palaeogeography, palaeoclimatology, palaeoecology · the paleoecology and taphonomy of amk (bed i,...

Palaeogeography, Palaeoclimatology, Palaeoecology 488 (2017) 35–49

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Palaeogeography, Palaeoclimatology, Palaeoecology

j ourna l homepage: www.e lsev ie r .com/ locate /pa laeo

The paleoecology and taphonomy of AMK (Bed I, Olduvai Gorge) and itscontributions to the understanding of the “Zinj” paleolandscape

Julia Aramendi a,b,⁎, David Uribelarrea c, Mari Carmen Arriaza b,d, Héctor Arráiz a,e, Doris Barboni e,José Yravedra a,b, María Cruz Ortega f, Agness Gidna b,g, Audax Mabulla g,Enrique Baquedano b,h, Manuel Domínguez-Rodrigo a,b

a Department of Prehistory, Complutense University, Prof. Aranguren s/n, 28040 Madrid, Spainb IDEA (Institute of Evolution in Africa), Covarrubias 36, 28010 Madrid, Spainc Departamento de Geodinámica, Universiadad Complutense, Avda. Complutense, Madrid, Spaind School of Geography, Archaeology and Environmental Studies, Evolutionary Studies Institute, University of the Witwatersrand, Private Bag 3, 2050, South Africae CEREGE (UMR6635 CNRS/Université Aix-Marseille), BP80, F-13545 Aix-en-Provence cedex 4, Francef Universidad Complutense de Madrid-Instituto Carlos III (UCM- ISCIII), Centro de Investigacion de la Evolucion y Comportamiento Humanos, Madrid, Spaing Archaeology Unit, University of Dar es Salaam, P.O. Box 35050, Dar es Salaam, United Republic of Tanzaniah Museo Arqueológico Regional, Plaza de las Bernardas s/n, 28801, Alcalá de Henares, Madrid, Spain

⁎ Corresponding author at: Department of PrehistoryAranguren s/n, 28040 Madrid, Spain.

E-mail address: [email protected] (J. Aramen

http://dx.doi.org/10.1016/j.palaeo.2017.02.0360031-0182/© 2017 Elsevier B.V. All rights reserved.

a b s t r a c t
a r t i c l e i n f o
Article history:Received 2 December 2016Received in revised form 24 February 2017Accepted 24 February 2017Available online 1 March 2017

AMK (Amin Mturi Korongo) is a newly discovered site situated under Tuff IC (Bed I, 1.84 Ma). It contains severalfossiliferous levels and the top one is situated on the same paleosurface as FLK-Zinj. For the first time this allowssampling the “Zinj” paleoenvironment well into the Secondary Gorge and expands the known area of thispaleolandscape. Fossils found at this site show exceptional preservation. Several articulated units have been dis-covered, indicating minimal postdepositional disturbance and rapid sedimentation. This assemblage allows ageneral estimation of time span (the most elusive variable in archaeological analyses) for the formation ofAMK. Phytolith analyses have discovered a dense palm forest at the site, expanding the forested area knownon the slightly elevated platform that contains the FLK-Zinj – FLK-NN – PTK sites. Although a few artifacts havebeen discovered in the vicinity of AMK, the site was mostly naturally (i.e., non-anthropogenically) formed. Thisis of major relevance to determine that factors other than forested habitats must have influenced the formationof anthropogenic sites on the same platform as AMK in the Olduvai lacustrine basin.

© 2017 Elsevier B.V. All rights reserved.

Keywords:Natural background scatterCarnivoreFLK-Zinj, PTKSite formation

1. Introduction

Since the discovery of the FLK-Zinj site (Bed I, Olduvai Gorge, Tanza-nia), several interpretations of it have been presented to understandhominin behavior around 2 Myr ago. This site was first interpreted asa living floor (Leakey, 1971), which according to Isaac (1978)corresponded to the selection of a specific location for toolmaking,butchery, and collective food consumption activities by Early Pleisto-cene hominins. A subsequent analysis of the site by Bunn (1982) ledIsaac (1983) to propose a new model (“central-place” foraging) forthis type of sites. In this model, hominins appeared as primary agentsin the acquisition of animal carcasses. In addition to raw materials,such carcasses were repeatedly brought to selected specific spots onthe landscape to be collectively exploited. This main idea presented in

, Complutense University, Prof.

di).

the “central-place” foraging model about carcass transportation hasbeen supported by several studies during the last decades (Bunn andKroll, 1986, 1988; Potts, 1988; Bunn and Ezzo, 1993; Oliver, 1994;Blumenschine, 1995; Capaldo, 1995, 1997; Rose and Marshall, 1996;Domínguez-Rodrigo, 1997; Domínguez-Rodrigo and Pickering, 2003).However, the kind of access that hominins might have had to thosetransported carcasses has been a source of debate for several years.The accumulation at FLK-Zinj has been interpreted as the result ofhominins acting as primary agents hunting and selectively transportingcarcasses (Isaac, 1978, 1983; Bunn, 1982, 1983, 1991; Bunn and Kroll,1986, 1988; Bunn and Ezzo, 1993; Oliver, 1994; Rose and Marshall,1996; Domínguez-Rodrigo, 1997; Domínguez-Rodrigo and Pickering,2003; Bunn and Pickering, 2010), but also as the result of hominins act-ing as secondary agents, either transporting previously partiallydefleshed carcasses (Capaldo, 1995, 1997) or scavenging bones fromdefleshed felid kills (Blumenschine, 1986, 1991). Most recent tapho-nomic analyses have provided compelling evidence that homininsmust have had primary access to fully-fleshed small and medium-sized carcasses (Domínguez-Rodrigo et al., 2007, 2010b, 2014).

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36 J. Aramendi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 488 (2017) 35–49

In addition to taphonomic evidence, geological and paleoecologicalstudies have been essential for the understanding of the site and the re-lated hominin carcass consumption behavior. The geological sedimentsaround FLK have long been known to have been dominated by lake-margin facies, corresponding to a shallow lake with predominantlyfine-grained sedimentation (Hay, 1976). According topaleoenvironmental reconstructions, FLK-Zinj occupied a location onthe southern edge of an elevated platformby the lake floodplain charac-terized by the presence of a mixed landscape with densely woodedareas (Ashley et al., 2010a; Leakey, 1965; Bonnefille, 1984; Plummerand Bishop, 1994; Sikes, 1994; Spencer, 1997; Fernández-Jalvo et al.,1998; Domínguez-Rodrigo et al., 2010b). Phytoliths analyses showedthat around FLK the vegetation was dominated by palm trees, whichwould have provided shelter from predation and easy access to water(Ashley et al., 2010a). Such a landscape concurs with the taphonomicwork carried out on the paleolandscape surrounding FLK-Zinj. The exca-vation of several trenches as well as the excavation of nearby sites suchas FLK-NN, showed that bone deposition in the vicinity of FLK-Zinj wassignificantly less dense and, further, that little to no evidence for carcassbutchering was preserved in the immediate surrounding environmentoutside the space containing the FLK archaeofaunal assemblage. Thissuggests that butchery was repeatedly carried out on the same spot,namely where FLK-Zinj was formed (Domínguez-Rodrigo et al., 2007,2010b; Uribelarrea et al., 2014). Thus, the site would have providedshelter from predation and easy access to water, both of which wouldhave encouraged prolonged stays by hominins at the site.

Only two newly discovered sites (PTK and DS) overlaid by Tuff ICand found on the samepaleosurface show similar bone and tool concen-tration rates to FLK-Zinj. DS (David's Site) was discovered in 2014 and iscurrently under study (see Domínguez-Rodrigo et al., submitted for fur-ther information). PTK (Phillip Tobias Korongo) preserves a clay stra-tum similar to that reported at FLK-Zinj, which contains twodifferentiated archaeological levels (Domínguez-Rodrigo et al., 2010b;Uribelarrea et al., 2014). The discovery of PTK proves two points: (1)there was no river channel crossing the area where the site is locatedas proposed by Blumenschine et al. (2012), and (2) there are particularand clearly distinguishable spots on the landscape that were selected byhominins to transport and utilize raw materials and exploit carcasses.Even though the research of the PTK and DS archaeofaunal assemblagesis in progress, preliminary data (i.e., abundance of hominin-modifiedbones) indicate that those sites were formed intentionally by hominins.

In the new paleoenvironmental reconstruction carried out by TOPPP(Uribelarrea et al., 2014), the Zinj paleolandscape appeared as a largearea subdivided into two subareas. To theWest, there was a raised plat-form,where themain Zinj complex sites are found (FLK-NN, FLK-N, PTK

Fig. 1. A: Location of Olduvai Gorge in Tanzania and the contemporary lowermost Bed I sites incorner: location of AMK in the Secondary Gorge and the closest anthropogenic sites FLK Zinj, PTPanoramic view of the four sites.

and FLK-Zinj) and to the East a shallow depression that extended to-wards the KK fault. In the model presented by Uribelarrea et al.(2014), the Zinj paleolandscape was further subdivided into four unitsbased on depositional energy, topography, and facies. One of theseareas was interpreted as a mudflat where the paleolake Olduvai was lo-cated and where several rivers discharged forming a well-establisheddrainage network upstream. The extension of the lake varied accordingto the rain and wet seasons. The archaeological sites are located nearbythe paleolake in a low energy depositional environment that corre-sponds to a lacustrine terrace; a relatively flat and elevated area. Higherin elevation than this terrace is the area where the site presented here(AMK) is located. This area corresponds to the supralittoral environ-mental belt, i.e. the external border of the lacustrine basin.

AMK (Amin Mturi Korongo) is a newly discovered site situatedunder Tuff IC (Bed I, 1.84 Ma) that contains several levels, the top claylevel being equivalent to the FLK-Zinj paleosurface (Fig. 1). AMK is situ-ated in the Secondary Gorge, at the eastern Olduvai paleolake alluvialplain (~500 m from FLK-Zinj and 200 m away from PTK). The site wasnamed AMK after the prominent Tanzanian archaeologist, Amin Mturi.A total of 16m2were excavated in 2013 and 2014, exposing several geo-logical units and three fossiliferous levels.

Here we present the taphonomic study of AMK and a more detailedgeological study of the site, including the results of new paleobotanicalanalyses, which together will contribute to a better understanding ofthe Zinj paleolandscape.

2. Methods and samples

2.1. Geological and paleobotanical reconstruction

AMK and its surroundings formed part of a previouspaleoenvironmental study focused on reconstructing the northern,eastern, and southern portions of the FLK-Zinj paleolandscape(Uribelarrea et al., 2014). The geological study was based on three dif-ferent techniques: a) stratigraphy and facies analysis, b) geomorpholo-gy, and c) structural tectonics (see Uribelarrea et al., 2014 formethodological issues). In this study we add a more detailed local geo-logical description based on the analysis of a stratigraphic cross-sectionunder Tuff ID at AMK.

The paleobotanical study rests on the analysis of phytoliths. To re-construct the paleovegetation, several samples were taken along theZinj landscape below Tuff IC (Arráiz et al., 2017). Two sampleswere col-lected in stratigraphic sections from AMK, right below Tuff IC, and werethen prepared for phytolith analyses as explained in Barboni et al.(2010). Here we show the paleobotanical results.

the junction of the Gorge. Upper-left corner: Digital Globe Foundation image. Upper-RightK and DS; the image was created using photogrammetric techniques using a dron. Below:

37J. Aramendi et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 488 (2017) 35–49

2.2. Orientation analyses

Orientation patterns of bones were taken into consideration to rein-force the geological and paleobotanical reconstruction of the site and toassess the impact of water flows on the documented faunal arrange-ment. Vertical and horizontal orientations were directly taken fromeach item using compasses and clinometers (Voorhies, 1969; Fiorillo,1991; Howard, 2007). Thesemeasurementswere taken along the longi-tudinal axis that divided the item more or less symmetrically and onlyin specimens that were at least twice as long as they arewide. This sym-metry axis has been proved to be taphonomically informative since longobjects tend to orient according to their longitudinal axes (Toots, 1965;Voorhies, 1969). Orientation data were statistically treated using the“circular” package in R software (http://www.r-project.org). Three dif-ferent tests were used to assess orientation patterns. A Kuiper's testand a Reyleigh's test were used to detect isotropy (Fisher, 1995),while a Watson test was performed to detect anisotropy. Statisticaltests were supplemented with graphical procedures. Orientation datawere displayed graphically using rose diagrams (ORIANA free soft-ware). Orientation analyses were only conducted for Levels 1 and 2,since sample size for Level 3 was too small. Levels 1 and 2 sampleswere bootstrapped 100 times each to ensure reliable results.

2.3. Taphonomic analysis

A total of 198 remains (including hominin and faunal fossils andlithics) were recovered at AMK. Stone tools and hominin bones werenot included in our analysis, so in the end 183 bone remains were ana-lyzed from a taphonomic perspective. Most fossils (N58%) presented agood cortical surface, some of them being exceptionally well preserved.Only around 17% of the remains were too deteriorated to be able to an-alyze bone surface modifications, but were included when possible tocomplete the skeletal part profiles and the study of taxa. Bad preserva-tion was mainly due to alterations related to soil humidity. Carbonates,mineralization and biochemical alterations related to bacteria are someof the most common non-biotic processes detected at AMK. A signifi-cant portion of the fossils discovered at AMK underwent some conser-vation treatments in order to ensure the preservation and enable thisstudy. Intervention strategies on the fossils undergo different stagesand always follow a criterion ofminimum intervention. Ourmain objec-tive for treatments applied at sites during excavation is the stabilizationof the fossils. The treatments applied in the field include preventativegauze coatings and/or the application of adhesive to surface areas; iso-lated adhesion of fragments to avoid their dispersion and/or loss; con-solidation to maintain cohesion and mechanical resistance; and theextraction with a packaging composed of tissue paper and aluminumfoil as a kind of frame. Treatments carried out in the laboratory includeunpacking, cleaning (removal of sediments and carbonatesmechanical-ly at the tip of scalpel and lathe and surface dirt with water and acetonein cases that are adhered and/or consolidated), consolidation, adhesionof fragments and/or reconstruction and packaging (Ortega et al., 2016).After having properly cleaned and treated the fossils, the AMK Levelswere studied independently. Level 1 includes 98 remains, Level 2 has70 and Level 3 contains 15 bone remains. Due to sample size, Level 3was excluded from the taphonomic analysis.We did not include themi-crofaunal remains in this study.

2.3.1. Skeletal part representationAll specimens were identified to element and carcass size whenever

possible. In order to analyze skeletal part profiles, carcasses were dividedinto three anatomical regions: skull (horn, cranium, mandible and teeth),axial (vertebrae, ribs, pelves, and scapulae) and appendicular (limbbones),with limbs further divided into upper (humeri and femora), inter-mediate (radii and tibiae), and lower (metapodials) units (Domínguez-Rodrigo, 1997). Pelves and scapulae were grouped with vertebrae andribs due to their similarity in bone texture and taphonomic properties

(Domínguez-Rodrigo et al., 2007). Skeletal profiles are based on the num-ber of identified specimens (NISP) and an estimate of theminimumnum-ber of elements (MNE). MNE estimates were based on an integrativeapproach that includes the analysis of shafts according to their bonesection, muscle insertion sites, cortical bone thickness, cross-sectionalshape, and medullary surface (Patou-Mathis, 1985; Munzel, 1988; Barbaand Domínguez-Rodrigo, 2005; Yravedra and Domínguez-Rodrigo,2009). Bone fragments were also identified according to orientation: cra-nial, caudal, lateral or medial.

Carcass size was classified using Bunn's (1982) methodology: smallcarcasses include sizes 1 (b25 kg, e.g. gazelle) and 2 (25–125 kg, e.g. im-pala), medium-sized carcasses are size 3 (125–350 kg, e.g. lesser kuduand zebra) and large carcasses are sizes 4 to 6 (350–1000 kg, e.g. buffa-lo; 1000–3000 kg, e.g. giraffe; N3000 kg, e.g. elephant). MNE estimationwas then used to calculate the minimum number of individuals (MNI),where criteria such as size, side, age and biometrics, usually used in acomprehensive analysis (Lyman, 1994), were taken into consideration.Age profiles were established based on unfused elements and dentalspecimens.

2.3.2. Breakage patternsBreakage planes were identified as green or dry following Villa and

Mahieu (1991) and were analyzed following the methods outlined byDomínguez-Rodrigo et al. (2007). The appearance of dry breakage pat-terns is associated with non-biotic processes such as geological forces(i.e., sediment compaction), while green fractures are related to the ac-tion of biotic agents (Pickering et al., 2005; Alcántara et al., 2006). Greenbreakage planes show smoother surfaces and are more likely to be ori-ented obliquely to the long axis of the bone. Green breakage can also besubdivided into patterns created by dynamic (impact using hammerstone) and static (pressure exerted by carnivore gnawing) loading ac-cording to the angle between the cortical and the medullary surface(Pickering et al., 2005; Alcántara et al., 2006). Dry breakage planeshave a rough texture characterized by the presence of micro-step frac-tures and tend to be longitudinal and/or transverse to the long axis ofthe bone, with a nearly 90° angle between the cortical and the medul-lary surface.

A complementary approach that allows the differentiation betweenbone-breakage processes (i.e., dynamic versus static loading) involvesthe study of notches. Experimental studies have shown that differenttypes of notchmorphologies can be identified and that their frequenciesrespond to the action of different agents (Capaldo and Blumenschine,1994; De Juana and Domínguez-Rodrigo, 2011). The identification ofnotches was performed as a proxy in the present study, following DeJuana and Domínguez-Rodrigo's (2011) classification; but no statisticalanalysis was performed since the sample was too small to obtain reli-able results.

Bone impact flakes were also examined. Bone impact flakes show asimilar morphology to lithic flakes and their frequency varies accordingto themodifying agent. Boneflakes aremuchmore common in anthropo-genic sites, whereas carnivores do not usually produce these kind of frag-ments, except for the hyenas, which do so in small numbers (Pickeringand Egeland, 2006; Domínguez-Rodrigo and Martínez-Navarro, 2012).

Shaft breakage patterns were analyzed following Bunn's (1982) cir-cumference classification, where Type 1 refers to shafts that preserveb50% of the circumference, Type 2 includes shafts with N50% of the sec-tion left, and Type 3 are complete cylinders. AMK circumference typesamong prey size 3 were compared with those found in the Syokimauhyena den (Egeland et al., 2008) and the Olduvai carnivore site (OCS),which has been proposed as a bone accumulation generated by lionsand scavenged by spotted hyenas (Arriaza et al., 2016). The three sitesare dominated by medium-sized bovids.

2.3.3. Bone surface modificationsSurface macroscopic and microscopic marks on bone surfaces were

studied and analyzed for each level independently. Marks were analyzed

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with the aid of hand lenses (15×–20×) under strong light (60 W) asoutlined by several authors (Blumenschine, 1988, 1995; Blumenschineand Selvaggio, 1988, 1991; Bunn, 1981; Domínguez-Rodrigo et al.,2009). The identification of the different surface modifications (e.g. cutmarks, percussion marks, tooth marks, biochemical marks) was carriedout following previous works (Binford, 1981; Bunn, 1981; Shipman andRose, 1983; Shipman et al., 1984; Blumenschine, 1988; Blumenschineand Selvaggio, 1988; Blumenschine et al., 1996).

In the case of AMK, special attention was paid to tooth marks sincethey were the most frequent modifications. Tooth mark morphology,size, frequency and anatomical location were recorded and analyzedfor each level independently. Modifications of the axial skeleton werealso carefully observed considering that felids and hyaenids alter verte-brae differently (Domínguez-Rodrigo, 1999; Parkinson et al., 2015).Among the four different classes of modifications usually left by carni-vores – pits, scores, punctures and furrowing (Bunn, 1981; Binford,1981; Blumenschine and Marean, 1993; Blumenschine, 1988, 1995) –pits (Andrews and Fernández-Jalvo, 1997; Selvaggio and Wilder,2001; Domínguez-Rodrigo and Piqueras, 2003; Andrés et al., 2012)and furrowing (Domínguez-Rodrigo et al., 2015) provide useful infor-mation in order to determine the kind of carnivore involved. Due tosample size, we only conducted a statistical analysis of pits (breadthand length). Pit dimensions documented at AMK were compared tothe most complete carnivore reference collection available (Andrés etal., 2012). Only the Maasai Mara hyena den and the lion sample wereused for the comparison since from the carnivores analyzed in Andréset al. (2012) these two species are the most likely agents at AMK dueto chronology and location of the site. Pits on medium-sized shaftswere selected in order to make AMK sample comparable to the dataavailable. Tooth pit dimensions were calculated using a two tailed con-fidence alpha value of 0.025, so that the variation of most of the sampleis represented as explained in Andrés et al. (2012).

2.3.4. Disarticulation patterns and site formationDue to the fossil spatial pattern detected during excavation, special at-

tention was paid to disarticulation patterns (Hill, 1979; Hill andBehrensmeyer, 1984), bone type and shape for potential transportabilityby physical processes (Voorhies, 1969; Boaz and Behrensmeyer, 1976).

Disarticulation patterns provide information about timeline exposure,especially if coupled with weathering stages (Hill, 1979; Hill andBehrensmeyer, 1984). Thepresence and/or absence of certain intact jointsin any given ungulate skeleton are indicative of the dismembering pro-cess. AMK Level 2was investigated for intact joints andweathering stagessince several complete elements appeared in anatomical connection.

Classic studies in vertebrate taphonomy (Voorhies, 1969) haveshown that there are some types of bonesmore prone to be transportedthan others. That means, that the predominance of certain bones mustbe taken into account in order to establish the autochthony orallochthony of a site (Boaz and Behrensmeyer, 1976).

The maximum length of each fragment was recorded to examinebone breakage patterns, size-sorting and possible preservation biasdue to postdepositional events, such as the presence ofwater or second-ary agents. Likewise, other macroscopic modifications such as polishingor abrasion (Thompson et al., 2011), biochemical modification (i.e.,fungi) (Marchiafava et al., 1974) and weathering (Behrensmeyer,1978) were also analyzed, since they can contain information aboutbiostratinomic processes and paleoenvironmental conditions. Further-more, they can also contain information of subaerial exposure times.

3. Results

3.1. Paleoenvironmental description

3.1.1. Geology and stratigraphyAMK is a Bed I site located in the Olduvai Secondary Gorge, 400 m

upstream from the Main Gorge (Fig. 1). The site is characterized by

lake-margin facies and consists of three fossiliferous levels. The upperfossiliferous level, Level 1, lies right below Tuff IC, dated around1.832+ 0.003Ma (Deino, 2012). According to previous paleoecologicalreconstructions of the FLK-Zinj paleolandscape (Uribelarrea et al.,2014), AMK is situated in the supralittoral area of the lake margin,very close to the southern limit of the basin. In comparison to other con-temporaneous and nearby sites like FLK-Zinj, FLK-N, PTK and DS, thearea occupied by AMK is less affected by lacustrine sedimentation pro-cesses and significantly influenced by external sediment inputs. As a re-sult, the local stratigraphic sequence is more complex than at the othersites.

The geological contextualization of the three fossiliferous levels ex-cavated at AMK was conducted based on a 1.5 m cross-section underTuff ID (Fig. 2). From bottom to top, the stratigraphic sequence beginswith a tuffaceous waxy-clay layer that corresponds to a lacustrine sedi-mentation process over a volcanic substratum of very fine-grained sed-iment (ash). Above this unit, a 50 cm layer of very fine volcanicreworked sands was deposited. These fine sands are pale yellow(2.5Y-8/2, according to theMunsell Soil Color Charts, 1994; revised edi-tion), and are interrupted by a) lenticular layers of coarse sands andsome 3–5 cm basalt fragments, and b) by a discontinuous and chan-nel-like level of coarse grey sands (5Y 6/1). In this unit, load cast,flame structures and bioturbation caused by roots are abundant, andtheir presence has notably modified the internal structure of the depos-it. In the uppermost 10 cm of this unit lies the oldest fossiliferous levelexposed at AMK, Level 3. As a whole, this lower unit corresponds to amid-distal alluvial fan, mainly formed by massive fine sediments,where pebbles have been sporadically detected and surface runoffswould have contributed with coarser sands. The presence of root andload casts indicates high standing of the water table at least during cer-tain periods. In this environment, there are also some local depressionsthat allow the decantation of silt and clay, as is the case of the secondgeological unit identified at AMK. This second unit consists of a 5 cmlayer of overlaying silts and clays that decreases its section to the Eastof the site and defines the paleosurface of AMK Level 2.

The second fossiliferous level was covered by a fast and low-energyevent, clearly not strong enough to transport the fossil remains. AboveAMK Level 2, five parallel stacked and fining upwards sequences (3–12 cm) were deposited. The first of them is characterized by a fine-grained sedimentation, with very fine sands and clayey silt; whereasthe upper sequences are formed by coarser sediments. Each sequencecorresponds to a different flooding event that generated diffuse runoffsover the alluvial system. The increase in thickness and grain size alongthe sequence might be indicating a progradation of the alluvial systemto the basin center in the North. According to previous studies, thisstratigraphic position is occupied by a laminar tuffaceous level (the Cha-pati Tuff) in the other lowermost Bed I sites located between the4th andthe FLK Fault (Uribelarrea et al., 2014).

Above these 5 sequences is the geological unit equivalent to ZinjLevel 22. In the case of AMK, this clay level contains a greater percentageof tuffaceous silt that is intensively laminated, especially in the lowerhalf (Fig. 3). Although the sedimentation process corresponds to ahigh standing of the lake level, the influence of the setting of AMK inthe supralittoral belt is evident in the form of coarser sediment inputs.In contrast to other pene-contemporaneous sites, where the Zinj clayis clearly divided into two levels (Levels 22a and 22b), here severalthin clay levels of millimetric thickness have been observed. Thus,while the rest of the sites are characterized by two clearly distinguish-able subaerial exposure phases (two paleosurfaces), AMK shows amore continuous sedimentation process, thoughwith a low sedimenta-tion rate. The fossiliferous Level 1was formed on top of the clay unit andwas directly buried by Tuff IC.

3.1.2. Phytolith resultsThe AMK vegetation was largely controlled by water table oscilla-

tions, as previously hypothesized by Uribelarrea et al. (2014).

Fig. 2. Stratigraphic cross-section of AMK,with the position of the three fossiliferous levels. Right: Views of the 2014 excavations at the sitewith the exact location of the three fossiliferouslevels. The small box in the inferior picture corresponds to Fig. 3.

Fig. 3. Detail picture of the clay unit under Tuff IC, where the thin lamination in the lowerhalf of the deposit can be appreciated.


Environmental fluctuations are also reflected in phytolith analyses. TheAMK site shows strong differences between samples. In one of the sam-ples (DB14-27C), palm phytoliths represent 56%, whereas in the othersample (DB14-29) they represent a mere 2%. However, both samples(Arráiz et al., 2017) indicate a densely wooded area, in which palmtrees would have been a regular component, but also other elements(e.g. grasses and sedges) would have been present forming a mixedvegetation cover (Table 1).

3.2. Orientation analysis

Orientation analyses of fossils for Levels 1 and 2 show a uniform dis-tribution. Both rose diagrams (Fig. 4) indicate that most of the azimuthconfidence interval shows no clearly identifiable anisotropic orienta-tion. Statistical tests support these results, as p values N 0.05 indicatethat the null hypothesis of uniform distribution cannot be rejected. ARayleigh test of a general unimodal alternative (Level 1: p = 0.9156;Level 2: p = 0.4706), a Kuiper test of uniformity (Level 1 and Level 2:p N 0.15), and a Watson goodness-of-fit test of circular uniformity(Level 1 and Level 2: p N 010) suggest that the assemblage is largely au-tochthonous. Thus, the hydraulic flows detected in the geology of the

Table 1Detailed phytolith counts in AMK site samples.

Description (size in microns) Taxonomy Samples

DB14-27C DB14-29(2.993333/35.347583)a (2.993361/35.3475)a

Globular echinate body with distinct spines 182 5Faceted, smooth, granulate and irregular bodies FIc 23 35Acicular body (Lb 16–20) 1 5Parallepiped body, bulliform-type texture (Lb 20–40) Grass/Sedge 3 0Parallepiped/cubic body, sinuous edges, psilate surface (Lb 10–40) FIc 5 56Parallelepiped/cubic/ovate body, granulate surface (Lb 15–20) FIc 6Blocky body cuneiform, likely silicified bulliform cell (Lb 20–30) Grass 1Paralleleipedal. Faceted, sharp edges (Lb 20–80) Fern 3 0Elongate tabular, irregular edges, rough surface (Lb 25–60) FIc 0 0Elongate cylindrical body, surface psilate to slightly lacunate (Lb 20–30) FIc 5Elongate tabular or slightly cylindrical, surface perforated-granulate (Lb 30) Grass/Sedge 16 0Elongate quadrangular section, body curved, surface spilate to slightly rugose(Lb 22–70) 4Elongate smooth glass-rod type straight body (Lb 15–30) 12 12Elongate cylindrical/subcylindrical body granulate (Lb 20–120) FIc 4Dubious elongates, altered. 36Grass Silica ShortCells (GSSC) 61 25Mesophyl dicot cell FIc 0 0Plate, thin, irregular contour, wavy surface, finely rugose (Lb 20–50) 2Thin plate, edge or surface lacunate (Lb 30) 13Tabular thick smooth, usually contorted (Lb 20–40) FIc 12Dubious phytoliths 1 89

Σ phytoliths 323 294

a Latitude S and Longitude E Coordinates.b L: length.c FI: Forest Indicator phytoliths.


site were not sufficiently intense to preferentially orient materials andcause a strong post-depositional disturbance.

3.3. Faunal analysis

3.3.1. Skeletal part profilesThe analyzed bone assemblage (Levels 1 and 2) is composed of 168

specimens, representing a significant number of taxa. The majority ofbone remains recovered could be attributed to carcass size, especiallyin Level 2 where fossils are less damaged. Most bones could be alsoassigned to animal families and in few cases to the species (Table 2).Level 1 bone assemblage is composed of 98 bone remains. Of these

Fig. 4. Rose diagrams for azimuth orientation in AMK Levels 1 and 2; and statistical tests an

fragments, 56 specimens could be identified to element, representingat least 12 individuals. Carcass size attribution was more difficult dueto high fragmentation. However, 18 elements could be attributed tome-dium-sized bovids, 3 are representing small bovids and only 1 elementcorresponds to a large carcass individual. The rest of the elements couldbe attributed to smaller not Bovidae species. The Level 2 bone assem-blage contains N57 identified specimens, representing different taxaand at least 13 individuals. A total of 34 elements (excluding bird andfish remains) could be attributed to carcass size: 30 elements belongto medium-sized carcasses and 4 to smaller individuals.

Among herbivores, most of the identified specimens are bovids,equids being limited to one bone. Medium-sized bovids are the most

d p values for orientation distribution, where p N 0.05 indicates significant anisotropy.

Table 2Taxonomic representation according to NISP, MNE and MNI at AMK.

Level 1 Level 2

NISP MNE MNI NISP MNE MNI

BovidaeBovid size 1–2 3 2 2 – – –Antilopini 1 1 1 – – –Bovid size 3a/b 30 18 3 35 26 5Reduncini – – – 3 3 1

EquidaeEquidae indet – – – 1 1 1

SuidaeSuidae indet 1 1 1 – – –

Herbivore indetIndet size 4–5 8 1 1 – – –

PrimatesPrimate indet 3 2 1 – – –Theropithecus – – – 2 2 1Hominidae 5 5 2 2 1 1

CarnivoraCarnivora indet 1 1 1 2 1 1

OthersAves indet 3 2 1 2 2 1Siluriformes – – – N10 N10 2

Total 56 33 12 N57 N46 13

Fig. 5. Skeletal part profiles according to MNE for AMK Levels 1 and 2.


represented type of carcasses in both levels. In Level 1, bovid represen-tation is more varied, with some remains that belong to small bovids(sizes 1 and 2). Only few dental remains have enabled a more preciseidentification, certifying the presence of a sub-adult Antilopini in Level1 and a sub-adult Reduncini in Level 2. In both levels small carnivoresare represented, but it was not possible to determine the species. Sever-al primates have also been identified in both levels, including hominins,a small-sized primate and a Theropithecus. Non-mammals do also ap-pear in both levels; at least one bird is represented in each level andtwo catfishes were recovered from Level 2, certifying the presence ofwater at AMK.

Regarding anatomical sections, differential preservation could be ob-served in Levels 1 and 2 (Fig. 5). Long bones are the best represented el-ements in Level 1; they make up half of the MNE (56%). Axial elementsand compact bones seem to behighly underrepresented asmeasured byMNE according to MNI. Axial elements are the most abundant bones inLevel 2, but they are still underrepresented according to MNI and MNEestimations. Long bones represent 33% of theMNE. In both levels, crani-al bones are poorly represented. The high representation of the appen-dicular bones (64% in Level 1, 53% in Level 2) suggests that theaccumulation is the result of the transport of partial carcasses or of inlocus death (kill) with subsequent postdepositional carnivore ravaging(deleting a substantial amount of axial elements). It is interesting that,overall, upper limb bones are better represented than metapodials inLevel 2, even though the latter are denser and, thus, tend to preservebetter than other long bones. Both humeri and femora are best repre-sented by their distal epiphyses. Axial bones (including pelves, scapulae,vertebrae and ribs) are more prominent than cranial specimens in bothlevels. In theory, axial and compact elements tend to disappear in carni-vore scavenged assemblages, creating skeletal profiles dominated bycranial and long limb bones that are generally better represented byshaft fragments.

3.3.2. Bone breakageFragmentation patterns vary at AMK, with higher degrees of break-

age in Level 1 and the preservation of entire elements in Level 2. InLevel 1, a total of 44 specimens could be identified as bearing greenbreakage and 31 showed dry breakage patterns. The assemblage is

highly fragmented, and specimens N4 cm predominate (70%). Breakageis also evident when looking at long bone fracture patterns. Most longbones (N69%) have b50% of the circumference left. The rest are com-plete and no long bone shows an incomplete shaft with N50% of the cir-cumference preserved (Fig. 6).

Level 2 includes 48 bones bearing green or/and dry breakage. Ofthese, 36 specimens exhibit dry breakage and only 12 specimensshow green breakage. Breakage is less intense in Level 2. N72% of thespecimens are N4 cm and the majority of long bones remain almostcomplete. Only 5 long bones preserve b50% of the circumference and1 N 50% (Fig. 6).

Theoretically, in both human and carnivore assemblages, and in theaccumulations created by the action of both agents, shaft circumferenceType 1 (b50% of the shaft is preserved) is predominant. However, carni-vore assemblages show a greater proportion of Types 2 (N50% of theshaft) and 3 (complete shaft) with a higher frequency of complete sec-tions (Bunn, 1982, 1983b; Marean et al., 2004). However, such patternsvary according to the carnivore involved in the modification of car-casses. Felids are known to be less destructive than hyenas. The compar-ison of the circumference types found at AMK with a hyena den and alion accumulation (Fig. 6) suggests that each AMK Level is representinga different pattern. While Level 1 results are more similar (though notexactly the same) to those found at Syokimau hyena den (Egeland etal., 2008), with a preponderance of Type 1 followed by complete cylin-ders; Level 2 shows almost the same pattern observed in the OCS, withmost long bones remaining complete, almost a third being highly frac-tured and a small percentage maintaining N50% of the circumferenceleft. According to Arriaza et al. (2016) the OCS assemblage is the resultof a large felid (most likely lion) primary access followed by minimalhyena scavenging. The presence of complete appendicular bones, as ob-served inmanyOlduvai Bed I sites (Potts, 1988), has been interpreted asa result of a meat surplus (Egeland and Byerly, 2005). That is, Type 3would be the result of an environment with little competition amongpredators with small impact of scavenging strategies. On the otherhand, predominance of Type 1 sections would be indicative of intensivemarrowexploitation and, therefore, higher competition.When Type 1 isas abundant as 2 and 3 together, or even smaller, it is assumed that thesite represents a low competition scenario where bone destruction wasreduced. This is the case in AMK Level 2 (Fig. 6). Meanwhile, the pres-ence of complete bones along with others that are very fragmented inLevel 1 (Fig. 6) suggests different depositional events with different car-nivores exploiting carcasses.

The frequencies of dry breakage identified in both levels (see above)indicate important non-biotic postdepositional modifications. The ratioof green breakage is also noteworthy, especially in Level 1. Green frac-tures might be interpreted in relation to other surface modifications

Fig. 6. Circumference type frequencies (Bunn, 1982) identified for medium sized bovid long bones in AMK Levels 1 and 2, compared with Syokimau (Egeland et al., 2008) and the OCS(Arriaza et al., 2016) samples. The different types refer to 1: b50%, 2: N50% and 3: complete shafts.


such as tooth marks (see below) and notches. The calculation of the ra-tios of notch Types C/A and D/A has been used to discern between an-thropic and carnivore assemblages (Domínguez-Rodrigo et al., 2007).Carnivores tend to generate more Type C notches, since they havemore than one contact point (using the cusps of their teeth). Opposingnotches appear when carnivores apply pressure with the upper andlower dentition at two opposing points (Egeland, 2010; De Juana andDomínguez-Rodrigo, 2011). Six specimens in Level 1 have been identi-fied as bearing Type C and D notches (Table 3), whereas no notcheshave been observed in Level 2. Some of the few notches identified atAMK appear associated with pits and scores, indicating a possible rela-tion.Micro-notches (b1 cm) have also been observed in Level 1. This ty-pology is also characteristic of carnivores (Capaldo and Blumenschine,1994). A Type C notch that is broader than deep (morphologymore typ-ical of dynamic loading) was observed on a metapodial shaft in associa-tion with a percussion mark. Usually Type C notches appear associatedwith percussion marks on denser bones, as a result of multiple impactsmade in order to fragment the bone and extract the marrow (De Juanaand Domínguez-Rodrigo, 2011). Although the sample of notches is notlarge enough to conduct reliable statistical analyses (Table 3), the pre-dominance of Type C (double overlapping) and D (double opposing)notches underscores that the breakage patterns identified in both levelscould be associated with static loadings, such as the ones carried out bycarnivores.

Table 3Types of notches identified at AMK Levels 1.

Notch typea No. Location

Level 1 Type A 1 LB shaftType B 4 Metapod shaft

Tibia shaftLB shaftAxial indet

Type C 3 LB shaftTibia shaft

Type D 3 Radius shaftTibia shaft

Micro-notch 4 Femur shaft

a Typologies are based on De Juana and Domínguez-Rodrigo (2011)

Finally, at AMK no bone impact flakes have been documented. Thisobservation indicates hominids were not a significant bone modifying/accumulating agent.

3.3.3. Bone surface modificationsMost bone remains had a good preservation, with cortical surfaces

that enabled the identification of conspicuous marks. However, thestate of preservation varied within each assemblage: some of thebones had biochemical alterations or different stages of weatheringand/or trampling and carbonate concretions (Fig. 7). This differential al-teration of the cortical surfaces might indicate multiple deposition epi-sodes of variable duration.

Trampling is the most common non-biotic alteration observed inboth levels. A total of 31 specimens (31.6%) in Level 1 and 20 in Level2 (28.6%) show trampling marks. This might be related to the higheramount of coarser sediments at the site as explained above. Biochemicalalterations are also significant in the form of shallow and irregular exfo-liations (Fig. 7D). N30% (30 specimens) of the remains recovered fromLevel 1 and around 27% of the bone fragments (19) from Level 2 showsome kind of biochemical alteration related to the presence of root etch-ing of associated fungi and bacteria (Fig. 7B), typical of humid environ-ments (Marchiafava et al., 1974; Sharmin et al., 2003;Domínguez-Rodrigo et al., 2007). The precipitation of carbonates is im-portant in the assemblage and is also linked to the presence of water.N27% (n = 27) of the assemblage in Level 1 and N17% (n = 12) inLevel 2 are affected by carbonates (Fig. 7A). Manganese alterations(Fig. 7C) have also been observed, but in a much lower frequency(n= 6 in Level 1, n= 9 in Level 2). Early stages of chemical weatheringhave been identified among some remains. Traces of subaerialweathering are marginal and refer to stage 1; most of the well-pre-served assemblage exhibit weathering stage 0 (Behrensmeyer, 1978).In sum, 59% (99 specimens) of the studied assemblage was well pre-served, 24% (40) had a regular preservation and 18% (29) showed badcortical surfaces. Only specimens that enable the identification of con-spicuousmicroscopic marks underwent a full scrutiny in order to deter-mine the action of biotic agents.

Our results indicate that anthropogenic activity is marginal at AMK.Only one percussionmark on a long bone shaft was identified in Level 1.No other taphonomic evidence for human activity was documented in

Fig. 7. Examples of abiotic bone modifications documented at AMK: (a) carbonateconcretions, (b) biochemical alterations related to the presence of root etching, (c)manganese alterations, (d) exfoliation.


any of the three fossiliferous levels. Thus, no functional link between thefew lithic tools recovered at AMK (one basalt and one gneiss artefacts inLevel 1, one quartzite artefact in Level 2) and the bone assemblage couldbe established.

In contrast, toothmarks appear in thewhole assemblage. The type oftooth marks and their frequencies differ between levels (Table 4).Scores (Fig. 8B) have been found on several long bones and axial ele-ments in Level 1: 34% of bovid elements bear at least one tooth score.Most tooth scores are on the diaphyses and do not appear in large num-bers on each element. There is only one ribwhere N10 tooth scoreswererecorded. By contrast, specimens with tooth scores are very scarce inLevel 2 (8%), appearing only on a long bone shaft and on a rib.

Pits are alsomore abundant in Level 1 (45%) than in Level 2 (27%). InLevel 1 more specimens bearing pits have been found, and also thenumber of pits per specimen is higher than in Level 2. Bones with carni-vore surface modifications are mainly long limb bones and axial ele-ments such as ribs, scapulae or coxals (Table 3). Among all the pitsrecovered, a few triangular pits on a tibia and a metapodial from Level1 stand out (Fig. 8A). The size of the pits was statistically analyzed andcompared to experimental analogues (see Andrews andFernández-Jalvo, 1997; Domínguez-Rodrigo and Piqueras, 2003;Andrés et al., 2012). Pit sizes of AMK were compared to the Maasai

Mara spotted hyena den and the lion samples published in Andrés etal. (2012). Pits on shafts from medium-sized carcasses were selectedin order to make the AMK sample comparable to the referential frame-works. Such selection largely reduced our sample, so that Level 2yieldedmean estimates since sample size was too small to derive confi-dence intervals (Table 5). Pits are scarce in Level 2 (n= 10) andmost ofthem are located on axial bones (ribs, scapula, coxal) and on long boneends. Level 1 pit dimensions are more similar to the ones observed inbones modified by spotted hyenas. The range of pit breadth and lengthin Level 1 fall within the confidence interval for the Maasai Mara hyenaden (Fig. 9), although the AMK pits are clearly smaller (see maximumvalues in Table 5). Level 2 results point to an increase in pit size butdata are too scarce to draw any conclusions (Fig. 9, Table 5).

Furrowing has been only detected on a few long bones (Table 4).Though in low frequencies, furrowing is more significant in Level 2than in Level 1. Thismight seem contradictory as carnivoremodificationis much more prominent in Level 1 than in Level 2, but this can be ex-plained by differential fragmentation patterns. While in Level 1 onlyfew long bones remain sufficiently undamaged to properly identifyfurrowing, several complete and unmodified long bones (Fig. 10) havebeen recovered from Level 2. Typical hyena modifications (seeDomínguez-Rodrigo et al., 2012a) have been recorded in a humerusfrom Level 1, where the proximal and distal epiphyses are missing (hu-merus on the right in Fig. 11). Equally, two other humerii in Level 2 (leftand middle humeri in Fig. 11) might have been furrowed by hyenas, asthey lack either the whole proximal epiphysis, or both ends. A femur(size 3a) in Level 2 does also show typical hyena furrowing as bothends have been deleted. In addition to epiphyseal deletion, some verte-bral apophyses are also missing in Level 2. The disappearance of verte-bral apophyses along with the absence of other kind of tooth marks isassociated with felid consumption (Domínguez-Rodrigo, 1999). A lum-bar vertebra in Level 2 exhibits exactly the patterns observed after felidaction, that is, modification of the apophyses and intact body (Fig. 8C).

No bonewith both human and carnivoremodificationswas found inany of the three levels. Thus, no clear evidence of their interaction on theassemblage could be found taphonomically.

3.3.4. Disarticulation patterns, specimen size and site formationAs explained above, specimens b4 cm form a significant part of the

assemblage (30% in Level 1, and 28% in Level 2). The presence of smallfragments might indicate that hydraulic forces were not strong enoughto generate a significant bias towards larger specimens. Along withthese small specimens, the presence of less dense and easily transport-able elements has been documented (Voorhies, 1969). Axial and com-pact bones, corresponding to Voorhies Group I and I&II, have beenfound in both AMK Levels. Cranial elements are poorly represented inboth levels (Fig. 5). Cranial bones belong to Voorhies Group III, theleast transportable group. The identification of elements of all Voorhiesgroups (Voorhies, 1969) indicates that the AMK accumulations repre-sent neither a hydraulically-transported nor a lag assemblage in any ofthe two levels, despite the limitations due to small sample sizes.

The presence of epiphyses varies between levels at AMK. WhileLevel 1 shows a lower frequency of long bone ends, in Level 2 a signifi-cant representation of epiphyses was recorded. The transportation ofepiphyses in relation to hydraulic forces has been discussed by severalauthors (see Behrensmeyer, 1975; Pante and Blumenschine, 2010;Domínguez-Rodrigo et al., 2012b). The fact that at AMK epiphysesmight have been subjected to different scavenging pressure and the dif-ferent breakage patterns in Levels 1 and 2,might explainwhy epiphysesare muchmore abundant in the latter level. Taking all these results intoaccount, postdepositional distortion, though present, must have beenlow or moderate, and in any case it was not constant during the forma-tion of the site. The absence of bones with abrasion or polishing marks,as well as the orientation analyses presented above, also supports thishypothesis. Thus, the accumulation at AMK seems to be autochthonous.

Table 4Distribution of surface marks identified on bovid bones at AMK.

Element Carnivore Anthropic Location

Pa Sb Fur.c PMd

Level 1 Hum. 1/1 1/1 1/1 0/1 DPe/EPf dist. + prox.Rad./Ul. 2/2 1/2 0/2 0/2 EPf + MEPg prox./DPe

Fem. 1/3 0/3 0/3 0/3 DPe

Tib. 3/5 2/5 0/5 0/5 DPe/MEPg prox.Met. 3/4 3/4 0/4 0/4 DPe

Vert. 0/1 0/1 0/1 0/1Rib 3/7 2/7 0/7 0/7Scap. 1/1 1/1 0/1 0/1Cran. 0/2 0/2 0/2 0/2Indet. 3/12 3/12 0/12 0/12 DPe

Total 17/38(45%) 13/38(34%) 1/38(3%) 0/38(0%)Level 2 Hum. 1/2 0/2 2/2 0/2 MEPg dist./EPf dist. + prox./EPf prox.

Rad./Ul. 1/1 0/1 0/1 0/1 EPf prox.Fem. 1/2 0/2 1/2 0/2 DPe/EPf dist. + prox.Tib. 0/1 0/1 0/1 0/1Met. 0/3 0/3 0/3 1/3 DPe

Vert. 0/3 0/3 1/3 0/3 APh

Coxal 1/4 0/4 0/4 0/4Rib 3/5 1/5 0/5 0/5Mand. 0/1 0/1 0/1 0/1Astra. 0/2 0/2 0/2 0/2Phal. 0/1 0/1 0/1 0/1Indet. 0/1 1/1 0/1 0/1 DPe

Total 7/26(27%) 2/26(8%) 4/26(15%) 1/26(4%)

-The numerator indicates the number of elements showing such modifications, and the denominator expresses the MNE in each level.-Location of the marks is indicated on the right:

a P: Pits.b S: Scores.c Fur.: Furrowing.d PM: Percussion Mark.e DP: Diaphysis.f EP: Epiphysis.g MEP: Metaepiphysis.h AP: Apophysis.


Carnivore ravaging of bones in Level 1 does not facilitate an estimateof the time of formation of the assemblage other than by subaerialweathering stages, which do not exhibit large variation from 0 to 1, al-though few of specimens display higher weathering stages. Chemicalweathering caused during diagenesis is documented in some specimenstoo. However, most of the assemblage seems to have a recent originprior to sedimentation. The discovery of a partial primate hand withseveral carpal, metacarpal and phalangeal specimens in connection in-dicates a short exposure period. Phalanges are disarticulated veryearly (in the first 5% of time exposure until complete carcass disarticula-tion) (Hill, 1979; Hill and Behrensmeyer, 1984). This is why elements inwhich phalanges are still in connection to the limbmost commonly dis-play weathering stages 0–1 (Hill, 1979; Hill and Behrensmeyer, 1984).

The presence of an almost complete articulated front limb of an ar-tiodactyl in Level 2 also suggests short exposure time. Front limbs getdisarticulated before hindlimbs. In this case, the humerus-radius-ulna,metacarpal and distal phalanx appear in anatomical connection. Thedistal phalanx was found a few cm away from the articulated limb.None of these elements display subaerial weathering stages beyond0–1. This indicates a short exposure and rather quick sedimentationprocess (Hill, 1979; Hill and Behrensmeyer, 1984).

4. Discussion

AMK is one of the densest concentrations of remains caused by non-anthropogenic agencies on the FLK-Zinj paleolandscape. Itsmostly pale-ontological nature shows a contrast in the density of remains per spaceunit (bones/m2) when compared to the core area of FLK-Zinj. At the lat-ter site, an area similar in extension to the excavated area in AMKcontained thousands of bones, in contrast with the 98 specimensfound in AMK Level 1 and the 70 specimens found in Level 2. In addition,

the taphonomic signatures of both assemblages also differ. Bones atFLK-Zinj display high frequencies of green-broken bones bearing abun-dant percussion and cut marks, whereas at AMK, the faunal assemblageis either fragmented with only tooth marks (Level 1) or moderatelyfragmented with a high survival of complete bones (Level 2). These re-sults suggest that AMKwas a palimpsest created by the action of severalagents (with amarginal hominin input) superimposed over time on thelandscape. To support this, only onehominin-mademark and lithics andthe recovery of some hominin remains provide evidence for homininpresence at AMK.

According to Bunn (1986), certain bone accumulations withoutlithics or with few artifacts like AMK should be considered natural back-ground scatters. The main difference between natural and archaeologi-cal sites is the density of bone remains and the functional association oflithics and bones in the latter. Natural background scatters show signif-icantly lower NISP, MNE and MNI estimations, as they are the result ofthe natural (i.e., as opposed to anthropogenic) deposition of carcassesin the landscape, instead of a repeated transport of carcasses to thesame spot in the landscape by either humans or any other bone-accu-mulating agent such as hyenas (Potts, 1988).

Natural deposits can be also distinguished by the absence of clearhuman traces and because they present high frequencies of cranialbones and axial elements (Bunn, 1986). This pattern is not completelyobserved at AMK, where cranial elements are rather scarce. Low energyrunoffs could explain the lack of some other elements that are expectedat such sites and that at the same time are easily transported by water.However, although present at AMK,flowingwater seems to have playeda small role in the assemblage configuration, not being themain cause ofthe bone accumulation and not altering the orientation pattern in a sig-nificantway. This is also supported by the presence of easily transportedbones such as axial and compact bones, and a high percentage of

Fig. 8. Examples of carnivore surfacemodifications observed in AMK. A: Triangular pits onmatapod shaft; B: Score on a shaft fragment; C: Lumbar vertebra lacking apophyses andany other carnivore modification.


fragments b4 cm in every level. At AMK, we did not find any signs ofpolishing or abrasion that could potentially support the transportationof part of the assemblage. The presence of animals of various sizes (in-cluding micromammals) has been documented. These facts suggest alocal origin for the faunal assemblage, despite the potential effect oflow-energy hydraulic rearrangements. It could be argued that the influ-ence of water could have caused a small bias towards less dense ele-ments; however, this could also be the result of carnivore post-depositional damage.

Table 5Length and breadth of tooth pits documented at AMK compared to a hyena and a lion samples

n Mean 95% confidence interval lower

Hyena shaft breadth⁎ 46 1.55 1.2Hyena shaft length⁎ 46 2.71 1.81Lion shaft breadth⁎⁎ 28 1.7 1.49Lion shaft length⁎⁎ 28 2.87 2.2Level 1 shaft breadth 22 1.2 1.01Level 1 shaft length 22 1.6 1.41Level 2 shaft breadth 2 1.4 –Level 2 shaft length 2 2.05 –

Data includemean values, 95% confidence interval, standarddeviation(a) andminimum(b) andmonly simple observations could be done.⁎ Maasai Mara hyena den in Andrés et al. (2012).⁎⁎ Lion sample in Andrés et al. (2012).

Taphonomic analyses reveal a clear predominance of carnivore be-havior at AMK, showing different behavioral patterns reflected in differ-ent fragmentation degrees and tooth mark frequencies. The frequencyof tooth marks, including pits and scores, varies substantially betweenLevel 1 and Level 2. Level 1 shows a higher frequency of tooth marksper specimen and according to MNE. The location of tooth marks ismainly concentrated on the long bone diaphyses. Fragmentation isreflected in longbone circumference types,which show similar patternsto those found in the Syokimau hyena den (Egeland et al., 2008). In con-trast, fossils found in Level 2 are barely modified, many of them beingcomplete or almost complete. This last pattern fits the skeletal andbone modification patterns documented in non-scavenged or onlyslightly scavenged felid-made bone accumulations, like the OCS(Olduvai Carnivore Site) (Arriaza et al., 2016, 2017). In the case oflarge and medium-sized carnivores, felids are the most specializedflesh-eaters, with teeth almost exclusively developed for meat-slicingand that barely leave marks on the cortical surfaces (Turner andAnton, 1997). Thus, it is understood that felids would produce fewertooth marks than durophagous carnivores like hyenas (Brain, 1981;Selvaggio, 1994). Almost 99% of all complete long bones from the car-casses consumed by lions in the wild bear less than ten marks on thesame shaft specimen (Gidna et al., 2014). This feature was also foundamong carcasses consumed by leopards: complete long bones rarelyshow N3–5 toothmarks (Domínguez-Rodrigo et al., 2007). The low fre-quency of tooth marks on midshafts of complete long bones is charac-teristic of felid primary access to carcasses. No specimen, including thecomplete long bones from Level 2 show N2 tooth marks on the sameshaft. In addition, a lumbar vertebra with an intact centrum and dam-aged apophyses (Cavallo, 1998; Domínguez-Rodrigo, 1999), such asthe one found in Level 2 supports our interpretation of felid interventionat the site.We conclude that differences between AMK Level 1 and 2 aremost likely the result of the action of different carnivores.

The frequency of toothmarks recorded at Level 1 could be the resultof the overlapping activity of felids and hyaenids. It seems plausible thata felid acted as themain accumulating agent leaving fewmarks on bonesurfaces, and, secondly, hyenas partially ravaged the assemblage in-creasing the frequency of tooth marks, the degree of bone fragmenta-tion and leaving typical hyena furrowing patterns. We propose thisscenario because the bone destruction at AMK is lower than the one ex-pected in an exclusively hyena-made bone accumulation. In Level 1,only 30.4% of the identified specimens bear some kind of carnivoremodification, while in Syokimau the percentage reaches the 34.8%(Egeland et al., 2008) and in Kisima Ngeda hyena den 44% of the assem-blage (Prendergast and Domínguez-Rodrigo, 2008). Although thesepercentages are not extremely different, it is noteworthy that Syokimauden is considered a low competition setting, with considerably lowerdestruction patterns than other open-air actualistic sites in East Africa(Egeland et al., 2008). Carnivore modification in Level 2 is even lower.In addition, some of the complete elements appear articulated. Thismay indicate the transport of complete animals or limbs by a carnivore.

.

95% confidence interval upper SDa Minb Maxc

1.9 1.2 0.21 8.73.41 3.11 0.33 9.11.91 1.41 0.6 7.253.54 1.8 1.29 9.21.39 0.44 0.3 2.01.79 0.44 1.0 2.5– – 0.5 2.3– – 1.0 3.1

aximum(c) values documented in each sample, except for Level 2where due to sample size

Fig. 9.Mean values and 95% confidence intervals of tooth pit breadth and length on shafts in the AMK sample and the data published for hyenas and lions in Andrés et al. (2012). AMK pitdimensions were calculated following Andrés et al. (2012), using a two tailed confidence alpha value of 0.025. Level 2 is only represented by its mean dimensions since sample size is toosmall to obtain reliable statistical results.


The discovery of other specimens that might belong to the same indi-vidual (axial and long bones from front and hind limbs) could be indi-cating that the complete skeleton was at some point in time at thesite. In that case, the question is whether it represents a natural deathor whether it was transported or hunted in locus. Felids have been

Fig. 10. Complete medium sized bovid long bones recovered from AMK Level 2. Left toright: tibia, radius and metacarpus.

documented to occasionally transport complete carcasses at a short dis-tance from kills (Brain, 1981; Ruiter and Berger, 2000;Domínguez-Rodrigo and Pickering, 2010; Arriaza et al., 2016).

Several Bed I sites in Olduvai Gorge have been generated by carni-vores, while hominin intervention on faunas is documented only spo-radically. This is the case for the FLK-N, FLK-NN and DK(Domínguez-Rodrigo, 2001; Domínguez-Rodrigo et al., 2007; Egeland,2007). The high specialization pattern of prey, low breakage, lowtooth mark frequencies, skeletal part representation and typical bonemodification of felids on the axial skeleton and long bone ends, suggeststhat the accumulating agent in some of these sites was a felid(Domínguez-Rodrigo et al., 2007; Arriaza and Domínguez-Rodrigo,2016). In someof these sites (e.g. DKor FLK-NN) subsequent scavengingby hyenas has been also detected (Domínguez-Rodrigo et al., 2007;

Fig. 11. Cylinders of humeri (size 3a) found at AMK showing intense deletion of endstypical of durophagous carnivores.


Egeland, 2007), showing similar patterns to those documented at AMKLevel 1.

The choice of a specific spot in the landscape used repeatedly by car-nivores is usually associated with locations that might have served asshelters (Cavallo, 1998). That means that the immediate environmentwould have provided sufficient safety for the consumption of carcasses.According to paleoenvironmental analyses, the habitat surroundingAMKwould have been characterized by an evenmore dense vegetationthan the one be found in the FLK-Zinj area (Arráiz et al., 2017). Such adense arboreal environment would have encouraged predators totransport their prey (if obtained in nearby more open spaces) to thesite, since it would have been an area of safe consumption. This typeof interpretation was provided for understanding the formation ofFLK-N (Domínguez-Rodrigo et al., 2010a). Similar places are currentlyspots of very low competition in modern African landscapes wherethe accumulation of appendicular elements is very high due to repeatedtransport and low postdepositional disturbance (Blumenschine, 1986;Cavallo, 1998; Domínguez-Rodrigo, 2001). In most of the FLK sites, bio-chemical modifications caused by fungi or bacteria have been detected.Such modifications were also observed at AMK. The presence of thesemarks suggests that the remains were deposited in moist and shadedareas (Marchiafava et al., 1974; Hackett, 1981; Piepenbrink, 1984;Child, 1995; Sharmin et al., 2003). These conditions rarely occur inopen floodplains, but are more widely documented on bones depositedin wooded environments near water sources.

Recent paleovegetation reconstruction of the FLK-Zinjpaleolandscape (Arráiz et al., 2017) contradicts previous paleoecologi-cal interpretations of the area. According to geological and paleobotan-ical analyses, Blumenschine et al. (2012) suggested the existence ofgrasses and sedges in the wetlands close to a river channel 50 to200 m southeast of FLK-Zinj site (between PTK and FLK-Zinj). Palmphytoliths, like those revealed by our analyses, are incompatible withthe river channel proposed by Blumenschine et al. (2012), since inthat case there would have been no mature soils enabling the growthof palm tree roots. The recent paleovegetation description matches theone published for FLK-NN (Ashley et al., 2010b) and is consistent withprevious studies that suggest that the vegetation of the areawas denselywooded during middle and uppermost Bed I times. The presence offerns in some of the phytoliths samples suggests shaded and humidhabitats, supporting the existence of a river input southeast of PTKsite, as indicated by Uribelarrea et al. (2014). The somewhat elevatedplatform in the Zinj paleolandscape seems to have been a mosaic dom-inated by palm trees, but also including wetland areas and some moreopen sedges and grasslands (Barboni et al., 2010). Such a mixed envi-ronment acted as focal point for a wide range of herbivores and, there-fore, also for carnivores and hominins (Domínguez-Rodrigo et al.,2010b).

The discovery and the interpretation of AMK presented here is alsoof major relevance to show that factors other than forested habitatsmust have influenced the formation of anthropogenic sites on theFLK-Zinj platform, since the dense arboreal vegetation was not limitedto the locations where those anthropogenic sites occurred.

5. Conclusions

AMK is a recently discovered Olduvai Bed I site consisting of threefossiliferous levels where human presence has been detected in formof human fossil remains and very few lithic artefacts. Due to its locationat the Eastern Olduvai paleolake side, an area characterized by densevegetation, AMK appears within a close and low-competition environ-ment, which predators may have used to consume their prey, andresulting in the deposition of carcass remains.

The superimposition of depositional events at AMKwould have cre-ated a palimpsest reflecting the independent actions of carnivores (pos-sibly felids and hyaenids) and themarginal though perceptible presenceof hominins. The discovery of such sites should make archaeologists

reflect on the functional relationship between artifacts and bonesfound together in the same fossiliferous levels. For instance, AMK repre-sents an accidental association between very few lithic tools and a sub-stantial number of bone remains.

According to the taphonomic results presented here, natural pro-cesses would have been responsible for the formation of AMK. Thissite could be described as a natural background scatter in a space inten-sively coveredwith vegetation. Assemblages like AMK Level 1 and 2 andFLK-NN1 and 3 show the contrast in density of remains between an-thropogenic (FLK-Zinj, PTK, DS) and non-anthropogenic sites and alsoshows that despite the occasional availability of marrow-bearingbones on the Olduvai paleolandscape, hominins disregarded themmost of the time, as more clearly documented slightly later at 1.8 Maat FLK-N.

Acknowledgments

We wish to thank The Ngorongoro Conservation Area Authorities,COSTECH and the Antiquities unit for permits to conduct research atOlduvai, andMuseo Arqueológico Regional de la Comunidad deMadrid.The authors greatly appreciate themajor funding provided by the Span-ish Ministry of Science and Innovation through the European projectI + D HAR2013-45246-C3-1P and the Spanish Ministry of Culturethrough the Heritage Institute and the Program of Funding for Archaeo-logical Projects Abroad. We would like to thank the entire TOPPP team,especially the local workers and Ainara Sistiaga for the help provided inthe conduction of the research at AMK. We highly appreciated the fieldwork conducted by the field school students during 2013 and 2014campaings. The corresponding author would like to thank FundaciónLa Caixa and the Spanish Education, Culture and Sports Ministry forfunding her postgraduate education programs. Finally, we would liketo thank two anonymous reviewers for their comments and sugges-tions. This paper highly benefited from their feedback.

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palaeogeography, palaeoclimatology, palaeoecology · the paleoecology and taphonomy of amk (bed i,...

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