Constraining time and ecology on the Zinj paleolandscape: Microwear and mesowear analyses of the archaeofaunal remains of FLK Zinj and DS (Bed I), compared to FLK North (Bed I) and BK (Bed II) at Olduvai Gorge (Tanzania)
Introduction
Although interpreting agency at the earliest archaeological sites has been controversial for several decades, understanding the timing and duration of specifically anthropogenic depositional processes at these sites has been even more challenging (Domínguez-Rodrigo et al., 2007). Regardless of which side of the hunting-scavenging debate one favors, current evidence suggests that a) hominin input was marginal in the accumulation of faunas at some sites, b) anthropogenic accumulations, in contrast, exist at other sites and, c) in the latter case, all or most of the fauna accumulated was transported by hominins targeting bulk defleshing and long bone marrow exploitation of small and medium-sized carcasses (Domínguez-Rodrigo and Barba, 2007; Domínguez-Rodrigo et al., 2007; Pobiner et al., 2008; Ferraro et al., 2013; Parkinson, 2013, 2018; Domínguez-Rodrigo, 2015; Domínguez-Rodrigo and Pickering, 2017). These anthropogenic loci remain the source of speculation regarding how much occupation time they represent and the type of depositional processes (single continuous or multiple discontinuous) that created them. Potts (1988) initially suggested that the extensively weathered assemblages from Olduvai Bed I sites indicated that deposition occurred intermittently over a time span involving many years. Domínguez-Rodrigo et al. (2007), in contrast, argued that bone weathering at these sites was mostly chemical and not subaerial, representing diagenetic modifications that were unrelated to depositional time spans. Most bones at the anthropogenic site of FLK Zinj unaffected by chemical weathering show no traces of subaerial weathering, indicating a relatively fast accumulation over one or two years (Bunn et al., 1986; Binford et al., 1988; Domínguez-Rodrigo et al., 2007).
Olduvai Gorge has played a prominent role in all these debates since it contains the best-preserved anthropogenic sites for the early Pleistocene. Some of these are vertically discrete concentrations of stone artifacts and fossil bones from a diversity of animals, where time-averaging can be better understood than in vertically-dispersed archaeological deposits. A thin (<20 cm) clay stratum situated under Tuff IC in Bed I contains a diverse set of pene-contemporaneous archaeological and paleontological sites over the same paleolandscape surface: FLK NN, FLK Zinj, AMK, PTK, DS and AGS (Uribelarrea et al., 2014; Aramendi et al., 2017; Domínguez-Rodrigo and Cobo-Sánchez, 2017; Domínguez-Rodrigo et al., 2017). This is a unique opportunity for understanding Oldowan hominin behavioral variability over a diverse set of habitats situated in lacustrine-alluvial ecotones.
FLK Zinj (Bed I, Olduvai Gorge, Tanzania) is famous for being the most widely debated early Pleistocene anthropogenic site. It is the second largest window onto an early Pleistocene paleo-surface occupied by hominins after DS (David's Site), which is situated on the same 1.84 Ma paleolandscape (Domínguez-Rodrigo et al., 2017). PTK (Philip Tobias Korongo) is also situated in the same paleosurface as FLK Zinj and DS (Fig. 1). At no other early Pleistocene site is there as much taphonomic evidence as at FLK Zinj and DS (work in progress) that the bulk of the faunal accumulation was carried out by hominins (see summary in Domínguez-Rodrigo, 2015; Domínguez-Rodrigo and Pickering, 2017). Interestingly, the amounts of bones and macrommamal animals unearthed at FLK Zinj and DS are more abundant than those documented in many modern hunter-gathererścamps (Bunn, 1983a, Bunn, 1983b; Bunn et al., 1988; Bartram et al., 1991; Lupo, 2001). This could imply more prolonged time span(s) of occupation at these sites and/or bigger group sizes. Recent modeling of the spatial dispersal of food refuse at FLK Zinj and the amount of animal food represented by the animal parts excavated at the site suggest -using ethnoarchaeological referential frameworks-that the amount of time represented by the assemblage may have been large and so may the number of potential hominins who occupied that space (Domínguez-Rodrigo et al., 2019). An estimate of a minimum of four months and between 16 and 28 individuals was produced using Yellen (1977) data on Kalahari foraging camps and updated regression formulas (Domínguez-Rodrigo et al., 2019). The time estimate was conservative, since it was based on just the preserved amount of food and not the carcass parts potentially deleted by post-depositional agents. This could imply an even longer length of occupation by hominins at the site. This would also indicate that occupations at FLK Zinj were either few and prolonged or short and multiple over a short time scale, as suggested by the taphonomic preservation of bone surfaces.
The application of the same regression formulas to the spatial analysis of PTK yielded similar estimates of occupants (average = 13; maximum = 20) and length of occupation ranging between one and five months (Cobo-Sánchez et al., 2018). Data from both sites suggest that group sizes were bigger than traditionally assumed for Oldowan hominins and that the occupation of the sites was not ephemeral. This similarity between both sites for the two types of estimates can also be applied to the similar spatial configuration of both assemblages, indicating a common behavioral pattern (Domínguez-Rodrigo and Cobo-Sánchez, 2017)
The Olduvai sites have been used to model seasonal foraging by hominins within lacustrine habitats, mostly during the dry season (Peters and Blumenschine, 1995; Blumenschine and Peters, 1998). If hominins were behaving as these models suggest, it would be expected that the occupation of sites by the paleo-lake would have been seasonal (i.e., short and possibly redundant). In contrast, if hominins were exploiting resources on the lacustrine basin all year round, it would be expected that the occupation of the Bed I sites would have been more prolonged and multi-seasonal. Evidence against the formation of FLK Zinj and DS1 during the dry season could be inferred from the taxonomic composition of both faunal assemblages, dominated by Kobus, Parmularius and Antidorcas, all of them potentially classifiable as local fauna. The virtual lack or underrepresentation of migratory taxa (i.e., wildebeest) argues against an intensive occupation during the dry season, unless these taxa were already migrating like they do today in the Serengeti, returning to the region during the short wet season.
Tooth enamel microwear analyses have been used to interpret diets from fossil ungulates (Solounias and Semprebon, 2002; Rivals and Deniaux, 2003; Rivals et al., 2007b; Solounias et al., 2010, 2013; Rivals and Semprebon, 2011; Uno et al., 2018). This technique has also been used to study seasonality and types of occupations by hominins during site formation (Rivals and Deniaux, 2005; Rivals et al., 2009, 2015a; 2015b; Sánchez-Hernández et al., 2014; Rodríguez-Hidalgo et al., 2016). A substantial amount of microwear research has been done via two-dimensional imaging under high or low magnification and identification and quantification of relevant enamel alteration features (namely, pits and scratches). This has been considered by some to involve a high degree of subjectivity (Scott et al., 2006). An alternative 3D method called microwear texture analysis (MTA) has been built with the intention of not involving any subjective assessment of enamel alteration features and has been applied in order to identify the diets of a wide array of faunal taxa (Scott et al., 2005; Ungar et al., 2007; Scott, 2012; Merceron et al., 2014; Williams, 2014; Souron et al., 2015; Calandra and Merceron, 2016; Ragni et al., 2017). However, the focus of MTA has been mostly on diet breadth and dietary niche reconstruction rather than on seasonality and length of occupation at anthropogenic sites. Importantly, microwear texture analyses have been applied to the artiodactyl faunas of some of the Olduvai Bed I sites, with results suggesting a prolonged year-round deposition of faunal materials at these sites (Gurtov, 2016).
Constraining the temporal framework during which site formation took place is of utmost relevance to interpret what these early Oldowan sites represent in terms of the behavior of those early humans. Rivals et al., 2015a, Rivals et al., 2015b, using alternative methods of microwear scoring and quantification of feature variability, elaborated a type of microwear analysis aimed at determining the duration of faunal depositional events in archaeological assemblages. We believe this approach is currently more adequate for assessing time and number of occupations in any given archaeofaunal assemblage than available techniques of MTA, since it has successfully been tested for this purpose with controlled samples.2 This is why we will adopt this technique here for testing alternative scenarios of site formation (single or multiple occupations involving short or prolonged time spans) in the anthropogenic sites of the Zinj paleolandscape; namely at FLK Zinj and DS. Although not pertaining to the Zinj paleolandscape, comparisons will also be made with fauna from the FLK N faunal assemblage (Upper Bed I), mostly created by carnivores (Domínguez-Rodrigo et al., 2007), to contrast hominin and non-hominin agency in site formation and use. Comparisons will be also extended to BK5, which corresponds to Leakey's (1971) excavation of this upper Bed II site where a “herd” of 24 Pelorovis was found. This large bovid assemblage was recently interpreted as a time-averaged deposit, therefore purportedly spanning carcass deposition at different intervals during a long time span (Organista et al., 2016).
A second target in the present work is to present new dietary inferences for the taxa most widely represented in these Bed I assemblages: Antidorcas recki, Kobus sigmoidalis, and Parmularius altidens. Additional taxa from Bed I (e.g., Connochaetes) and Bed II (Pelorovis)oldowayensis were also included for comparative reasons. For this purpose, we will combine microwear and mesowear techniques. Microwear analysis uses the proportion of abrasive features (i.e., pits and scratches) to differentiate between grazing, browsing, and mixed diets on short-term temporal frameworks (Grine, 1986; Solounias and Moelleken, 1992; Solounias and Hayek, 1993; Solounias and Semprebon, 2002; Rivals and Deniaux, 2003; Rivals et al., 2007b; Solounias et al., 2010, 2013; Rivals and Semprebon, 2011; Uno et al., 2018). Mesowear analysis focuses on the attritional and abrasive wear of teeth, which is reflected on the particular topography of their occlusal surfaces (Kaiser and Solounias, 2003; Fortelius and Solounias, 2000; Franz-Odendaal and Kaiser, 2003; Clauss et al., 2007; Louys et al., 2011; Kaiser et al., 2013). The springbok (Antidorcas marsupialis) is the closest living relative of the prehistoric Antidorcas recki. Antidorcas marsupialis is mainly a browser, feeding on leaves, shrubs, and succulents. It lives in dry areas occupying grasslands, bushlands, and shrublands. Modern waterbucks (i.e.,Kobus ellipsiprymnus) are the closest modern phenetic relatives to Olduvai's Kobus sigmoidalis. Kobus ellipsiprymnus is highly dependent on water and leaves in riverine and lacustrine settings, although it feeds mainly on grasses. Parmularius altidens is an extinct Pleistocene alcelaphini genus/species, which has been compared to modern topi (Damaliscus), hartebeest (Alcelaphus), and hirola (Beatragus hunteri). These three alcelaphines are mainly grazers adapted to open habitats, but they resort to browsing substantially during the dry season. Here, we will test if these fossil taxa fed like their modern counterparts and were adapted differently to the savannah biome. This has important repercussions for our understanding of the environments where hominins lived at Olduvai Gorge and also for the timing of interaction between hominins and these ungulates.
Section snippets
Microwear analysis and type of site occupation
Standard analytical approaches to the study of tooth enamel microwear were initially based on the use of only the upper and/or lower second molar for quantifying microwear features (Solounias and Moelleken, 1992, Solounias and Semprebon, 2002; Rivals and Deniaux, 2003; Semprebon et al., 2004b; Rivals and Semprebon, 2011; Rivals, 2012; Sánchez-Hernández et al., 2014). Subsequently, it has also been applied to other molars (e.g., Rivals and Deniaux, 2005; Rivals et al., 2009, 2015a; 2015b;
Microwear analysis and duration of site occupation
Data on the microwear features of the selected Olduvai sample are shown in Table 1. The FLK Zinj Antidorcas microwear scratch pattern with high CV and SD occurs within the error zone of Area C, suggesting a probable redundant deposition in different times of the year. The FLK Zinj Kobus sample falls within the error zone of area A, indicating a potential (but insecure) single depositional moment, spanning no more than one season. Alternatively, it could represent several visits at the same time
Microwear analysis and type of site occupation
MTA studies have had very limited success in determining season of death and even differentiating generalist diets from mixed diets among ungulates (Scott, 2012). MTA also has not built a proper referential analogical framework to interpret length of occupation and seasonality efficiently. For this reason, Gurtov (2016) argued that such an analogical framework was necessary and she elaborated one using impala (Aepyceros melampus) from Lake Eyasi (Tanzania) as a proxy to assess potential
Conclusions
Hominins responsible for the anthropogenic sites of FLK Zinj and DS were not determined by vegetation in their selection of loci to process animal carcasses and make stone artifacts in the lacustrine ecosystem. Most of the fauna that they exploited was local and probably underwent very little transportation. This is supported by an analysis of skeletal abundances of elements most likely to resist attritional processes (i.e., high-survival skeletal set) (Marean and Cleghorn, 2003; Cleghorn and
Conflict of interest
Authors have no conflict of interest.
Acknowledgements
This work was carried out with support from a Research Salvador Madariaga grant to MDR (Ministry of Education, Culture and Sport, Spain. Ref PRX16/00010). We thank the Tanzanian Commission for Science and Technology (COSTECH), the Department of Antiquities, the Ngorongoro Conservation Area Authority in the Ministry of Natural Resources and Tourism and the National Museums of Tanzania for permission to conduct research at Olduvai Gorge and at the National Museum in Dar es Salaam. We also thank
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