present the advantage of being largely independent from one another. Univariate and multivariate statistical analyses were applied to these data. We present here the results of two PCA (Principal Component Analysis) with convex hulls for each group of gravels. We obtained those PCA via the FactoMineR package [START_REF] Lê | FactoMineR: an R package for multivariate analysis[END_REF].

Amplitude parameters (characterization of the z axis perpendicular to the surface)

Sq µm

Root mean square height, standard deviation of heights. A low roughness will have a Sq value equal to 0 while a high roughness will have a Sq value superior to 0.

Table 4. Three ISO 25178 parameters used to measure the textural surfaces.

We selected 2 amplitudes parameters: the root mean square height (Sq), the degree of bias of the shape of the roughness (Ssk), and one hybrid parameter: the developed surface ratio (Sdr).

Ethnographic survey of small pebble use

To explore historical and contemporary uses of small stones similar to the lustrous gravels examined, we used the electronic Human Relations Area Files (eHRAF), a collection Because each paragraph is usually tagged with multiple OCM codes, we developed a bespoke coding scheme for specificity.

All material in a sample of 82 cultures (all 38 hunter-gatherer societies in the SCCS plus 44 others stratified by region, c.20% in each totalling 10488 paragraphs) was read and coded for how pebbles were used. We noted if pebbles were present in multiples of four or more at one time (Yes, No, Unclear), and any information about bags or other carrying items. We did not code keyword mentions that were irrelevant to our study, especially where stones were described as larger than a fist (e.g. grinding or cooking stones, cairns, commercial use) or when the keywords were associated with technologies not relevant for our study (e.g. pebbles associated with metal or glass in ornaments, pebbles with holes for stringing, for use as weights in commercial trade etc.). Other rare mentions, such as being described in fantastical folklore, or stones referring to the seeds of fruit, were also not assigned to a code.

Ssk

Skewness. Degree of bias of the shape of the roughness (asperity). A low roughness will have a Ssk value under 0 (surface dominated by valleys) while for high roughness le Ssk value will have a value superior to 0 (surface dominated by peaks).

Hybrid parameters (information present on the three axes x, y and z of the surface) Sdr % Developed surface ratio. 0% indicates a flat and smooth surface, 100% indicates a complex surface with large peaks and valleys.

Results

Distribution and spatial association of lustrous gravels at Landry

Lustrous gravels are concentrated in squares R11, R12, and Q11, sector 3, and to a lesser extent in adjacent squares (Fig 7 ). A few gravels are present in sector 2. They are virtually absent in the rest of the excavated area. This is confirmed by a dispersion index of 9.7, much higher than that expected for a uniform or random distributions (Fig 7).

The spatial distribution of flints and metamorphic rocks appears strongly correlated with that of the lustrous gravels (Fig 8). As indicated by a significant difference in association indices (Fig 9), the average distance between the gravels and the objects in the other categories is greater than if the category had been randomly assigned to objects with the same geographic coordinates. This result establishes that the concentration of gravels is stronger than that of the other categories of remains.

Morphometry and colorimetry

Morphometric analysis identifies significant differences (p < 0.05) (see SM Table 1) between lustrous gravels found at Landry, Casserole, and a part of those from Fourneau du Diable, which are smaller and more spherical (Fig 10). These are also different from those at Pech de la Boissière, Laugerie Haute, a part of those from Fourneau du Diable, those of Landry retrieved outside the concentrations and those from natural sites, which are more elongated and more variable in size. The gravels from the Fourneau du Diable, Pech de la Boissière and Laugerie Haute sites are also more homogenous in color; less so for the others (Fig 11 andSM Table 1). At Landry no significant differences are observed in the color of lustrous and notlustrous gravels, since they are all made of quartz.

Microscopic analysis

The archaeological lustrous gravels are characterized by homogeneous surfaces on which depressions are either absent, or rare and of dimensions not exceeding 10µm. Curved micro-cracks corresponding to incipient fracture cones are also discernable (Fig 12). The edges of these fracture lines occasionally reveal chipping and conchoidal micro removals. The frequency of depressions and the degree of surface homogeneity is dependent on the degree of luster observed to the naked eye. The natural gravels (Fig 13) which are visually matte are characterized at the microscopic scale by more uneven surfaces than those observed on the archaeological pieces. The natural gravels present small homogenous areas exclusively on prominent zones. This is particularly evident on pieces from river terrace deposits. Gravels collected on beaches show more intense pitting associated, in some cases, with incipient fracture cones not flattened by wear, as is the case for archaeological gravels. The surfaces of gravels tumbled in barrels are similar to those on archaeological specimens without reaching the same level of homogeneity (Fig 14). Notably, treatment with tumblers covered with ochred skin produces the surface modification comparable to those observed on archaeological gravels.

Abrasing natural gravels with a polishing machine covered with wet silt produces lustrous areas entirely covered with 20-50µm wide parallel striations.

Surface texture analysis

Roughness parameters identify significant differences (p < 0.05) (see SM

Multivariate analysis

The principal component analysis of all recorded parameters (Fig 17) highlights a clear difference between gravels found at archaeological sites excavated in the past, natural gravels and gravels found at Landry both inside and outside the concentrations of archaeological artifacts. While very few gravels from the Landry concentrations display features compatible with those recorded on natural populations, a consistent portion of those found outside the concentrations are comparable with the natural ones. A clear difference appears also between the lustrous gravels of Landry and those from the other archaeological sites. The former are more circular, whiter in color, and display comparatively rougher surfaces.

Experimental results

As expected, all experimental treatments significantly reduced the roughness of the gravels. The tumbling process, entailing impact of the gravels on each other and their contact with the skin, causes a gradual flattening of the surface resulting in a shiny appearance (Fig 18).

Abrasion of gravel against Landry silt also produces a shine.

At microscopic scale, however, the area smoothed by abrasion is covered by microstriations not found on lustrous gravels from Landry, the other archaeological sites and natural

Ethnographic survey results

Uses were first documented at a low level (e.g. "as luck charm"), and then on completion of the sample, our coding system aggregated initial categories into a higher level (e.g. charm, divination etc. as "ritual"), as described in Table 5. Some cultures had many different uses categorized from keyword mentions; roughly a quarter (19 cultures, of which 13 were huntergatherers) had no relevant uses despite the mention of the keyword in a paragraph. A small number of uses crossed multiple categories (e.g. medicine and ritual, counting and social coordination). The final judgement considered the immediate functions to be primary. There is a very strong magic power in these stones, and their form is regarded as variable. For instance, if such a stone is left uncovered it may grow legs and run away; so they are worn hanging from the neck in little sacks.

[Aranda]

Games

Used as game tokens, or in other childhood activities

Thirty-three stones are arranged in a line which may be straight, but is more often curved or crooked. The player must touch each stone and say "Too" for all 33 stones in order, at a high rate of speed. [Kpelle] Tool Used in hunting, fishing, craft, headshrinking, or as weights Dressing a hide is generally woman's work; sometimes men do this work, using woman's tools, which include a small sack of pebbles held behind the haft of the leg bone scraper to give added force to each push of the scraper removing fat and detritus from the hide.

[Innu] Weapon Stones explicitly thrown at other people in defence or offence

The sling stones were not worked but were selected from waterworn pebbles (ala) of appropriate size.

[Samoan] Medicine Primary functional use given in a medical remedy

Pour over the head of the sufferer an infusion of bark from the top of the trunk of the Bugare. But before pouring the liquid, it must be frothed. This is done by throwing little stones in and mixing it with a stick.

[Azande]

Music

Used to make musical instruments or noisemakers (including ritual functions)

A woman's dance rattle contains many small pebbles and the handle passes entirely through the gourd.

[Kuna]

Counting Explicit mention as a counting tool Pebbles are occasionally employed as a mnemonic aid to counting warp threads when weaving; each pebble represents 10 threads.

[Aymara]

Other

Various uses as ornament, social coordination, genealogical tool etc He talks about pedigrees. Our expert … gathers a heap of small stones and says: "This is Ancestor Bangáchon and this is Gáyon, his wife". Meanwhile he places two small stones, side by side in front of his big toes. "They had four children", he continues... and four stones are put at a short distance of the two first ones.

[A]nd so it goes on; names and more names, stones and more stones..

. [Ifugao] None

Paragraphs contained the words stone* or pebble* but no relevant uses

In the full sample, ritual uses (as charms, divination, or other magico-religious functions)

were the most commonly described (n=44), followed by games (n=25) and music (n=18). A range of other uses accounted for the remainder. Hunter-gatherer societies differed in the most common use (none, n=13) but followed the trend for music (n=11), ritual (n=10) and games (n=9) as most common (Table 6). Eating/ingestion as one hypothesized source of shine for pebbles was not noted in the sample; some medicinal uses mentioned pebbles placed in a shaman's mouth but then removed [START_REF] Chewings | Clark GA. Research traditions in paleoanthropology: An archaeological perspective[END_REF]. Table 1 in the supplementary materials gives full information including the bibliographic sources, paragraph text, and main subsistence, for each case. A bag for carrying is mentioned in four cases; polish is mentioned in one. Separate counts are given for the 38 hunter-gatherer cultures of the full sample.

'Multiples' indicates if pebbles were found or inferred to be in groups of four or more, with percentages of Yes:No:Unclear (rounded to nearest integer), and it makes it clear that uses of multiples pebbles together is largely dominant.

Discussion

Polished pebbles have been found repeatedly at archaeological, including ancient, sites [START_REF] Boivin | Sensual, material, and technological understanding: exploring prehistoric soundscapes in South India[END_REF][START_REF] Moncel | Non-utilitarian lithic objects from the European Paleolithic[END_REF]. Few are the cases, however, in which complementary analyses were conducted to eliminate the possibility of a natural origin of the polish or that, the case of Landry, provide contextual information to assess their nature. A remarkable exception are the polished pebbles found at the Jiahu site, Henan Province, China [START_REF] Zhang | Oldest playable musical instruments found at Jiahu early Neolithic site in China[END_REF]. At this site, the skeleton of a male individual with the skull missing, dated to c. 9,000 BP, is associated with two seven-hole bone flutes and a turtle shell with polished pebbles inside. The latter is interpreted as a musical instrument to produce a rhythmic sound.

The geoarchaeological study of the Landry site indicates that the spatial distribution of archaeological remains has not been marginally affected by post-depositional disturbances [START_REF] Brenet | The Late Solutrean open-air site of Landry (Aquitaine, France). A preliminary spatio-temporal analysis[END_REF]. Landry is currently the only site for which unequivocal information is available on the stratigraphic and the spatial provenance of the lustrous gravels, their spatial relationship to the other archaeological remains, and their cultural affiliation. The lustrous gravels are clearly associated with the other archaeological remains and dispersed within the archaeological layer, which make their attribution to the Solutrean undisputable. They are highly concentrated in an area interpreted as reflecting domestic activities connected to the use of metamorphic rocks. This area has also yielded few incised dolerite blocks and pebbles [START_REF] Brenet | The Late Solutrean open-air site of Landry (Aquitaine, France). A preliminary spatio-temporal analysis[END_REF]. For the archaeological sites excavated earlier, only the cultural and, in few cases, the stratigraphic provenance are known. Theoretically, the best available information on the gravel cultural attribution come from Fourneau du Diable (D. [START_REF] Peyrony | Rapport de D. Peyrony. Chargé de mission aux Eyzies-de-Tayac[END_REF] where

Peyrony is formal in attributing them to both the Gravettian and the Solutrean. The 136 gravels arranged one next to the other to form a rectangle (Fig 4) were apparently found in Gravettian layers, the others in layers attributed to the Upper Solutrean. Although left a drawing of this chest piece, it is difficult at present, in absence of additional information, to assess the reality of this surprising findings. At Laugerie Haute the gravels were also apparently retrieved in Upper Solutrean layers; at Pech de la Boissière and Casserole, they were recovered in Solutrean layers. Our spatial analysis has demonstrated that the distribution of lustrous gravels at Landry is not random nor uniform. They are concentrated in an area corresponding to a portion of the zone with the highest density of the other archaeological remains. On the contrary, the nonlustrous gravels are evenly distributed on the excavated area. Unlike the lustrous gravels, all made of white quartz, they are more variable in size, morphology and color. This finding contradicts the hypothesis that the distribution of the lustrous gravels is the result of a natural process and argues in favor of humans being the main or the exclusive accumulation factor.

Apparently Landry visitors selected small, round, white gravels and left or disposed them inside their main activity area. Although the lack of direct comparative samples makes it difficult to establish what criteria guided the choice of the gravels at the other archaeological sites, their comparatively low dimensional variability and roundness suggest these were certainly key selection criteria. It can be therefore reasonably inferred that humans were the major gravel accumulation factor at the other sites as well.

At all the analyzed sites, what makes the gravels peculiar is also their shiny appearance.

Since the mode of formation of the sedimentary layers is not different at Landry between the concentrations and the areas outside them, there is no reason for thinking that the site formation processes could have affected the gravels inside and outside the concentrations in a different way and create a gloss only on the former. No evidence of chemical alteration or a similar gloss is observer on the remainder of the archaeological remains found inside the concentrations of artifacts [START_REF] Brenet | fouille d'un campement paléolithique solutréen[END_REF][START_REF] Mesa | Interaction outillage en silex/macro-outils passifs en dolérite sur le site Solutréen de Landry[END_REF]. In addition, experimental polishing of quartz gravels from the Landry sieving rules out the possibility that the action of silt may produce the gloss observed on the gravels found in the archaeological concentrations. The hypothesis of gastroliths originating from animal carcasses can also be discarded since the lustrous gravels differ in size, morphology and surface condition from such gravels [START_REF] Tryon | Exotic minerals or ostrich gastroliths? An alternative explanation for some early evidence of hominin non-utilitarian behavior at Wonderwerk Cave, South Africa[END_REF][START_REF] Wings | Identification, distribution, and function of gastroliths in dinosaurs and extant birds with emphasis on ostriches (Struthio camelus)[END_REF].

This supports the hypothesis that differences in the appearance of gravels found inside and outside the concentrations are due to human agency. Roughness analysis has identified three significantly different types of surfaces: rough, typical of gravels collected in natural accumulations and in the Landry sieving; glossy, only found on archaeological gravels from sites excavated in the past; moderately glossy, characteristic of the Landry gravels associated with concentrations of cultural remains. Experimental tumbling of Landry gravels collected outside the archaeological concentrations has produced a shine absent in natural accumulations and similar to that of gravels found at archaeological sites excavated in the past and some lustrous gravels from Landry.

In summary, the roughness analysis of natural, experimental and archaeological gravels, supports the hypothesis that after being selected by their size and shape, they have been subjected to a human action that has strongly polished them. The wear could have been produced deliberately or be the unintended consequence of their use or transport. These three possibilities are not mutually exclusive. The differences in roughness observed between the lustrous gravels from Landry and other archaeological sites may be due to: the fact that only the most lustrous gravels were collected in early excavations; the fact that at Landry non-or low-gloss gravels were naturally present in archaeological concentrations, which created a sample that included both lustered and non-lustered gravels; the fact that the activity that lustered the gravels at Landry was not as extensive as at other sites; or the fact that due to their mineralogical composition, the Landry gravels could not reach the degree of lustering observed at other sites, where the gravels are composed of other rocks, while being subjected to the same modifications. Selective collection of gravels from older excavations is possible, but two of the collections examined were from recent excavations in which the sediment was systematically sieved with water. The degree of luster of the gravels from these sites is not substantially different from that observed in the ancient collections, which contradicts the hypothesis of selective gathering. The possibility that the rock of which they are composed may determine the lower luster of the Landry gravels seems partly contradicted by the fact that the experimentally tumbled gravels, composed of quartz, attained a degree of luster similar to that observed on the lustered gravels from other archaeological sites. Although it is difficult to rule out the possibility that some of the discarded factors may be a contributing factor to the differences in luster between Landry and the other sites, the most likely hypothesis appears to be that the luster on the Landry gravels is either the result of the same action to which the gravels from the other sites were subjected, but for a shorter period of time, or the result of a slightly different action. The fact that experimental tumbling of gravels produced wear that was comparable to that of archaeological gravels suggests that prolonged contact with a soft material such as hide was the cause of the luster. The experimental results also indicate that variations in the treatment of soft materials in contact with the gravels may be responsible for the differences in roughness observed between sites. The long duration of the experiment necessary to produce the luster seems to contradict the hypothesis that sporadic contact with soft material or human skin, for example when handling the gravels, can produce such a developed luster.

These results do not allow us to identify with certainty the action to which the gravels were subjected to acquire the luster and it is possible that this action tells us little about the function for which the gravels were intended. The luster may, for example, be the result of repeated transport of the gravel from one site to another in leather containers, but this does not tell us the function for which the gravel was intended once the Solutreans arrived on site, as this function may have produced little or no luster. In reverse, the luster could be the direct consequence of the function of the gravel. In this case it could have been produced deliberately by tumbling the gravel into a leather bag to then exploit its lustrous appearance, for example in an adornment device as suggested by Peyrony, or unintentionally, for example by tumbling the gravel into a leather container to produce a rhythmic sound. These last two cases do not eliminate the possibility that the transport of gravels from one site to another may also have contributed to the production of the luster. The likely multi-causal nature of the luster and its variation between sites, as well as the potential equifinality of different process, which could produce the same luster, reduces the likelihood that further experimentation will be able to identify precisely the action that produced the luster, let alone the function of the gravels. One cannot rule out, however, the possibility that future experiments will identify more precisely the material to which the archaeological gravels have been in contact during their social life.

It is worthwhile for this reason to turn to the ethnographic and archaeological record. The former may inform us on the uses of gravels in different traditional human cultures. The latter may identify cases in which contextual information would allow us to understand the function that gravels had in past populations. Both records open the possibility of analyzing in the future, with the same methodologies applied in our study, ethnographic and archaeological gravels and compare the results of these analyses with ours.

Our ethnographic survey shows that the most common functions for small pebbles are magico-religious ritual uses, and as game pieces. Worldwide, pebbles are used as talismans, amulets, and charms [START_REF] Bunzel | Introduction to Zuñi ceremonialism[END_REF][START_REF] Fortune | Manus religion: an ethnological study of the Manus natives of the Admiralty Islands[END_REF][START_REF] Róheim | The eternal ones of the dream: a psychoanalytic interpretation of Australian myth and ritual[END_REF] (see SM Table 1 for examples), appear in divination or sorcery kits [START_REF]The head-hunters of Western Amazonas: the life and culture of the Jibaro Indians of eastern Ecuador and Peru[END_REF][START_REF] Little | The Mende of Sierra Leone[END_REF], and often embody spirits and ancestors [START_REF] Lehtisalo | Sketch of a mythology of the Yurak Samoyed[END_REF]Olson et al., 2001;[START_REF] Preuss | on the Huichols. people of the peyote: huichol indian history, religion, & survival[END_REF].

When used in games, pebbles are most often in multiples and handled frequently, including by children [START_REF] Anderson | Children's play and games in rural Thailand: a study in enculturation and socialization[END_REF][START_REF] Junod | The Life of a South African tribe: vol. 1. 2d edition revised and enlarged[END_REF][START_REF] Opler | An Apache life-way: The economic, social, and religious institutions of the Chiricahua Indians[END_REF]Taylor, 1962). The use of gravels in musical instruments mimics experimental tumbling and can obviously produce the same effects: in our survey we noted drum/rattle containers made from dried fruit [START_REF] Larken | An account of the Zande[END_REF], cocoons [START_REF] Kaufmann | The ?Auin: a contribution to the study of the Bushmen[END_REF], animal shells (Birket-Smith & De Laguna, 1938), birch bark [START_REF] Skinner | Notes on the eastern Cree and Northern Saulteaux[END_REF], as well as hides and gourds [START_REF] Gayton | Tulare Lake, Southern Valley, and Central Foothill Yokuts[END_REF]. Further uses include tools or weapons, elements in medicinal treatments, ornaments, and in counting (see SM Table 6 for examples). All these functions involve the transport of gravels, which can produce their gradual polishing. A use for games or shamanism/divination also implies a more or less prolonged handling, which can contribute to their polishing-possibly, in the case of games, from hide/leather surfaces on which pieces are scattered. Some of these functions (games, music, medicine, and counting) are more consistent with the use of many gravels than others (weapons, tools and 'other' uses are often single items), but ritual uses can involve single or multiple stones with equal likelihood (Table 6).

While many of the ethnographically attested functions imply a choice of gravels on the basis of size, shape, weight and raw material criteria, exact remarks on size were uncommon in the sources. Anthropologists occasionally noted qualitative features such as color or shape e.g.

"marble-like stones" [START_REF] Westermann | The Shilluk People, Their Language and Folklore[END_REF], "preferably those of a black color" [START_REF] Teit | Field notes on the Tahltan and Kaska Indians: 1912-1915[END_REF].

However, these remarks were too infrequent (only 12/152 cases) to support generalizations regarding specific functions for the archaeological gravels studied in our work. It is however possible that the smaller and rounder gravels from the Landry and Casserole sites had another function than the larger gravels of the Fourneau du Diable, Laugerie Haute and Pech de la Boissière. However, this is only partially consistent with the result of the textural analysis, which rather highlights a difference between Landry and the other sites. By ethnographic analogy, ritual and gaming uses are most likely, with the uniformity of the gravels indicating a deliberate aesthetic preference. Although our cross-cultural survey indicates the use of gravels in counting systems is relatively uncommon (in our sample, 3 of 128 uses or 2.5%), their use for other functions (games, ornaments, divination etc.) certainly implies their approximate and, especially in the case of games, possibly precise quantification. In both cases, the creation of a cultural innovation involving the use of small, visually homologous objects may have played an adjunct role in the complexification of modalities for precise quantification (verbal, finger counting, notational) or may have been a reflection of contemporary or prior advances in these fields [START_REF] Bender | Numeration systems as cultural tools for numerical cognition[END_REF][START_REF] Rosso | Patterns of change and continuity in ochre use during the late Middle Stone Age of the Horn of Africa: The Porc-Epic Cave record[END_REF][START_REF] Núñez | Terrestrial ecoregions of the world: a new map of life on earth a new global map of terrestrial ecoregions provides an innovative tool for conserving biodiversity[END_REF].

Conclusion

The results of our study indicate that there are means to distinguish objects modified by natural processes from artifacts minimally modified by humans. Use-wear analysis has been widely applied to prehistoric artefacts to identify their function. However, this approach has been implemented on artefacts recognized as the outcome of human agency. In our case, instead, we have faced the challenge of identifying such agency exclusively from surface modification. The chosen means to pursue this endeavor involved evaluation of contextual data, creation of natural and experimental frames of reference, and characterization of natural, experimental and archaeological material using adapted analytical tools. These analyses leave no doubt: the lustrous gravels discovered in several Gravettian and Solutrean sites in southwestern France were lost or abandoned in these sites after having been selected on the basis of size and shape criteria and having been involved in one or more specific activities that produced their lustrous appearance. These gravels were not discovered in older Gravettian layers during the numerous excavations conducted in the region despite the fact that many of these excavations were conducted in recent times, with modern excavation techniques and by archaeologists aware of the possibility of discovering glossy gravels in Upper Palaeolithic archaeological layers. This leads us to conclude that the activity in which the gravels were involved, which is impossible to identify precisely but could involve a long-time use or curation in a leather bag, reflect a cultural innovation that developed during the Gravettian period and continued into the Solutrean period at the least.

Fig 1 .

 1 Fig 1. Lustrous gravels discovered in several Palaeolithic sites in southwestern France. (A) Late Solutrean gravels from Landry; (B): Late Solutrean gravels from Laugerie Haute West; (C) Late Solutrean gravels from Laugerie Haute East; (D) Solutrean gravels from Pech de la Boissière; (E) Solutrean gravels from the Abri Casserole; (F) Gravettian gravels from Fourneau du Diable; (G) Solutrean gravels from Fourneau du Diable and (H) Late Solutrean from Fourneau du Diable.

Fig 2 .

 2 Fig 2. (A) Geographical map of France locating the Solutrean site of Landry in Dordogne (yellow star), the archaeological sites (red dots) and the natural sites (green dots). (B) Synthetic stratigraphic profile and location of OSL dated samples to the west of the excavated area, according to Bertran [modified after [20]].The topography of the site slopes gently toward the Isle River and masks an alluvial terrace. Aeolian silts cover the layer on which the Solutrean hunter-gatherers settled. (A) Map created with QGIS 3.30.2 "s-Hertogenbosch", background maps created with Natural Earth. Free vector and raster map data @ naturalearthdata.com.

Fig 3 .

 3 Fig 3. Spatial distribution of flint and metamorphic rock artifacts highlighting the 5 different activity sectors and size clusters (Brenet et al., 2018).

Fig 4 .

 4 Fig 4. Drawing by Denis Peyrony of the gravels discovered at Fourneau du Diable (D. Peyrony, 1922). Retranscription: « Les autres devraient être juxtaposées à l'intérieur. Fig. 13 -Disposition des 136 petites billes trouvées dans la couche inférieure du gisement du Fourneau ». Translation: "The other 153 should be juxtaposed inside. Fig. 13 -Arrangement of the 136 small balls found in the lower layer 154 of the Fourneau site".

  5). The gravels from natural accumulations were collected by two of us (LG and MB) in 2018 (Fig 2A and 5). They come from several Mediterranean beaches located in the Hérault, dated from the Pliocene, Cap d'Agde beaches, Maguelone beaches, Oligocene alluvial terraces located at Croignon, Gironde and Upper Cretaceous alluvial terraces close to Campagne, Dordogne.

Fig 5 .

 5 Fig 5. Examples of gravels from natural sites. (A) Gravels from the tertiary terraces of Campagne (Dordogne, France); (B) marine gravels from Maguelone (Hérault, France); (C) marine gravels from Cap d'Agde (Hérault, France) (D) gravels from the Quaternary alluvium and vineyards of Croignon (Gironde, France) and (E) gravels from the Landry's sieving. These gravels were selected and collected according to previous observations of the Landry gravels and before having had knowledge of the lustrous gravels of the other archaeological sites whose raw material is visibly different. Scale = 1 cm.

Fig 6 .

 6 Fig 6. Presentation of the three experiments. (A) Material used for experiment 1, the animal skin used and the cylinder of the polishing drum. (B) Material used for experiment 2, the ochred and greased animal skin. Illustration of the chaîne opératoire reproducing an ochre tanning: obtaining the ochre powder and applying the ochre/grease paste on the hide. (C) Equipment used for experiment 3, the soil polish. A mechanical polisher covered with silt collected on the Landry site. Presentation of the manual (bottom left) then mechanical (bottom right) method of gravel gripping.

  //www.ehrafworldcultures.yale.edu/). A text search on ["pebble* OR stone*" NOT subject:building and construction] was performed (the term "gravel*" yielded only 15 descriptions of material used for paths). The search was restricted to the 186 Standard Cross Cultural Sample (SCCS) cultures, a subset of eHRAF with the most detailed and robust ethnographic descriptions. This yielded 23022 paragraphs across 175 cultures. The eHRAF Outline of Cultural Materials (OCM) has subject codes tagged to each paragraph. The top five for this search were Sacred Objects and Places, Prayers and Sacrifices; Mythology; Literary Texts; Spirits and Gods. Utensils, Tools, Weapons, and Burial Practices were in the top 20.

Fig 7 .

 7 Fig 7. Graphics presenting the results of the spatial analysis. (A) Count of gravels in the Landry grid. This area corresponds to sector 1 (squares columns I through J), sector 2 (columns K through P), and sector 3 (columns P through V). (B) Simulation of a uniform spatial distribution of gravel on sector 1, 2 and 3. (C) Distribution of dispersion indices corresponding to a uniform distribution (500 random draws) and on the right in purple, dispersion index associated with real glossy gravel's distribution.

Fig 8 .

 8 Fig 8. Spatial distribution observed on sector 3 of different flints tools and lustrous gravels (A) and of the latter and metamorphic stone tools (B).

Fig 10 .

 10 Fig 10. Comparison of gravel length (A), width (B) and circularity index (C) for each site. The circularity index is close to 1 for a round object and 0 for an object of more complex morphology.

Fig 11 .

 11 Fig 11. Representation of gravel color according to the parameters L* (luminescence, 0 = black, 100 = white), a* (green-red axis) and b* (blue-yellow axis). The two parameters range from +127 to -128.

Fig 12 .

 12 Fig 12. Macroscopic (left) and microscopic (right) photos of gravels from archaeological sites illustrating the lustre on surfaces. (A-B) Gravel from Landry; (C-D) gravel from Casserole and (E-F) gravel from Fourneau du Diable.

Fig 13 .

 13 Fig 13. Macroscopic (left) and microscopic (right) photos of gravels from natural sites illustrating the variability of surfaces. (A-B) Gravel from Campagne; (C-D) gravel from Croignon; (E-F) gravel from Cap d'Agde and (G-H) gravel from Maguelone.

Fig 14 .

 14 Fig 14. Macroscopic (left) and microscopic (left) photos of experimental gravels. (A-B) Gravel from the experiment 1 processing the animal skin; (C-D) gravel from the experiment 2 processing the ochred skin with animal fat; (E-F) gravel from experiment 3 processing wet silt from Landry.

  Fig 15. (A) The distribution of Sq (µm) parameter, (B) the distribution of the Sdr (%) parameter and (C) the distribution of Ssk parameter.

Fig 17 .

 17 Fig 17. Principal component analyses taking into account morphometric, roughness and colorimetric variables. The PCA in (A) and (B) compares natural, experimental and archaeological gravels. Gravels from Landry and those retrieved during the sieving of the sediments from this site are plotted separately. The PCA in (C) and (D) compares experimental gravels with those found at each archaeological sites. Dots correspond to one of the four measurements performed on each gravel: 35 natural, 69 archaeological and 22 experimental gravels.

  accumulations(Fig 18). Gravels tumbled in a barrel coated with untreated animal skin reached Sq values not substantially different from those recorded on gravels tumbled in a barrel covered with skin treated with ochre and fat. Similar Sq values are also seen on facets modified by abrasion against Landry silt. Wear produced by tumbling with untreated skin and abrading on silt, however, display a more intense gloss, reflected by lower Sdr values. Both tumbling with treated and untreated skin and without ochre and fat produce a smoothness comparable to that recorded on archaeological gravels from Fourneau du Diable, Casserole, Pech de la Boissière and Laugerie Haute and stronger than that measured on Landry lustrous gravels. The roughness values recorded on experimentally tumbled and abraded gravels fall outside the variability of gravels from natural accumulations.

Fig 18 .

 18 Fig 18. Three-dimensional visualization of the surfaces before and after experimentation. Process 1 (gravels used on animal skin) has erased the reliefs in favor of hollows and valleys and the polish is more homogeneous. (A) Shows the 3D surface of the experimental gravel before the experiment and (B) shows the surface of the same gravel after the experiment. The picture to the right is taken with petrographic microscope and shows the surface after the process. Process 2 (gravels used on ochred and greased animal skin) has smoothed the surface leaving some hollows and valleys (in blue). (C) Shows the 3D surface of the experimental gravel before the experiment and (D) shows the surface of the same gravel after the experiment. The picture to the right is taken with petrographic microscope and shows the surface after the process. Process 3 (gravels polished with silts from Landry) is the only one to produce striations. (E) Shows the 3D surface of the experimental gravel before the experiment and (F) shows the surface of the same gravel after the experiment. The picture to the right is taken with a petrographic microscope Leica 2055P. All treatments produced a strong downward shift on the Z scale.

Table 1 . Contextual data on Upper Palaeolithic sites with lustrous gravels.

 1

	Site	Type	Culture	Ages	Layer	NB	Sedimentary	Excavator(s)	Year(s)
				(BP)			processes
	Landry	Open air Late Solutrean 21000	Upper level, sector 3 106	Solifluxion, acid	Brenet	2012
							attacks
	Fourneau du	Rock-	Gravettian	29000-	Lower terrace	40	Screes before/after	Peyrony	1912-1929
	Diable	shelter		22000			occupations, erosion,
							colluvium,
							sedimentary
			Solutrean	26000-	Lower terrace	16	reorganization		1912-1929
				23000
			Late Solutrean 24000-	Upper terrace	409		Baumann	2015
				23000
	Pech de la		Solutrean	26000-	No information	30	Screes, surface	Peyrony	1929-1934
	Boissière			23000			runoff
	Casserole		Solutrean	23300-	No information	14	Block collapse,	Kelley,	1939,
				20500			trampling,	Chadoune, Breuil,	1991-1992
							percolation,	Dutrain
							bioturbation,
							gelifraction
	Laugerie		Late Solutrean 23500-	East sector, layer 25 19	Block collapse,	Lartet-Christy,
	Haute			19600			trampling,	Massenat-Girod,
							percolation,	Capitan, Peyrony,
							bioturbation,	Bordes, Smith
							gelifraction

  These gravels are kept in the Musée Fernand des Moulins in Brantôme, Dordogne, France. New excavations, conducted by[START_REF] Baumann | Le gisement préhistorique du Fourneau du Diable (Bourdeilles, Dordogne). Rapport de fin d'opération annuelle, Gironde, UMR 5199-PACEA[END_REF], led to the discovery of 465 new lustrous gravels(Fig 1). They were all recovered when sieving sediment from Gravettian and Solutrean layers. Forty gravels come from Gravettian layers and 16 from Solutrean layers, both located in the lower terrace. As many as 409 gravels were discovered in Late Solutrean layers located in the upper terrace. Gravels from this last excavation are curated in the Musée National de Préhistoire, Les-Eyzies-de-Tayac, Dordogne, France.

). The pattern was interpreted by Peyrony as the remnant of a chest piece made of perishable material on which they were originally attached.

Table 2 . Known natural processes producing luster on gravels. Wind erosion Water Ground or ice polishes Chemical alterations Animals Environments

 2

		Desert, coast,	River, sea		Periglacial region		Soil	Various (sea lions,
		periglacial						galliformes, birds)
		region,
		mountains
	Phenomena	Abrasion		Transport	(rolling,	Solifluxion	,	Patina,	Geophagy, attack of
				saltation, suspension),	cryoturbation,		corosion	gastric	juices,
				sedimentation		gelifluxion		abrasion-polishing by
									organic or vegetable
									elements
	Macroscopic	Shiny	facets,	Smoothed and shiny	Unworn shiny pebbles	Gastroliths	(small
	stigmata	sinuous ridges	pebbles		with	accentuated	-	matted gravel)
						reliefs
	Microscopic	Wide grooves	V-shaped	impact	Micro-polished,		Polished	surfaces,
	stigmata	with preferential	cupules, preferentially	conchoidal fractures,	microstriations
		orientation	oriented		parallel	striations,	-
		along the wind	microstriations,		circular	grinding
		direction		conchoidal fractures	traces, impact cupules
	Data from (Abrahams

Table 3 . Number of gravels measured in the different approaches of this study for each archaeological sites including those collected in the sieve refusals, different natural environments and the experimental gravels.

 3

		Concretions	Morphometry	Colorimetry	Surface texture analysis
	Archaeological site
	Landry	11	102	102	41
	Casserole	1	14	14	4
	Fourneau du Diable	288	465	465	11
	Pech de la Boissière	2	9	9	3
	Laugerie Haute East	2	19	19	5
	Laugerie Haute West	3	18	18	5
	Natural gravels
	Landry from sieving		42	42	22
	Cap d'Agde		2	2	2
	Campagne		3	3	3
	Croignon		3	3	3
	Maguelone		5	5	5
	Experimental

gravels Process 1: gravels used on animal skin 10 0 10 Process 2: gravels used on ochred and greased animal skin

		10	0	10
	Process 3: gravels polished with silts from Landry	2	0	2

Table 1

 1 in a dull appearance. The lustrous gravels from Landry are significantly rougher than those found in the other archaeological sites (p < 0.05) (see SM Table1). They are, on the contrary, not significantly smoother (p > 0.05) (see SM Table1) than those from natural sites but significantly smoother than those recovered at Landry outside the archaeological concentrations. This intermediate state of surface for the Landry lustrous gravels

	)

complex surfaces

(Fig 16)

. The positive reliefs of the micro-topography appear erased, leaving a flat surface with hollows and valleys

(Fig 16C)

. This wear pattern gives these gravels a strong shine. In contrast, the gravels from the Landry sieving and from natural sites have rougher surfaces, resulting

Table 5 . Coding scheme for pebble use with representative examples.

 5

	Use	Definition		Example
	Ritual	Use is given as charms, amulets,
		religious symbols, curse stones,
		embodiments	of	spirits/gods,
		funeral tokens; or otherwise used as
		a magic ritual

Table 6 . Uses of pebbles in 82 cultures of the Standard Cross Cultural sample.

 6

	Use	Full sample	Hunter-gatherers	Multiples (%)
		(n=82)	(n=38)	Yes No Unclear
	Ritual	44	10	39	39	22
	Games	25	9	84	12	04
	Music	18	11	100 00	00
	Tool	12	3	67	25	08
	Weapon	11	4	64	18	18
	Medicine	9	5	78	22	00
	Counting	3	1	100 00	00
	Other	10	5	40	30	30
	None	19	13		NA

Acknowledgments

We thank Célia-Fat-Chung for collecting and conditioning the Landry gravels during sediment sieving, Catherine Cretin, Peggy Jacquement, Stéphane Madeleine and Bernard Nicolas for facilitating our analysis of archaeological material kept at the Musée National de Préhistoire, Les-Eyzies-de-Tayac, Malvina Baumann and Luc Detrain for kindly communicating information on the gravels from Fourneau du Diable and Abri Casserole, Véronique Laroulandie for giving access to gastroliths. We would also like to Frédéric Santos for coding the Berry's permutation function and Anthony Sécher for assisting us

Author's contributions

Lila Geis: conceptualization, methodology, formal analysis, investigation, visualization, writingoriginal draft. Francesco d'Errico: conceptualization, methodology, formal analysis, investigation, writing-original draft, supervision, funding acquisition. Fiona M. Jordan: methodology, formal analysis, investigation, writing-original draft. Michel Brenet: excavation, funding acquisition. Alain Queffelec: conceptualization, methodology, formal analysis, investigation, visualization, writing-original draft. All authors have read the manuscript and the submitted manuscript and contributed comments. during statistical analyses. We are grateful to Morgan Collard for his help with tumbling experiments. The archaeological excavation of Landry was financed by SITA Suez and the operation of sieving and sorting the archaeological sediments was financed by the SRA (Service Régional d'Archéologie) of the Nouvelle Aquitaine.

Supplementary materials

R script and data to produce figures 7-11, 15 and 17, and supplementary tables 1 and 2 are available online at the OSF repository https://osf.io/9wpn7/.