Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia

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Journal of Human Evolution xxx (2015) 1e24

Contents lists available at ScienceDirect

Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol

Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia Bobe c, Denne Reed d, Jessica C. Thompson a, *, Shannon P. McPherron b, Rene W. Andrew Barr c, Jonathan G. Wynn e, Curtis W. Marean f, g, Denis Geraads b, h, Zeresenay Alemseged i a

Department of Anthropology, Emory University, 1557 Dickey Drive, Atlanta, GA 30322, USA Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, D-04103 Leipzig, Germany Department of Anthropology and Center for the Advanced Study of Human Paleobiology, The George Washington University, 2110 G St., NW, Washington, DC 20052, USA d Department of Anthropology, University of Texas at Austin, 2201 Speedway, Stop C3200, Austin, TX 78712, USA e School of Geosciences, University of South Florida, 4202 E Fowler Ave, NES107, Tampa, FL 33620, USA f Institute of Human Origins, School of Human Evolution and Social Change, Arizona State University, PO Box 874101, Tempe, AZ 85287-4101, USA g Faculty of Science, Nelson Mandela Metropolitan University, Port Elizabeth, Eastern Cape 6031, South Africa h Centre de Recherche sur la Pal eobiodiversit e et les Pal eoenvironnements (UMR 7207), Sorbonne Universit es, MNHN, CNRS, UPMC, CP 38, 8 rue Buffon, 75231 PARIS Cedex 05, France i Department of Anthropology, California Academy of Sciences, 55 Concourse Drive, San Francisco, CA 94118, USA b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 March 2015 Accepted 30 June 2015 Available online xxx

Two fossil specimens from the DIK-55 locality in the Hadar Formation at Dikika, Ethiopia, are contemporaneous with the earliest documented stone tools, and they collectively bear twelve marks interpreted to be characteristic of stone tool butchery damage. An alternative interpretation of the marks has been that they were caused by trampling animals and do not provide evidence of stone tool use or large ungulate exploitation by Australopithecus-grade hominins. Thus, resolving which agents created marks on fossils in deposits from Dikika is an essential step in understanding the ecological and taphonomic contexts of the hominin-bearing deposits in this region and establishing their relevance for investigations of the earliest stone tool use. This paper presents results of microscopic scrutiny of all nonhominin fossils collected from the Hadar Formation at Dikika, including additional fossils from DIK-55, and describes in detail seven assemblages from sieved surface sediment samples. The study is the ﬁrst taphonomic description of Pliocene fossil assemblages from open-air deposits in Africa that were collected without using only methods that emphasize the selective retention of taxonomicallyinformative specimens. The sieved assemblages show distinctive differences in faunal representation and taphonomic modiﬁcations that suggest they sample a range of depositional environments in the Pliocene Hadar Lake Basin, and have implications for how landscape-based taphonomy can be used to infer past microhabitats. The surface modiﬁcation data show that no marks on any other fossils resemble in size or shape those on the two specimens from DIK-55 that were interpreted to bear stone tool inﬂicted damage. A large sample of marks from the sieved collections has characteristics that match modern trampling damage, but these marks are signiﬁcantly smaller than those on the DIK-55 specimens and have different suites of characteristics. Most are not visible without magniﬁcation. The data show that the DIK-55 marks are outliers amongst bone surface damage in the Dikika area, and that trampling is not the most parsimonious interpretation of their origin. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Taphonomy Early hominin subsistence Pliocene hominin behavior Cut marks Trampling marks

* Corresponding author. E-mail addresses: [email protected] (J.C. Thompson), [email protected] (S.P. McPherron), [email protected] (R. Bobe), [email protected] (D. Reed), [email protected] (W.A. Barr), [email protected] (J.G. Wynn), [email protected] (C.W. Marean), [email protected] (D. Geraads), zalemseged@ calacademy.org (Z. Alemseged). http://dx.doi.org/10.1016/j.jhevol.2015.06.013 0047-2484/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Thompson, J.C., et al., Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia, Journal of Human Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.013

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J.C. Thompson et al. / Journal of Human Evolution xxx (2015) 1e24

1. Introduction Tool-assisted butchery is one of many taphonomic processes that may act on bones while they still contain nutritive value in the form of meat, marrow, or bone grease (i.e. a nutritive state). Butchery leaves marks on bone surfaces that provide direct evidence for hominin processing of animal tissues in the past. However, taphonomic processes are prone to equiﬁnality e where different processes lead to overlapping or indistinguishable end results. Butchery traces can be difﬁcult to distinguish from marks left by very different processes that bring stone into contact with bone, such as trampling (Behrensmeyer et al., 1986; Olsen and Shipman, 1988; Domínguez-Rodrigo et al., 2009) or natural rock falls (Oliver, 1989; Fernandez-Jalvo, 2012; Karr and Outram, 2012). At a superﬁcial level, carnivore tooth marks may also resemble certain types of butchery marks, such as percussion marks (Blumenschine and Selvaggio, 1988), and carnivore tooth marks may also be mimicked in turn by processes such as microbial bioerosion (Domínguez-Rodrigo and Barba, 2006). In most cases, microscopic studies using modern taphonomic reference collections allow for the correct diagnosis of these modiﬁcations (Blumenschine et al., 1996). However, when disagreements do arise regarding the agent(s) behind their production, this can lead to quite different interpretations of the ecological and behavioral contexts of the fossils on which the modiﬁcations are found. This has been the case at DIK-55, a Pliocene locality within the Dikika Research Project (DRP) area of the Lower Awash Valley in Ethiopia (Fig. 1). Here, we examine alternative interpretations of the modiﬁed DIK-55 fossils (Domínguez-Rodrigo et al., 2010; McPherron et al., 2010; Domínguez-Rodrigo et al., 2012) through study of surface modiﬁcations on all non-hominin fossils collected from the Hadar Formation at Dikika. The sample includes additional fossils from DIK-55, as well as seven assemblages from sieved

surface sediment samples. This enables comparisons of the size, shape, and attributes of the marks reported in 2010 to a large sample of other marks from the same deposits. We also present data on bone weathering, rounding, and other post-depositional processes that illustrate the utility of landscape taphonomic approaches to understanding the context of fossils found on Pliocene depositional landscapes. 2. Background 2.1. Ecological context of the DIK-55 specimens At DIK-55, two large-mammal fossils bear surface marks (DIK55-2, an adult rib; and DIK-55-3, a juvenile femur) interpreted by McPherron et al. (2010) to be a mix of cutting and percussion damage from stone tools, along with some modiﬁcations deemed unidentiﬁable. Two other fossil specimens that were examined did not preserve any modiﬁcation considered to be attributable to hominin butchery. However, because these fossils date to over 3.39 million years ago (Ma), any butchery marks on them would indicate that hominins wielded stone tools to process large mammal resources prior to the emergence of Homo. Although this is roughly contemporaneous with stone tools dated to 3.3 Ma at Lomekwi 3, Kenya (Harmand et al., 2015), large ungulate resource exploitation invokes scenarios of australopith diet, ecology, and social behavior that depart from those of great apes and from models of earlier hominin ecology such as those proposed for Ardipithecus spp. (White et al., 2009). Thus, if the DIK-55 marks were caused by stone tool butchery, a revised model of early hominin behavior and ecology would be required. Since ﬂaked stone artifacts or un-ﬂaked but utilized stone have not been found at DIK-55, they would also provide an independent way of identifying ancient archaeological occurrences on Pliocene landscapes.

Figure 1. Overview map of the DRP area relative to other study areas.

Please cite this article in press as: Thompson, J.C., et al., Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia, Journal of Human Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.013

J.C. Thompson et al. / Journal of Human Evolution xxx (2015) 1e24

Reconstructions of australopith diet and the environments in which they subsisted have been critical areas of paleoanthropological research for decades (Behrensmeyer and Reed, 2013; Stewart, 2014), which is why it is important to fully explore the utility of bone surface modiﬁcations as an additional line of inquiry. Current evidence suggests that a shift in diet occurred in the hominin lineage between 3 and 4 Ma, which may have derived from either a change in the foods that were selected within the same resource regime, or a change in the resources that were available overall (Lee-Thorp et al., 2010; Sponheimer et al., 2013). At this time, hominins began to more intensively exploit C4 based resources, although grassland environments had been present across northeastern Africa since the Late Miocene (Cerling et al., 1993, 2013; Feakins et al., 2013). Carbon isotopic evidence from Australopithecus afarensis indicates a diet that was highly variable between individuals, including some individuals with diets largely based on foods which use the C4 and/or CAM photosynthetic pathway (tropical grasses, sedges, and succulents), and/or herbivores/insects that fed upon those foods (Wynn et al., 2013). This contrasts with results from extant African apes and earlier hominins, such as Ardipithecus ramidus (White et al., 2009) and even from the morphologically similar and presumed parent species of Au. afarensis, Australopithecus anamensis, which consumed relatively few C4 and/or CAM foods in comparison to later hominins e a signature more consistent with closed-habitat foraging (Cerling et al., 2013; Sponheimer et al., 2013). The facial architecture of Australopithecus spp. suggests adaptations for heavy chewing loads (Strait et al., 2009). Fallback foods have been proposed as primary selective drivers for shaping morphological adaptations, behavior, and socioecology in primates (Marshall and Wrangham, 2007), and hard-object feeding (such as on nuts) has been proposed as an important fallback strategy that would have been accommodated by both australopith facial musculature and tooth topography (Ungar, 2004). However, dental microwear evidence from both Au. africanus (Scott et al., 2005) and Au. afarensis (Ungar et al., 2010) shows a highly variable diet with last meals mostly comprising either soft or tough materials. Animal tissues may be quite tough and require signiﬁcant masticatory effort (Wrangham and Conklin-Brittain, 2003), but this is not problematic if only small amounts of meat were eaten (Hardus et al., 2012). Thus, the degree to which ungulate tissues were incorporated into the diet remains unknown, but its consumption is not precluded by any of the anatomical features or geochemical dietary reconstructions in Australopithecus.

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(Domínguez-Rodrigo et al., 2009). One of the long cuts (mark A2) has an embedded rock fragment determined by secondary electron imaging (SEI) and energy dispersive X-ray (EDX) to be volcanic and its origin was interpreted to result from impact during the butchery phase. Such embedded rock fragments are common in chop and percussions marks. Thus, the argument was made from the combined evidence of embedded rock fragments and mark morphology determined by expert knowledge deployed with reference to the morphologies of experimental (known agent) and inferred (fossil) marks. It is not contested that the damage is ancient, i.e. that it occurred before the time of fossilization. Based on published photographs by McPherron et al. (2010), Domínguez-Rodrigo et al. (2010, 2012) alternatively interpreted the marks from DIK-55 as trampling damage. They also later added that the marks do not resemble those made by other possible agents such as vultures or crocodiles (Domínguez-Rodrigo et al., 2012). Although Domínguez-Rodrigo et al. (2010: 20933) assign all the marks to trampling, they agree that “Marks A1 and A2 on DIK-55e2 are morphologically compelling in their similarity to veriﬁed cut marks created by stone tools used in experimental butcheries: the marks show deep, V-shaped cross-sections and contain microstriations. In a less contentious context, the marks would likely be accepted as genuine cut marks”. Thus, although their argument is also largely based on the morphologies of the marks, they believe the context of the ﬁnds makes it difﬁcult to accept them as butchery marks “because of their singularity and because of the inferred age of the fossils” (Domínguez-Rodrigo et al., 2010: 20933). Although the age of the fossils is a less relevant point now that ﬂaked stone tools have been reported that date to 3.3 Ma (Harmand et al., 2015), no other marked fossils as old as the Dikika marks have since been reported. McPherron et al. (2011) noted that the main source of controversy is not over the morphology of the marks, but instead over the implications the ﬁnds would have for australopith tool use and diet. Domínguez-Rodrigo et al. (2011, 2012) responded that it is interpretation of the individual specimens that is at issue, and that trampling damage should be the null hypothesis for the marks from DIK-55. We argue here that an insufﬁcient sample of bone surface modiﬁcations has been studied from deposits dating to this time period to determine if the DIK-55 marks are indeed singular, and we use a large sample of other marks from the Hadar Formation at Dikika to provide a ﬁrst step toward achieving that quantiﬁcation.

2.3. Theoretical and methodological problems 2.2. The DIK-55 specimens McPherron et al. (2010) interpreted most (but not all) of the marks on the two DIK-55 specimens as inﬂicted by stone tools at or before the time of fossilization. The interpretation of twelve of the marks as being stone-tool inﬂicted was based on microscopic observations of their morphologies made blindly between experienced researchers, each of whom had passed with at least 95% accuracy blind tests of bone surface modiﬁcations on experimental assemblages where the source of the marks was known (Blumenschine et al., 1996). They converged in their interpretation of all twelve marks as being stone-tool inﬂicted and in six cases also converged more speciﬁcally on whether the action was cutting, percussion, or a combination of both (McPherron et al., 2010; their Supplementary Table 3). In addition to the morphological features of the marks and their anatomical placements, both scanning electron microscopy (SEM) and light microscopy showed a high incidence of microstriations, which are more common in stone-tool inﬂicted marks than in carnivore tooth marks (Blumenschine et al., 1996), and are present but less common in trampling marks

The paleoenvironment of Australopithecus spp. can be reconstructed to a certain degree, but as parts of an extinct ecosystem it is difﬁcult to understand the speciﬁc roles and behaviors of australopiths if we use only the modern analogs we have at hand. Researchers must be able to imagine and test scenarios that do not exist in the present day, and thus ambiguities in the DIK-55 marks may be attributable to combinations of behaviors and effectors that have no modern example (Thompson et al., 2011). Once such scenarios are imagined, in most cases we should be able to design model systems to simulate those processes to meet the stringent casual chain demanded by actualistic studies (Gifford-Gonzales, 1991). In addition to potentially taking a different form, evidence of the earliest butchery behavior is also not likely to be commonly encountered. This is because the behavior may initially have been so rare as to be effectively invisible in the archaeological record e much as is predicted to be the case for the earliest ﬂaked stone tools (Panger et al., 2002). As a related point, rarity of archaeological encounters does not in and of itself provide evidence for lack of a behavior, particularly if that behavior is subject to taphonomic bias,

Please cite this article in press as: Thompson, J.C., et al., Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia, Journal of Human Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.013

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J.C. Thompson et al. / Journal of Human Evolution xxx (2015) 1e24

spread over long time spans, and with a wide potential geographic distribution (Surovell and Grund, 2012). Domínguez-Rodrigo et al. (2012) performed experiments with unmodiﬁed stones to assess if the DIK-55 marks resemble marks made with unﬂaked stone. They maintained that the produced range of variability in mark morphology was too great to reliably assign any assemblage or individual subset of marks to them. Therefore, in their estimation, trampling must remain the null hypothesis because it can be more easily discerned from butchery damage. This approach not only excludes the possibility of identifying an entire class of potentially signiﬁcant ancestral behaviors (unmodiﬁed stone tool use), but it has been inconsistently applied to the zooarchaeological record. Domínguez-Rodrigo et al. (2011, 2012) did not consider that trampling should be a null hypothesis for the next-oldest purported butchery marks from Gona, Ethiopia (at 2.6 Ma), because some of those specimens were found in situ in deposits comprising ﬁner silt and clay particles that also contain stone tools. At other Plio-Pleistocene localities such as Bouri, also in Ethiopia, ex situ ﬁnds with purported butchery marks are not contested because although they are not associated with stone tools they date to a time when stone tools have been found elsewhere (de Heinzelin et al., 1999). This returns the argument back to a contextual one, rather than a morphological one, and illustrates how the ﬁeld of taphonomy would be well-served by developing more consensus and standardization on the role of context in the diagnosis of bone surface modiﬁcations (Njau, 2012; James and Thompson, 2015). We test trampling as the null hypothesis by quantifying: 1) how many other fossils recovered from the Hadar Formation at Dikika show marks with morphology similar to the contested DIK-55 marks; 2) how the morphologies of marks from a large sample of sieved fossils (e.g. a collection unbiased by paleontological selection) compares to the contested marks; and 3) how the characteristics from the DIK-55 marks compare to other marks from Dikika fossils inferred to be trampling marks, as well as to known trampling marks from published experimental assemblages. We also employ a comprehensive taphonomic analysis to explore the potential for bone surface modiﬁcation in Pliocene deposits to provide information about the broader taphonomic contexts of ﬁnds such as those from DIK-55, and to aid in the reconstruction of timeaveraged but distinctive habitats across the paleolandscape. Although much informative work has been done with analyses of skeletal element representation and fragmentation (Alemseged, 2003; Su and Harrison, 2008; Behrensmeyer and Reed, 2013), taphonomic analyses of fossils from landscapes of this age are typically not based on complete assemblages of all bones recovered from samples of sieved sediments, but are restricted to relatively complete and/or diagnostic fossils that are useful for taxonomic

identiﬁcation, ecomorphological analysis, or description/curation of rarely-preserved taxa (including hominins). Table 1 shows the typical paleontological collecting protocols within the Dikika Project Area, with speciﬁc criteria for completeness. This collecting strategy is problematic for bone surface modiﬁcation studies because bones or bone portions that are not taxonomically diagnostic, such as long bone midshaft fragments and ribs, can be some of the most informative for understanding the interactions of different bone-modifying agents (Pante et al., 2012). Detailed site formation processes are also difﬁcult to reconstruct from selected datasets, although some taphonomic patterning in variables such as skeletal part preservation or overall bone integrity is still discernible at different environmental, spatial, and temporal scales. We therefore propose that the broader value of this study is that it is the ﬁrst to include detailed taphonomic work (including bone surface modiﬁcation work) across sieved samples of sediment taken from open-air localities in Pliocene hominin-bearing deposits. This augments sedimentological data used to infer depositional environments, and allows for more nuance in reconstructing habitat assemblages at speciﬁc localities that may have presented different foraging opportunities for australopiths. 2.4. Dikika Research Project area description and depositional context The DRP area is located in the Lower Awash Valley (Ethiopia). It is bordered on the north by Gona, Hadar, and Ledi-Geraru and on the south by the Middle Awash research areas (Fig. 1). Work in the DRP area began in 1999 and has focused on survey in the Hadar (>3.8e2.9 Ma) and Busidima (2.7e0.15 Ma) Formations. In addition to the reported discovery of Pliocene-aged fossils bearing butchery marks (McPherron et al., 2010), the work has resulted in the discovery of a diverse and well preserved fauna, the discovery of several hominin fossils including a nearly complete juvenile Au. afarensis (DIK-1-1; Alemseged et al., 2006; Wynn et al., 2006), the only known hominin from the Basal Member of the Hadar Formation (Alemseged et al., 2005), and a complete geological description of the hominin-bearing Hadar Formation (Wynn et al., 2008). The lower contact of the Hadar Formation disconformably overlies an eroded and weathered surface of the Dahla Series Basalts, while its upper boundary is deﬁned by an angular unconformity to the overlying Busidima Formation (Wynn et al., 2008). The Hadar Formation contains four tephras which provide direct age information (Walter, 1981; Campisano and Feibel, 2008), three of which further divide the formation into stratigraphic members (from bottom to top: Basal, Sidi Hakoma, Denen Dora and Kada Hadar Members). In bulk, the sediments of the Hadar Formation

Table 1 Description of the criteria a fossil must meet in order to be collected under Protocol 1 or Protocol 2. Protocol number 1

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Description Primates regardless of preservation All mammalian cranial elements identiﬁable to family e except Elephantidae and Hippopotamidae, which are recorded All horn core and ossicone fragments All mammalian isolated teeth at least half preserved All complete crocodilian teeth in a bulk sample for each locality Fish, crocodile, or turtle crania that are relatively complete All astragali at least half complete All calcaneus, scapula, humerus, radius, ulna, femur, tibia, ﬁbula, and metapodial fragments at least 3/4 of the complete bone e except Elephantidae and Hippopotamidae Lizard or snake vertebrae Make observations on anything left behind (crocodile, turtle, ﬁsh, uncollected long bone fragments identiﬁable to family) Everything collected under Protocol 1 Any long bone with at least 1 articular surface

Please cite this article in press as: Thompson, J.C., et al., Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia, Journal of Human Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.013

J.C. Thompson et al. / Journal of Human Evolution xxx (2015) 1e24

represent the ancient deposits of Lake Hadar, a large persistent lake basin that had its depocenter northeast of Dikika (Campisano and Feibel, 2008; Wynn et al., 2008). Thus, during times of high lake levels widespread laminated clays and diatomites were deposited, such as those which characterize much of the deposition of the Basal Member and Denen Dora Members. Meanwhile, during relative lake lowstands, the shallow lake and delta clays were subject to soil formation on low-gradient mudﬂats and distributary river channels that traversed through the swampy delta plains prior to terminating at the lake's shoreline (Aronson and Taieb, 1981; Campisano and Feibel, 2008; Wynn et al., 2008). With this bigpicture view of depositional environments in mind, one can represent individual exposures of the Hadar Formation as from a range of depositional settings including permanent lake, meandering ﬂuvial channels and their ﬂoodplains, distributary delta channels and their delta plains with swamps, and shallowgradient soils (vertisols) in the mudﬂats.

3. Materials and methods 3.1. Field methods Collection protocols for general paleontological purposes focused on specimens that were at least ¾ complete, and which have been shown to offer information about taxonomic abundances or the ecomorphology of members of the fossil community (Plummer and Bishop, 1994). Collecting Protocols 1 and 2 are described in Table 1. For the present study, a new Protocol was also established and used. This “Protocol 3” is also referred to here as the “circle collections”. The majority of fossil ﬁnds in the DRP area are surface ﬁnds and it is useful to understand the taphonomic processes in operation on such specimens, without restricting analysis to any particular sedimentary context. Using Protocol 3, six samples were collected from two different stratigraphic intervals of the Sidi Hakoma Member (DIK-41-170, DIK-42-23, DIK-43-52, DIK-48-26, DIK-49-13, and DIK-50-50), and one was collected from an interval of the Basal Member (DIK-58-20) of the Hadar Formation. Beyond this, the speciﬁc locations were chosen at random. These are illustrated in Figure 2, and Table 2 provides a summary of their sedimentological characteristics. Note that localities DIK-49 and DIK-50 cluster together stratigraphically in comparison to the other Sidi Hakoma Member localities. Once a general locality was selected for sampling, the collector randomly tossed a marker onto the ground behind them. This provided the center point for the collection area, which was measured as a circle with a radius of 3 m. All surface ﬁnds were collected using Protocol 1 and then the top 2e3 cm of loose sediments within this circle were sieved through a 5 mm mesh and all fossil fragments were collected. No excavation into the intact sediments below was undertaken. Thus, each collection area included all the fossils within an area of ca. 28 m2 and to a maximum of approximately 3 cm depth. The recovered fossils are considered to be time-averaged samples of what would be found in the depositional environments represented in the exposed sections at each locality and summarized in Table 2, and not necessarily related to what would be found in underlying sediments. Once in the laboratory, all fragments were washed in clean water and all specimens >5 mm in the maximum dimension were analyzed.

3.2. Data collection and analytical methods Data were collected and analyzed in order to achieve the following aims:

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1) Provide a description of the taxonomic and skeletal part abundances found in assemblages that were not selected only for their taxonomic or ecomorphological utility; 2) Describe the sedimentological features of deposits containing the circle collections; 3) Describe the range of taphonomic modiﬁcations to bones across the DRP area; 4) Characterize the post-depositional histories of fossils from different localities; 5) Characterize the peri-depositional histories of fossils (i.e., what agents interacted with them when they were in a nutritive state); 6) Provide an assessment of the attributes of the modiﬁcations on the DIK-55 specimens relative to other modiﬁcations found across the DRP area. Laboratory analysis included two phases: basic examination of the surfaces of all non-hominin fossils collected from the DRP area under Protocols 1 and 2 and curated in the National Museum of Ethiopia, and detailed study and recording of all specimens recovered from the seven circle collections. All specimens were ﬁrst subjected to microscopic scrutiny under a 10e20 hand lens with bright incident light shining obliquely across the surface. This enabled a qualitative overview of whether any specimens exhibited surface modiﬁcation resembling those from DIK-55. The process simulated the means by which the two modiﬁed specimens from DIK-55 were recovered and initially identiﬁed, in that their surfaces were inspected and diagnosed using an expert knowledge approach by an analyst familiar with both experimental and fossil bone surface modiﬁcations. In total, 1086 bones were examined in this way. This procedure allowed for examination of bone surfaces from larger and more complete specimens recovered from 113 localities at Dikika, as well as an overview of what morphologies bone modiﬁcations exhibited across a range of depositional environments found in the Hadar Formation of the DRP area. The second phase of analysis focused on the circle collections (collection Protocol 3), where samples of all bone fragments larger than 5 mm in the maximum dimension could be assessed. This allowed quantiﬁcation of taxonomic and skeletal element representation within assemblages that did not comprise specimens selected only for their completeness or taxonomic utility. It also provided the basis for assessing how commonly marks of different morphologies and sizes occurred within these assemblages. All specimens were examined under a 10e40 binocular zoom microscope with bright incident light from an illuminator shining obliquely across the surface. This is one procedure that has been shown to maximize the ability to locate and describe bone surface modiﬁcations (Blumenschine et al., 1996). Specimens were subjected to brief reﬁtting to understand the potential for over-representation of taxa because of single highly fragmented elements. Specimens were then coded according to skeletal part and lowest possible taxonomic afﬁnity. Mammalian specimens were given individual records in the database if they could be identiﬁed to speciﬁc skeletal part or if they exhibited any surface modiﬁcation that could be discerned under low-powered microscopy. Reptile and ﬁsh remains were given individual records if they exhibited any surface modiﬁcation, but detailed taxonomic or skeletal part assignation will be conducted by the relevant specialist in a future study. All remaining specimens were bulk-recorded according to a series of summary taphonomic attributes presented in Table 3. The total studied number of identiﬁed specimens (NISP) from the circle collections including both bulkcollected and individually-collected data was 2926. For the Dikika fossils, each individual surface modiﬁcation was given its own record. In an effort to produce an objective method

Please cite this article in press as: Thompson, J.C., et al., Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia, Journal of Human Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.013

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J.C. Thompson et al. / Journal of Human Evolution xxx (2015) 1e24

Figure 2. Detailed map of study area showing geographic locations of circle collections.

for recording marks, Domínguez-Rodrigo et al. (2009) introduced a protocol for differentiating experimentally produced trampling marks from cut marks based on multivariate analysis of sixteen mark attributes, fourteen of which are based on categorical variables, one on a discrete variable, and one on a continuous variable. Linear marks (deﬁned as marks with a length:breadth ratio 2) were coded from the Dikika specimens according to all the attributes advocated by Domínguez-Rodrigo et al. (2009). This allowed for a descriptive evaluation of the frequency with which marks occur in these deposits that resemble those found on the two DIK55 specimens, as well as direct comparison to published experimental cutting and trampling data. Amorphous marks (deﬁned as marks with a length:breadth ratio < 2) were coded using the criteria described in Table 4. Marks with components that had both

morphologies were coded under both systems (e.g. a mark with a puncture and a trailing score from it). Marks were also assigned a qualitative assessment of the agent to which it would be diagnosed under a more traditional expertknowledge approach to mark identiﬁcation. For example, if their morphology was that of a tooth mark, percussion mark, or cut mark as deﬁned by Blumenschine et al. (1996), or if they had trample mark morphology as described by Behrensmeyer et al. (1986) then this was noted. Individual mark records were not created for marks that had morphologies resembling fungal etching (DomínguezRodrigo and Barba, 2007), insect etching (Backwell et al., 2012), or other forms of bioerosion such as root etchings (Fisher, 1995), but their presence was noted for each specimen. It was also noted at what level of conﬁdence (e.g. high or medium) each mark would

Please cite this article in press as: Thompson, J.C., et al., Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia, Journal of Human Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.013

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Table 2 Location of circle collections and DIK-55, the stratigraphic location of fossils, sedimentological features, and interpretation of depositional environments represented. Locality

Location

Position

58

Lebalahale

Basal Mb.

55

Andedo

lower SH Mb.

41,42,43

Andedo

lower SH Mb.

48

Andedo

lower SH Mb.

49

Andedo

mid SH Mb.

50

Andedo

mid SH Mb.

Stratigraphic description of potential fossil-bearing sediments

Sedimentological description

Interpretation of depositional environments represented

Short section just above weathered upper surface of DSB. Most fossils derive from zeolitiferous greenegray sands at the extreme base of the Basal Member. Locality exposes only sediments between SHT & SH-lm. Many fossils are found bl SHT. In most cases, an unconsolidated gray sand ~3 m ab SHT appears to produce most fossils, especially those with especially clean surfaces (no adhering matrix which is commonly observed on fossils from DIK-1 sands). Gastropod-bearing sands (B-g), and other sands below the SHT may produce some of the specimens found below the SHT. Predominantly sediments between SHT & SH-lm. Several local ridges expose up to DIK-1A sand, and some fossils may derive from above this unit. Many fossils are found below SHT, but are still likely to derive from the SH Member based on circumstantial stratigraphic evidence. Unconsolidated gray sand ~3 m above SHT appears to produce most fossils, has produced fossils in situ, and preserves many with especially clean surfaces (no adhering matrix which is commonly observed on fossils from DIK-1 sands). Predominantly sediments between SHT & SH-lm. Several local ridges expose up to DIK-1A sand, and some fossils may derive from this unit. Many fossils are found bl SHT. In most cases, an unconsolidated gray sand ~3 m ab SHT appears to produce most fossils, especially those with especially clean surfaces (no adhering matrix which is commonly observed on fossils from DIK-1 sands). However, it is clear that gastropod-bearing sands (B-g), and other sands below the SHT may produce some of the specimens found below the SHT. A local fault with 3 m offset cuts through the locality. Similar section to that at DIK-1 (Wynn et al., 2006); locality exposes from below DIK-1A sand to above DIK-1B. Most fossils, however, likely derive from one of the two sands, or thin intervening sand units. Fossils are cemented with carbonate cemented matrix, and often distorted by postburial fracture & cementation. Locality exposes between DIK-1A and DIK-1B sands. Distinct gravel lag at the base of trough cross sets produce ﬂagstones of conglomerate (grit) of DIK-1B sand. All fossils presumed to derive from between DIK-1A and DIK-1B sands.

Massive to cross-bedded, poorly sorted, subangular, juvenile litharenitic sands with average grain size 1e24. Scour and ﬁll structures with lower surfaces sharp and erosional contacts. Sequence of weakly bedded to massive clays, limestones containing ﬁsh scales and plant fragments as well as a diatomaceous bed. Unconsolidated gray sands is massive and poorly sorted with average grain size 1e24 (medium grained), with particles that range from ﬁne sand to ﬁne gravel (3 to 34). Thin beds containing well-rounded gravel clasts form lag deposits at the base of erosional scours.

Ephemeral streams with shallow, ﬂashy ﬂow locally redepositing juvenile sediment deriving from weathered surfaces of basalt.

have been ascribed to that agent if the diagnosis was based purely on morphological criteria. In previous work, only high-conﬁdence marks have been published for zooarchaeological assemblages, while medium-conﬁdence marks provide additional data about the closest morphological match to other marks in an assemblage

Shallow lake, muddy shoreline, and sheetwash sand deposits which protrude into lake produce most fossils.

Sequence of weakly bedded to massive clays, limestones containing ﬁsh scales and plant fragments as well as a diatomaceous bed. Unconsolidated gray sands as in locality 55. Some parts of locality may contain medium-grained, subangular, consolidated sand with trough cross-bedding from uppermost section (see description of locality 49 e50 for detailed description).

Shallow lake, muddy shoreline, and sheetwash sand deposits which protrude into lake produce most fossils. Some parts of locality may contain distributary delta plain deposits.

Sequence of weakly bedded to massive clays, limestones containing ﬁsh scales and plant fragments as well as a diatomaceous bed. Unconsolidated gray sands as in locality 55.

Shallow lake, muddy shoreline, and sheetwash sand deposits which protrude into lake produce most fossils.

Sequence of cumulative moderate to weakly developed black Vertisols with abundant discrete carbonate nodules. At least two distinct, extensive, tabular, cemented sand bodies are medium- to coarse-grained (0e24), subrounded with 5e25 cm scale trough cross-sets and occasional vertical calcareous rhizoliths. Sequence of cumulative moderate to weakly developed black Vertisols with abundant discrete carbonate nodules. At least two distinct, extensive, tabular, cemented sand bodies are medium- to coarse-grained (0e24), subrounded with 5e25 cm scale trough cross-sets and occasional vertical calcareous rhizoliths.

Distributary delta channel & associated mudﬂat paleosols (Vertisols) of delta plains. Fossils attributed to distributary delta sands.

Distributary delta channel & associated mudﬂat paleosols (Vertisols) of delta plains. Fossils attributed to distributary delta sands.

(Marean et al., 2000; Thompson, 2010; Thompson and Henshilwood, 2011). These deﬁnitions were also employed here. Different cut mark and tooth mark morphologies were described in terms of their general shape (Table 4), again to simulate a scenario in which the diagnosis was based purely on morphological grounds

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Table 3 Attributes used for bulk recording of taphonomic processes on unmarked and undiagnostic specimens in this study. Attribute Taxon Fragment type Fragment size class Recent break present Weathering stage Rounding stage Notes

Variables Mammal, ﬁsh, crocodile, turtle/tortoise or more speciﬁc where possible Non-ID, enamel fragment, tusk fragment, or more speciﬁc where possible 0.5e0.9 cm, 1.0e1.9 cm, and 1 cm increments up to 10 cm Yes/no 0e5 following Behrensmeyer (1978) 0 ¼ no evidence of rounding; 1 ¼ some edge rounding; 2 ¼ moderate edge rounding; 3 ¼ rounding apparent across entire fragment Any relevant notes speciﬁc to the entry

rather than a combination of morphological and contextual grounds, as advocated by Blumenschine et al. (1996). Because Domínguez-Rodrigo et al. (2009) only described the attributes of marks they could see with the naked eye, a note was made if each mark on the Dikika specimens was visible without magniﬁcation or not. The total number of recorded marks was 482 from 256 marked specimens. Statistical tests were all performed using the software PAST, and included the ManneWhitney U test to assess pairwise differences in the median values of two populations, the KruskaleWallis test to determine if a series of median values differed signiﬁcantly across several populations, the Chisquared test to determine if there were signiﬁcant differences

between the distributions of data between two populations, Fisher's exact test to assess differences between two proportions (in a two-by-two distribution table), and Spearman's rho to determine if two variables correlated with one another (Hammer et al., 2001). 4. Results 4.1. Paleontological collections Some data on the taxonomic composition of the Sidi Hakoma and Basal Members based on paleontological collecting from the

Table 4 Attributes used to record marks. Attribute Cut mark morphology (adapted from the program described in Abe et al., 2002 and from Blumenschine et al., 1996)

Tooth mark morphology (adapted from Njau and Blumenschine, 2006)

Total number of damage patches Number of small pits within the main mark Location of microstriations relative to main mark

Location of main mark

Maximum length of main mark Maximum breadth of main mark Main mark damage type (as many as apply)

Bruising

Character state

Description

Cut Slice Shave Scrape Puncture Puncture þ Drag Chop Saw Pit Score Pit þ score Puncture Puncture þ score Star puncture Triangular score Hook Bisected pit Integer 1

Incision perpendicular to the bone surface Incision at angle to bone surface Small curls of bone peeling away from a slice Broad, shallow ﬁelds often with dimpling Cortical surface has been breached Cortical surface has been breached and a linear mark emanates from the puncture Short, deep cut Multiple striae occurring in a patch Bone surface is pitted downward in a round depression Bone surface has been removed in a linear, U-shaped mark A score emanates from a pit The cortical bone surface is completely breached The cortical bone surface is completely breached and a score emanates from the puncture The cortical bone surface is completely breached and the puncture is bisected A score begins with a triangle shape and ends in a long tail A score traces a hook-shaped trajectory The pit has been bisected Total number of discrete areas of damage that occur within 2 mm of one another

Integer 1

Number of discrete pits within the main mark (e.g. largest mark)

Inside only Outside only

Microstriations only present within the boundaries of the main mark Microstriations only present outside the boundaries of the main mark (within up to 2 mm of any associated damage patch) Microstriations are continuous between inside and outside the boundaries of the main mark Microstriations present both inside and outside the boundaries of the main mark No microstriations observed Main mark occurs at edge of a complete element portion, for example the rim of a carpal Main mark occurs within 1 mm of a crack in the bone Main mark occurs within 1 mm of the edge of a fracture Main mark occurs within 1 mm of a notch on a fracture edge Main mark is isolated at least 1 mm away from any fragment portion described above Measured using callipers Measured using callipers Shape is round Shape is oval and has one end deeper than the other Bone has been displaced in a circle around a point Microstriations occur in a patch Bone is crushed and characterized by micro-cracking Bone is peeling away Bone is displaced horizontally with no apparent shape Bone is displaced vertically with no apparent shape Bone has been discolored with an origin point at the main mark

Emanating Inside and outside Absent Element edge At crack Fracture edge In notch Isolated To nearest 0.1 mm To nearest 0.1 mm Pit Gouge Divot Microstriation patch Crushing Delamination Displacement Compaction Yes or No

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DRP area have been presented elsewhere (Alemseged et al., 2005; Wynn et al., 2006; McPherron et al., 2010). Specimens collected using Protocols 1 and 2 were scrutinized to provide a general overview of surface modiﬁcations across the DRP area. This revealed several types of marks, including large bisected pits and punctures typical of crocodile damage (Njau and Blumenschine, 2006; Baquedano et al., 2012), smaller pits and scores more typical of mammalian carnivore tooth marks (Blumenschine and Selvaggio, 1988; Blumenschine et al., 1996), dendritic etchings indicative of root, microbial, or fungal activity (Fisher, 1995; Thompson, 2005; Domínguez-Rodrigo and Barba, 2006), shallow and wandering striae typical of trampling damage (Behrensmeyer et al., 1986; Domínguez-Rodrigo et al., 2009), and a range of other modiﬁcations such as cracking and exfoliation suggestive of subaerial exposure (Behrensmeyer, 1978), and matrix invasion of cracks that likely propagated in situ from sediment pressure (Villa and Mahieu, 1991). Representative images of notable modiﬁcations are provided in Figure 3. Apart from the two originally described DIK-55 fossils (McPherron et al., 2010), no specimens from any locality in the DRP area had surface modiﬁcations that had attributes describable as stone tool butchery marks. None of the surface marks in the remainder of the paleontological collections was characterized by a combination of V-shaped cross-sections, straight trajectories, shoulder effects, sub-parallel microstriations both internally within a groove and as a shoulder effect alongside a main groove (Domínguez-Rodrigo et al., 2009), pitting in association with internal and external microstriations (Pickering and Egeland, 2006),

9

or any of these suites of characteristics on parts of the bones that show anatomical placements indicative of ﬂesh removal or marrow extraction (Bunn and Stanford, 2001). Some specimens did show marks with some of these features, but never all in the same mark and usually occurring near modiﬁcation more typical of nonhuman agents such as abrading sediments, crocodiles, or mammalian carnivores (Figure 4).

4.2. Circle collections The taxonomic and skeletal element representation of the circle collections is provided in Table 5. They represent a diverse vertebrate fauna comprising small and large mammals, small and large reptiles, ﬁsh, and birds e as well as one freshwater mollusk. Coprolites were present in DIK-41, DIK-42, DIK-43, and DIK-58. The majority of all specimens could be classiﬁed as aquatic in their habitat preferences and 35% of all recovered specimens were ﬁsh. Although the ﬁsh taxa and skeletal element abundances were not speciﬁcally quantiﬁed, the majority of identiﬁed specimens were fragments of cranial armor rather than individual diagnostic specimens. This suggests a high degree of fragmentation rather than a large minimum number of individuals. Both aquatic turtles and land tortoises were present. Although generally the two types of chelonians could not be distinguished based on small fragments of carapace and plastron, 88% of those that could be identiﬁed were aquatic turtles with carapace patterning typical of the family Pelomedusidae.

Figure 3. Representative images of surface modiﬁcations across the DRP area: (aec) punctures, linear marks, and check marks (respectively), all considered diagnostic of crocodile damage; (dee) scratches considered diagnostic of trample damage; (f) parallel grooves diagnostic of rodent gnawing; (g) gastric etching; (hei) carnivore tooth marks more typical of mammalian tooth damage, but overlapping in morphology with experimental crocodile damage.

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Figure 4. Examples of marks with some ambiguous features: (a) subparallel incisions with V-shaped cross-sections and abundant microstriations on a mammal long bone, which is typical of cut and percussion damage but which in this case is inferred to be crocodile damage from the associated puncture and sharp “jag” to the bottom left; (b) subparallel lines on a ﬁsh element that superﬁcially resemble cut marks but which are too shallow and broad to interpret as “high conﬁdence”; (c) bifurcated V-shaped mark on a mammal rib fragment that under high magniﬁcation shows a “tearing away” of the mark walls in a way more typical of tooth damage.

Only 5% of all specimens by NISP were identiﬁed as crocodile, although in many cases highly fragmented chunks of bones from large animals could not be easily separated into either mammal or reptile. Thus, crocodile specimens comprised 16% of all specimens that could be identiﬁed as either large mammal or crocodile. Amongst mammals, 61% of all specimens identiﬁable to the family level or below were from Hippopotamidae. This number is likely an overestimate because of a large number of fragments from a single innominate from DIK-43 that was inferred to conjoin. The same is likely true for elephants, which were represented by a series of enamel fragments from a single locality (DIK-58). Bovids comprised 41% of the identiﬁable mammal specimens that were not hippopotamuses, but these could generally not be identiﬁed to the tribal level or below. The single specimen that could be identiﬁed to the genus level (Damalborea) was from the tribe Alcelaphini, modern representatives of which prefer semi-arid grasslands. Thirteen

specimens assigned to Suidae and one to Equidae also support the presence of some local grassland, but sample sizes of identiﬁable taxa were too small to distinguish between localities. The relative representation of aquatic fauna is likely to be useful in categorizing localities according to their depositional characteristics, but the Protocol 3 collecting strategy must be paired with Protocols 1 and 2 if a larger sample of more speciﬁc taxonomic abundance data is desired. Long bone breakage patterns can inform if most fragmentation in an assemblage took place while the bone was in a fresh (nutritive) state or a dry (non-nutritive) state (Villa and Mahieu, 1991). Unfortunately, there were insufﬁcient numbers of long bones in the Dikika assemblages to assess fragmentation in this way. The degree to which bones were broken after becoming fossilized, likely during their recent exposure history, could be assessed through examination of fracture edges of all bones. This is relevant for

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11

Table 5 Number of Identiﬁed Specimens (NISP) in each sample. Shading indicates fragments identiﬁed to the same family level listed immediately above it. Phylum

Class

Order

Family

Fragment Type

DIK-41

DIK-42

DIK-43

DIK-48

UM

M

UM

0 0 0 0 205 34 309 12 6 0 0 0

UMa Mb UM Mollusca Not determined Chordata Osteichthyes Reptilia

Not determined Not determined Testudines

Not determined Not determined Pelomedusidae

Complete Not determined Marginal

Testudinidae

M

DIK-49

DIK-50

M UM

M UM

0 54 0

0 7 0

0 19 0

0 0 0

1 26 0

M

DIK-58 UM

Total

M

0 105 0

0 2 0

Xiphiplastron Carapace/Plastron Marginal

0 1 0

0 0 0

0 18 0

1 7 0

0 0 0

0 0 0

0 2 1

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

1 28 1

Carapace/Plastron Ilium

1 0

0 0

0 0

0 0

0 0

0 0

2 0

0 0

0 0

0 0

0 0

0 0

0 1

1 0

4 1

Not determined

Limb Fragment Vertebra Neural Costal Marginal Hyo/Hypoplastron Xiphiplastron Carapace/Plastron Cranial

0 0 0 0 0 0 0 13 0

0 0 0 0 0 0 0 3 0

2 1 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0

0 0 0 3 1 0 0 7 29

0 0 0 0 0 0 0 4 5

0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 1 0

0 0 0 0 0 0 0 0 0

0 1 1 0 0 0 0 0 1 0 0 1 0 0 6 10 0 0

1 0 1 0 1 0 0 0 0 0 0 0 1 0 77 12 0 0

4 3 1 3 2 1 1 133 34

Not determined Elephantidae

Mandible Dental Scute Other Long Bone Dental

0 1 1 2 0 0

0 0 0 0 0 0

0 2 1 0 1 0

0 0 0 0 0 0

0 50 2 13 0 0

3 0 0 1 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 1 1 0 0 0

0 0 0 0 0 0

0 38 9 0 0 23

0 0 0 0 1 0

3 92 14 16 2 23

Metapodial Dental Horn Core

0 1 0

0 0 0

0 0 0

0 0 0

0 0 2

0 0 0

0 0 3

0 0 0

0 0 3

0 0 0

0 0 1

0 0 1

1 0 3

0 0 0

1 1 13

4 0 0 0 0 0 0 3

0 1 0 0 0 0 0 0

0 0 0 1 0 1 0 0

0 0 1 0 1 0 0 0

1 0 2 0 0 0 0 5

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

1 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

1 0 1 0 0 0 1 0

0 0 0 0 0 0 0 0

1 0 0 0 0 0 0 17

0 0 0 0 0 0 0 0

8 1 4 1 1 1 1 25

Suidae Not determined

Alveolus þ dental Femur Innominatec Magnum Dental Cranial

0 0 0 0 5 0

0 0 0 0 0 1

0 0 0 0 0 0

0 0 0 0 0 1

0 0 0 0 54 22 0 1 1 0 0 0

1 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 1 1 0 7 0

0 0 0 0 0 0

1 1 77 1 13 2

Not determined

Alveolus Maxilla Dental Tusk* Cervical vertebra Thoracic vertebra Lumbar vertebra Vertebra (indet.) Rib Scapula Humerus Femur Tibia Long Bone Compact Bone Non-ID Cortical Non-ID Spongy Cranial

0 0 2 0 0 1 0 1 0 0 0 0 0 4 0 2 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 0 0

0 0 25 0 0 0 0 0 0 0 0 0 0 1 0 47 49 0

2 0 0 0 0 129 0 0 0 0 0 0 0 0 0 1 2 0 0 0 1 0 0 0 0 0 3 1 1 0 0 6 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

3 1 185 219 6 3 3 22 7 1 2 2 1 23 2 122 51 2

Not determined

Not determined

Vertebra Long Bone Non-ID Cortical Calcaneus

0 0 0 0

0 0 0 0

Mammalia/Aves Not determined Mammalia/Reptilia Not determined

Not determined Not determined

Tibia Long Bone Cranial

0 0 0

Alveolus Mandible

0 0

Testudinidae/ Pelomedusidae

Crocodilia

Aves Mammalia (>4 kg fossil)

Not determined Proboscidea

Carnivora/Primates Not determined Primates Cercopithecidae Artiodactyla Bovidae

Dental Humerus Metapodial Astragalus 2nd Phalanx Equidae Dental Bovidae/Equidae Dental Hippopotamidae Dental

Not determined

Mammalia (>4 kg recent)

Mammalia (<4 kg)

Not determined

0 0 0 1 8 212 35 1028 0 0 0 6

0 0 1 0 0 9 0 219 0 6 0 2 0 3 0 16 0 1 0 0 0 0 0 0 0 1 3 1 0 0 5 0 0 0 0 0

1 0 0 0 0 0 0 4 1 1 0 1 0 2 0 2 0 0

0 0 3 0 0 0 0 0 0 0 0 0 0 2 1 2 0 2

0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0

0 0 2 0 0 0 0 0 1 0 0 0 0 0 0 52 2 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 15 0 0 0 0 0 1 0 1 1 0 6 0 4 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

1 4 1 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 1

0 0 0 0

1 4 1 1

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

1 1 1

0 0 0

1 1 1

0 0

0 0

0 0

2 0

0 1

0 0

0 0

0 0

0 0

0 0

0 0

1 0

0 0

3 1

(continued on next page)

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J.C. Thompson et al. / Journal of Human Evolution xxx (2015) 1e24

Table 5 (continued ) Phylum

Class

Order

Family

Fragment Type

DIK-41

DIK-42

UMa Mb UM

Not determined

Not determined

Not determined

Rib Vertebra Non-ID Cortical Non-ID Spongy Not determined

Total a b c

0 1 18 16 0 182

0 0 2 0 0 11

M

DIK-43

DIK-48

UM

UM

M

0 0 1 0 0 0 0 0 0 0 2 3 108 16 19 0 0 35 0 6 10 0 0 0 0 372 57 883 77 104

DIK-49

DIK-50

M UM

M UM

0 1 0 0 0 9

0 0 0 0 0 0

1 0 0 0 0 82

M

DIK-58 UM

Total

M

0 0 4 0 6 0 0 1 0 3 47 6 150 11 382 11 0 234 0 302 0 0 0 0 10 127 36 926 60 2926

Unmarked. Marked. Inferred to all conjoin from a single specimen.

understanding the degree to which a given circle collection includes fossils that have been exposed at the surface for different periods. Where recent breaks occurred, the break surface had a different appearance from the rest of the fossil surface, and lacked adhering matrix where matrix was present. This could not be easily determined for spongy bone fragments, but all data for the remainder of the assemblage are provided in Figure 5, and show signiﬁcant differences between localities (X2 ¼ 340.58; DF ¼ 6; p < 0.0001). Weathering stage data show that different localities produced fossils with varying exposure histories. Localities can generally be grouped according to whether or not three-quarters or more of their fossils fall into the pristine weathering category of Stage 0 or if there is a wider range across categories (Fig. 6). The Basal Member collection from DIK-58 has a clearly bimodal distribution, with nearly 40% of the fossils in the least extreme category of no weathering (Stage 0) and approximately 25% of the fossils in the most extreme category of heavy weathering (Stage 5). Although some elements, such as crocodile scutes, do not weather in comparable ways to other elements, these are so rarely represented that they should not affect the weathering data. Of the Sidi Hakoma Member samples, DIK-49 had a very high proportion of Stage 5 fossils, and DIK-50 had a more even distribution in the later weathering stages but with the majority still falling in the lower stages overall. Fragment sizes were generally small in comparison to the paleontological collections, with 33% of all fragments less than 2 cm in the maximum dimension (Table 6), and a median fragment size class of 2 (1.0e1.9 cm). There were some signiﬁcant differences between localities (KruskaleWallis H ¼ 123.7; p < 0.0001). DIK-49 and DIK-50 had signiﬁcantly larger fragment sizes than the other collections but were indistinguishable from one another (ManneWhitney U ¼ 5710; p ¼ 0.5577). These two localities also cluster together stratigraphically, and have different depositional

Figure 5. Recent break data. Fragments with a recent break are labelled “yes” and those without are labelled “no”. Numbers of fragments are provided within each bar.

environments as inferred by the sedimentology than do the other Sidi Hakoma Member localities. The abundance of ﬁsh fragments in the circle collections provide an obvious explanation for why so many fragments are small, but even when ﬁsh are removed from the analysis the median size class remains 2, and the same size class patterning across the collections remains apparent. The distribution of edge rounding on the fossils differs between localities (Fig. 7a). Although most localities had some specimens that fell into the most extreme rounding class (Class 3), localities such as DIK-42 and DIK-43 almost exclusively had specimens in class 0. The Basal Member locality of DIK-58 had the highest proportions of more heavily-rounded specimens, followed closely by the Sidi Hakoma Member localities DIK-49 and DIK-50. Some of the edge rounding data may be affected by the proportions of ﬁsh in the

Figure 6. Weathering stage data: (a) localities where most fragments have been barely exposed; and (b) localities with more variable exposure histories. Weathering stages 0e5 after Behrensmeyer (1978). Numbers in embedded tables represent numbers of fragments in each stage at each locality.

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Table 6 Fragment size classes. Size class

1 2 3 4 5 6 7 8 9 10 11

Cm

0.5e0.9 1.0e1.9 2.0e2.9 3.0e3.9 4.0e4.9 5.0e5.9 6.0e6.9 7.0e7.9 8.0e8.9 9.0e9.9 10.0

DIK-41

DIK-42

DIK-43

DIK-48

DIK-49

DIK-50

DIK-58

All

No Fish

All

No Fish

All

No Fish

All

No Fish

All

No Fish

All

No Fish

All

No Fish

38 94 42 9 6 2 1 0 0 0 1

5 32 33 7 5 2 1 0 0 0 1

152 182 61 19 7 3 3 0 0 1 1

54 63 47 13 5 3 3 0 0 1 1

204 353 244 79 34 21 11 2 4 2 6

54 242 191 72 34 21 11 2 4 2 6

30 48 21 2 5 4 0 0 2 1 0

12 16 10 2 5 4 0 0 2 1 0

9 26 12 7 8 6 0 4 7 1 2

6 13 10 6 8 6 0 4 7 1 2

19 62 31 23 9 8 6 2 1 0 2

7 45 27 23 8 8 6 2 1 0 2

158 591 172 41 14 3 1 0 4 1 1

59 458 160 38 14 3 1 0 4 1 1

Figure 7. Proportions of fragments at each locality in various rounding stages, with 0 ¼ not rounded, 1 ¼ slight rounding on edges, 2 ¼ rounding compromising the original shape of the fragment, and 3 ¼ original shape of fragment completely lost. (a) includes ﬁsh; and (b) does not include ﬁsh.

assemblage, as ﬁsh bone fragments are generally smaller and harder than mammal bone (Archer and Braun, 2013). When ﬁsh are removed from analysis, the lack of edge rounding at DIK-42 and DIK-43 remains apparent, while the variability in rounding classes at the other localities becomes more prominent (Fig. 7b). A range of surface modiﬁcation morphologies were observed within the circle collections (Table 7). The maximum number of marks on any single specimen was 24 in one case, although the rest of the specimens had between one and nine, with a median value of one. Only 256 (8.7%) of all specimens had any mark (including general microabrasion) on them; 237 of these marks were assigned to an agent with “high conﬁdence (HC)” and the rest were “medium conﬁdence (MC)”. Most marks were extremely small. For those specimens that could be measured (n ¼ 450; measured as either the maximum length of the main groove for linear marks or the maximum length of the main mark for amorphous marks), the mean length was 4.2 mm for linear marks and 2.5 mm for amorphous marks. However, the two mark types have very different distributions in their sizes, with amorphous marks being heavily left-skewed and with a median maximum length of 1.7 mm (average area 5.6 mm2 and median area 1.6 mm2), and linear marks having a roughly normal distribution with a median maximum length of 3.8 mm. The generally small size of marks is also evidenced by the fact that only 41% of all recorded marks were visible to the naked eye. Sixty six percent of the marks had tooth mark morphology (n ¼ 307), and 60% of these were visible to the naked eye. Eighteen percent of marks were a closest match for trampling damage (n ¼ 86) but only 4 (5%) of these were visible to the naked eye. Patches that did not ﬁt the deﬁnition of any known agent and generalized microabrasion with no associated mark were the only other damage types recorded, and in both cases it was rare for them to be noted without the aid of magniﬁcation. Only three marks had cut mark morphology, and of these only one was visible without magniﬁcation. The specimen bearing this mark was from DIK-43. Bones at this locality generally had a brilliant white surface, which made it difﬁcult to discern recent from ancient damage based on color. The mark with cut morphology appeared most likely to be recent on the basis of its shiny, compact damage ﬂoor relative to the appearance of the remainder of the bone surface. Although the majority of its attributes were similar to cut marks (e.g. a straight microstriation trajectory, ﬂaking on the shoulder, and a narrow V shape), it also had some attributes that were more typical of trampling damage (e.g. a sinuous groove trajectory). Similarly, only six marks had percussion mark morphology and of these again only one (also from DIK-43) could be seen without the aid of magniﬁcation. It was located near a modern fracture edge and from the slight sheen within the mark it also appeared to be recent in origin (Fig. 8). Across the entire sieved sample, no marks had morphologies that were an excellent match

Please cite this article in press as: Thompson, J.C., et al., Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia, Journal of Human Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.013

14

J.C. Thompson et al. / Journal of Human Evolution xxx (2015) 1e24

Table 7 General description of marks visible without magniﬁcation. Mark morphology

DIK-41

DIK-42

DIK-43

DIK-48

DIK-49

DIK-50

DIK-58

Total

Visible to naked eye?

No

Yes

No

Yes

No

Yes

No

Yes

No

Yes

No

Yes

No

Yes

Cut Mark Non-ID Patch Percussion Mark Tooth Mark Trample Mark Microabrasion only Total

0 2 0 3 3 2 10

0 0 0 24 0 0 24

0 (2)a 11 0 (2) 41 (6) 37 (3) 5 94 (13)

0 0 0 0 0 0 0

0 3 1 32 (1) 1 11 48 (1)

0 (1) 1 1 90 (2) 4 0 96 (3)

0 2 0 3 1 1 7

0 0 0 8 0 0 8

0 0 0 0 0 0 0

0 0 0 0 0 0 0

0 3 0 (1) 3 14 (1) 9 29 (2)

0 2 0 42 (2) 0 0 44 (2)

0 14 0 (1) 39 22 3 78 (1)

0 1 0 21 0 0 22

0 (3) 39 2 (4) 306 (11) 82 (4) 31 460 (22)

a Numbers in parentheses indicate marks that are medium-conﬁdence only; their morphology most closely resembles the assigned agent but it would not be published as a high-conﬁdence mark.

Figure 8. Marks with cut (a) and percussion (b) morphology.

for either cutting or percussion damage, and those that were the closest matches appeared recent. None was therefore considered “high conﬁdence” under the criteria deﬁned here. Seventeen percent (n ¼ 80) of all marks were classiﬁed as linear only, while 64% (n ¼ 310) were classiﬁed as amorphous only. Thirteen percent (n ¼ 61) had attributes of both, and the remaining 6% (n ¼ 31) were only instances of microabrasion with no additional marks on the same specimen. Of the total sample of 2926 bones, only 119 (4%) had microabrasion on them, and most cases of microabrasion (94%) were associated with other marks on the same specimen. Within the tooth mark morphology categories, the majority of marks were not speciﬁc to any particular taxonomic category, although 9% (n ¼ 29) were characteristic of crocodile damage (Njau and Blumenschine, 2006; Baquedano et al., 2012). Notably, these did not occur at all localities (Table 8). Marks within both morphology classes were generally small (median area for crocodile damage was 1.17 mm2 and median area for generic tooth damage was 2.49 mm2), and the differences between their median

areas was not signiﬁcantly different (U ¼ 2154; p ¼ 0.1534). Experimentally-generated crocodile marks appear to be larger than those observed on the circle-collected fragments (Njau and Blumenschine, 2006; Baquedano et al., 2012). It is a well-known phenomenon that as fragment sizes increase, they become more likely to exhibit marks (Abe et al., 2002); future work with an extended sample of larger specimens will determine if mark size also increases with fragment size. Most tooth marks were classiﬁed as amorphous marks, although some were scores or gouges and received additional or only data coding with respect to the portion of the mark that was linear. Characteristics of all amorphous marks are provided in Table 9, along with the same data coding for the two DIK-55 specimens that were previously published (McPherron et al., 2010). In 99% of cases visible to the naked eye, amorphous marks consisted of a single damage area with no small pits within the damage area. More than 95% of those visible without magniﬁcation had no microstriations, and most of the ones that did had microstriations constrained to

Please cite this article in press as: Thompson, J.C., et al., Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia, Journal of Human Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.013

J.C. Thompson et al. / Journal of Human Evolution xxx (2015) 1e24 Table 8 Numbers of modiﬁcations within different tooth mark morphology categories. DIK-41 DIK-42 DIK-43 DIK-48 DIK-50 DIK-58 Total Generic Morphology Pit Pit þ Score Puncture Puncture þ Score Score

21 1 0 0 5

17 3 2 2 15

56 18 4 1 35

5 1 1 0 0

20 2 3 1 13

52 1 6 0 1

171 26 16 4 69

Total

27

39

114

7

39

60

286

Bisected Pit Check Hook Star Puncture Triangular Pit Triangular Score

0 0 0 0 0 0

4 0 0 0 1 1

5 2 3 1 0 0

1 0 0 3 0 0

0 0 0 2 0 6

0 0 0 0 0 0

10 2 3 6 1 7

Total

0

6

11

4

8

0

29

Crocodile Damage

Table 9 Distribution of mark attributes for amorphous marks for the circle collections and DIK-55. All marks N

%

Total damage areas 1 365 98.4% 2 5 1.3% 5 1 0.3% Pits in main markb 0 364 98.1% 2 2 0.5% 3 4 1.1% 4 0 0.0% 5 1 0.3% Microstriation location Inside 47 12.7% Outside 0 0.0% Emanating 1 0.3% Inside and outside 4 1.1% Absent 319 86.0% Location of mark At crack 27 7.3% Element edge 30 8.1% Fracture edge 155 41.8% In notch 2 0.5% Isolated 157 42.3% Main mark dominant damage type Compaction 2 0.5% Crushing 4 1.1% Delamination 1 0.3% Displacement 10 2.7% Divot 2 0.5% Gouge 75 20.2% Microstriation patch 47 12.7% Pit 230 62.0% Bruising Present 4 1.1%

Without Magniﬁcation

DIK-55a

N

%

N

192 2 0

99.0% 1.0% 0.0%

7 2 1

191 0 2 0 1

98.5% 0.0% 1.0% 0.0% 0.5%

4 3 1 2 0

7 0 1 1 185

3.6% 0.0% 0.5% 0.5% 95.4%

10 0 0 0 0

18 17 85 1 73

9.3% 8.8% 43.8% 0.5% 37.6%

0 0 7 0 3

0 1 0 5 0 45 6 137

0.0% 0.5% 0.0% 2.6% 0.0% 23.2% 3.1% 70.6%

4 4 2 0 0 0 0 0

1

0.5%

0

a

Percent is not supplied as the sample size is too small to be meaningful. This may also include some examples of a single pit present, but merging to become part of the main mark.

15

Domínguez-Rodrigo et al. (2009) showed that within their experimental assemblages, ﬁve of their 16 proposed characteristics could be used to differentiate assemblages of marks made by simple stone ﬂakes versus trampling: trajectory of the main groove, shape of the main groove, presence of microstriations, trajectory of microstriations, and location of microstriations. Shoulder effect and ﬂaking on the shoulder were also considered to be relevant. The results of these seven criteria as recorded for the Dikika circle collections are given in Table 10, along with the same criteria recorded for the previously published DIK-55 marks (McPherron et al., 2010). Results are provided for marks that can be seen with the naked eye (to make them comparable to the experimental sample), and for all marks (to take advantage of the larger sample size). It is worth bearing in mind that these experimental characteristics were designed to differentiate trampling marks from cut marks, and do not include attributes of other linear marks, such as tooth scores. The experimental datasets also do not include percussion damage, which can overlap with cutting damage and may cause some differences when being compared to the fossil datasets. 5. Discussion “Landscape-scale taphonomy” that includes analysis of bone surface modiﬁcations is an approach that has been implemented at Early Pleistocene deposits that contain ﬂaked stone artifacts (Blumenschine and Peters, 1998; Potts et al., 1999; DomínguezRodrigo et al., 2002; Tappen et al., 2002; McCoy, 2009; Blumenschine et al., 2012), but not for sites older than about 2 Ma. More broadly, bone surface modiﬁcation studies of Plioceneaged fossils are uncommon. Most that have been done in association with Australopithecus are from South Africa, at cave sites where recovery methods are more akin to archaeological recovery (Pickering et al., 2004a; 2004b). The beneﬁt to these analyses is that they include more complete assemblages, while a drawback is that they represent depositional environments in which australopith remains are preserved, but not places where they habitually lived. Within deposits of tectonically-driven sedimentary basins, surface modiﬁcation studies have been performed at only a few sites older than 2 Ma: Gona (Domínguez-Rodrigo et al., 2005), Bouri (de Heinzelin et al., 1999), and Dikika (McPherron et al., 2010). In these cases, the focus has been on individual description of specimens bearing marks interpreted to be inﬂicted by hominin tool use, and from speciﬁc sites of interest, rather than more complete assemblages sampled from across the landscape. This approach does not document the range of variation and frequency of bone modiﬁcations that would appear in fully-recovered assemblages, and makes it impossible to establish if purported butchery marks are unusual in their characteristics relative to the background population. Understanding which marks are unusual in size or shape is especially germane to diagnosing trampling damage, which should have been a widespread process that did not discriminate by bone or bone portion. 5.1. Assessment of the trampling argument

b

inside the mark boundaries only. Marks consisting of microstriation patches were more commonly observed with the aid of magniﬁcation. Forty-four percent of macroscopically visible amorphous marks occurred at the fracture edge of a fragment, 38% occurred in an isolated part of the fragment, and most of the remainder occurred at cracks or at element edges. These are characteristic contexts of both experimentally observed tooth mark and percussion mark damage (Blumenschine et al., 1996).

Domínguez-Rodrigo et al. (2010, 2011, 2012) exhaustively compared images of each DIK-55 mark to experimental mark morphologies and concluded that they are a best ﬁt for trampling damage, but then asserted such comparisons cannot be used to diagnose individual marks but could only apply at the level of the total mark-sample. This problem is now more visibly exposed as one that requires methodological scrutiny, because diagnosis of individual marks is standard in taphonomic research, and is an approach accepted for Plio-Pleistocene sites such as Gona (Domínguez-Rodrigo et al., 2005) and Bouri (de Heinzelin et al.,

Please cite this article in press as: Thompson, J.C., et al., Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia, Journal of Human Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.013

16

Domínguez-Rodrigo et al. (2009) Trampling Groove trajectory Straight 75 Curvy 42 Sinuous 134 Groove shape V 10 \_/ 241 Internal microstriations Present 188 Absent 63 Microstriation trajectoryb Continous 169 Discontinuous 82 Location of microstriations Walls 7 Bottom 219 Both 25 Shoulder effect Present 15 Absent 236 Flaking on shoulder Present 7 Absent 244

This study

Domínguez-Rodrigo et al. (2009) a

Trampling

Unretouched

7 6 0

29.8% 16.7% 53.4%

93.5% 6.5% 0.0%

97.1% 0.0% 2.9%

31.2% 31.9% 36.9%

34.1% 26.8% 39.0%

53.8% 46.2% 0.0%

4 37

5 8

4.0% 96.0%

96.7% 3.3%

5.7% 94.3%

39.7% 60.3%

9.8% 90.2%

38.5% 61.5%

4 137

2 39

12 1

75.0% 25.0%

77.2% 22.8%

100.0% 0.0%

2.8% 97.2%

4.9% 95.1%

92.3% 7.7%

105 0

3 1

2 0

12 0

67.3% 32.7%

100.0% 0.0%

100.0% 0.0%

75.0% 25.0%

100.0% 0.0%

100.0% 0.0%

180 0 10

3 93 9

0 3 1

0 1 1

0 4 8

2.8% 87.2% 10.0%

73.2% 0.0% 4.1%

2.9% 88.6% 8.6%

0.0% 75.0% 25.0%

0.0% 50.0% 50.0%

0.0% 33.3% 66.7%

81 165

78 27

30 111

4 37

0 13

5.9% 94.1%

32.9% 67.1%

74.3% 25.7%

21.3% 78.7%

9.8% 90.2%

0.0% 100.0%

36 210

54 51

15 126

5 36

5 8

2.7% 97.3%

14.6% 85.4%

51.4% 48.6%

10.6% 89.4%

12.2% 87.8%

38.5% 61.5%

Unretouched

Retouched

Dikika Circles All

Dikika Circles Visible

230 16 0

102 0 3

44 45 52

14 11 16

238 8

6 99

56 85

190 56

105 0

190 0

DIK-55

Retouched

This study Dikika Circles All

Dikika Circles Visible

DIK-55

a This only includes those specimens from DIK-55 that were published with the interpretation that they were butchery-inﬂicted (McPherron et al., 2010); n ¼ 13 because only linear marks were included and marks DIK-55-3D, DIK-55-3-H, and DIK-55-2-A have multiple non-overlapping components that were combined in the interpretation of 12 individual butchery actions and these are each coded separately here to match the experimental and circle collection data. b Only applicable if microstriations are present.

J.C. Thompson et al. / Journal of Human Evolution xxx (2015) 1e24

Please cite this article in press as: Thompson, J.C., et al., Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia, Journal of Human Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.013

Table 10 Distribution of mark attributes for linear marks for published experimental collections (Domínguez-Rodrigo et al., 2009), the circle collections, and DIK-55.

J.C. Thompson et al. / Journal of Human Evolution xxx (2015) 1e24

1999), at a time when purported butchery damage is also very rare. When discerning the origin of a mark, researchers therefore assign different e and usually unspeciﬁed e weightings to the morphologies of bone surface modiﬁcations versus the contexts from which they are recovered (Njau, 2012; James and Thompson, 2015). The fact that this approach is accepted for Gona and Bouri but contested at Dikika does not mean that the DIK-55 marks should also be accepted as butchery damage without scrutiny. It does mean that the sum evidence for hominin butchery activities prior to 2 Ma e and thus their dietary and behavioral signiﬁcance in hominin evolution e cannot be evaluated until the same level of detail in reporting has been applied to all sites. The issues exposed by the debates surrounding DIK-55 have broader implications than simply resolving what caused the marks on the two specimens reported as hominin-modiﬁed. Much work remains at the methodological, theoretical, experimental, and empirical levels. Experimental parameters in zooarchaeological research are poorly standardized and frequently differ in the bone subjects that are used, the method of their preparation, and the variables that are included in the experiments (James and Thompson, 2015). Researchers often test quite speciﬁc hypotheses with their experimental assemblages, which may make some experiments more applicable to the interpretation of some fossil assemblages than others (Domínguez-Rodrigo, 2008; Pobiner, 2008). Therefore, although experimental data are extremely useful, in their current manifestation they are variable in their direct applicability to speciﬁc fossil assemblages, in their comparability to one another, and in their ability to capture the complexity of real patterns and processes. Middle Range approaches (e.g. those that tie present-day observations to the traces they leave and then apply them to traces left in the past) are best complemented by data derived directly from the fossil record, and interpretations are strongest when lines of evidence interpreted within strict uniformitarian actualistic logic are better contextualized within the range of variation expressed within a fossil sample. Our study adds a contextual angle to the interpretation of the DIK-55 specimens, which acts in complement to experimental and naturalistic research (collectively, “actualism”) that has become a staple for much taphonomic work (Marean, 1995). This is necessary because assemblages of fossils have complex taphonomic histories and (unlike with modern experimental samples) the total-mark sample is likely to contain a range of marks caused by several different agents and may not easily fall into distinct categories. Although experimental and naturalistic research provide essential inferential links between the observable present and traces left from past processes, they have several limitations in their operation. An actualistic study conducted within a brief time interval cannot easily replicate slow, long-term processes or palimpsests of slow processes that occur over the course of fossilization. It also is difﬁcult to replicate modiﬁcations to bone surfaces that are unique to the long-term process of fossilization, and this area of research demands that new models be developed to accommodate such processes. Given that the speciﬁc taphonomic pathways within a fossil assemblage will each have their own unique combinations of circumstances, it is impossible to experimentally replicate every possible combination of variables and scenarios that may have occurred in the past (DomínguezRodrigo and Yravedra, 2009). However, as actualistic research continues to build, it is possible to replicate every extant agent, model extinct agents, and then computationally simulate the possible combinations to determine statistically where the fossil sample best ﬁts (Cleghorn, 2006). The fossil record itself provides clues about the speciﬁc agents that should be modelled for speciﬁc environments.

17

Experimental and naturalistic work can only be performed within modern environments, modern ecosystems, and by modern organisms (or models of extinct ones), so that the current state of actualistic research has not equipped researchers with the full range of interpretive analogs for reconstructing the range of processes that operated in the past (Gifford, 1981; Gifford-Gonzales, 1991). For example, there were extinct carnivores in the Pliocene that had unique tooth morphologies that may have created novel marks that are currently undocumented (Marean and Ehrhardt, 1995; Brochu et al., 2010). By joining observations from the fossil record to novel actualistic research and statistical modeling, future taphonomic studies of Pliocene hominin-bearing deposits will be able to provide even better tools for the interpretation of these past environments. Here, we began that work through taphonomic analysis of all fossils recovered from the DRP area, including detailed analysis of the surfaces of bone from seven assemblages that were recovered using sieved collection methods and complete analysis of the entire population of marks on them. In addition to a contextual approach, our study also explicitly took a probabilistic stance, in that trampling damage should be the null hypothesis for bone modiﬁcation at DIK-55 only if similar damage is common across the project area where sandy deposits occur. Moreover, where such deposits occur, damage should resemble the DIK-55 specimens in form, size, and frequency. Domínguez-Rodrigo et al. (2010, 2011, 2012) maintain that trampling damage should be the null hypothesis for the marks from DIK-55 because the specimens are ex situ ﬁnds that may have derived from sandy deposits, the marks are morphologically similar to some experimental trampling marks, and the specimens appear to exhibit microabrasion on their surfaces. Here, we address each of these arguments and provide a contextual framework using new data on the morphology of other bone surface modiﬁcations from the same deposits. First, the argument that DIK-55 ﬁnds are ex situ does not add any evidence for or against the likelihood of trampling in an abrasive sediment. The stratigraphic association of the two specimens was originally reconstructed to a sandy deposit based on embedded matrix, with the minimum age of the entire section providing the conservative minimum age for the specimens (McPherron et al., 2010). That sandy sediment may provide a more abrasive medium than ﬁner sediments, but the fact that the specimens are ex situ does not contribute to this argument because the surface modiﬁcations in question were shown to be ancient in origin. Furthermore, the sandy sediments at DIK-55 contain coarse particles but the shape of those particles is generally rounded. The inﬂuence of particle shape, rather than simply size, is not a nuance that is captured in any published experimental trampling studies. The second argument, that the marks on the DIK-55 specimens morphologically resemble trampling marks, now can be addressed in several ways. Qualitative observations of over 1,000 paleontologically-collected specimens of what are predominately large mammal elements, and qualitative and quantitative observations of nearly 3,000 specimens that represent a mixture of elements from mammals and other taxa recovered from sieved sediments, show that no other specimens recovered from the Hadar Formation in the DRP area exhibit marks that resemble those on the DIK-55 specimens. The unusual appearance of the DIK-55 specimens was what prompted their initial collection, and the data show that in both categorical and metric variables they are indeed outliers relative to a large population of marks from a range of depositional environments. Further assessment can also now be made at a population level using the criteria argued by DomínguezRodrigo et al. (2009) to differentiate between cut and trample marks.

Please cite this article in press as: Thompson, J.C., et al., Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia, Journal of Human Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.013

18

J.C. Thompson et al. / Journal of Human Evolution xxx (2015) 1e24

The methodology behind the trampling experiments involved the production of cut marks with both retouched and unretouched ﬂakes, and then the trampling of those same specimens. Cut marks were distinguished from trample marks by indicating them with colored marker prior to trampling, although confounding effects may still have arisen because existing cut marks can be modiﬁed through the trampling process (Behrensmeyer et al., 1986). Domínguez-Rodrigo et al. (2009:2654) note that their criteria are “best applicable in cases of low-intensity trampling”, because “it seems that prolonged exposures to trampling further reduce (rather than increase) the similarities between trampling marks and butchery marks.” Of the original 16 variables that were recorded, seven of the categorical variables were found to differentiate, at a population level, between samples of cut marks and samples of trample marks. Associated microabrasion was not one of these. Domínguez-Rodrigo et al. (2012) later revised the criteria that were useful for differentiating trampling from stone tool butchery damage by referring to 14 original variables and only four that offer signiﬁcant results. They performed further butchery experiments on chicken and sheep bones using unﬂaked stones and then compared the resultant marks to experimental trampling marks using these four variables and an additional four that differentiated between marks made by replica Acheulean handaxes and simple ﬂakes. Domínguez-Rodrigo et al. (2012) found that there is significant overlap between linear marks caused by trampling and by butchery with an unmodiﬁed stone. For this reason, they counsel against assigning linear marks of unknown origin to unmodiﬁed stone as a possible effector. However, if an argument is employed that the morphology of the DIK-55 marks matches trampling damage, and if trampling damage cannot be differentiated from unmodiﬁed stone damage, then the morphologies of the DIK-55 marks cannot be logically used to infer one process in preference to another. Furthermore, this study has detailed how linear marks are not the dominant form of mark in the Hadar Formation deposits in the DRP area, and in fact most of the marks on the DIK-55 specimens are also not linear (or exclusively linear) in form. Only summary data have been published for the experimental trampling and cut mark datasets, which restricts the number and type of comparative analyses that can be performed. However, where they do occur they are a good match at the assemblage level for marks produced through a moderate degree of experimental trampling. Although trampling marks are not as common on Hadar Formation fossils as modiﬁcations with other morphologies (especially tooth marks), only two of the seven criteria that distinguish experimental trampling marks from cut marks were signiﬁcantly different in their distributions on the linear circle collection marks (Table 11). The two signiﬁcant results occurred in

attributes that are extremely subtle (presence of microstriations and shoulder ﬂaking), and which would be expected to occur in lower numbers on fossil collections because they are less resistant to erasure through subsequent taphonomic process. In contrast, the DIK-55 marks are not a good match for experimental trampling damage. Sample sizes were too low to use X2 analysis to compare the distributions of diagnostic mark attributes between the DIK-55 specimens and other datasets, but there is statistical support for the qualitative observation that the DIK-55 marks differ on the whole from experimental trampling marks using Fisher's exact test on basic 2 2 comparisons of each attribute that was advocated by Domínguez-Rodrigo et al. (2009) to differentiate trampling from butchery damage (Table 11). The test showed signiﬁcant differences between the proportions of attributes in the DIK-55 marks and experimental trampling marks in four of the seven criteria, and signiﬁcant differences between the proportions of attributes in the DIK-55 marks and the rest of the circle collections in three of the seven criteria (one was not available for testing). In terms of size, the marks on the DIK-55 specimens were far larger than marks in the sieved samples from the Hadar Formation. Linear marks from the circle collections that were visible without magniﬁcation were on average 5.2 mm long (4.3 mm median length), and amorphous marks had an average area of 5.8 mm2 (median area 2.2 mm2). The DIK-55 marks had an average maximum length of 7.9 mm (median 6.0 mm) for linear marks and an average area of 37.3 mm2 (median 16.8 mm2) for amorphous marks e all of which could be seen without magniﬁcation. That the DIK-55 marks are size outliers in comparison to a large sample of “background” marks is supported by resampling using a permutations test with 10,000 iterations on the combined circle and DIK55 samples. This showed that the probability of drawing a sample of marks with the mean mark length found on the DIK-55 modiﬁed bones is p < 0.00. A similar permutation test on mark area also yielded a result of p < 0.00 (Fig. 9). Domínguez-Rodrigo et al. (2009) do not provide length data on their trample and cut marks, but the marks they illustrate all appear to be quite small based on the supplied scale bars. It is also worth noting that all trampling experiments to date are focused on the analysis of linear marks, and many of the DIK-55 specimens have marks that are large and amorphous (or have an amorphous component), and therefore are not directly comparable in many ways to trampling damage as it has been documented experimentally to date. The presence of internal microstriations were once considered to be key features separating stone tool damage from other sorts of damage, such as carnivore tooth damage (Blumenschine et al., 1996), and it is notable that the fossil specimens from the circle

Table 11 Comparison of the distributions of linear mark attributes between published experimental collections (Domínguez-Rodrigo et al., 2009), the circle collections, and DIK-55.a Circle vs. experiment

DIK-55 vs. circle

DIK-55 vs. experiment

0.1170 <0.0001 0.0554 n too small for X2

0.0282 0.0001 N/A 1.0000

0.0003 0.1986 0.0206 <0.0001

0.3200 0.0161

0.5618 0.0484

1.0000 <0.0001

Differentiates trampling from simple ﬂake Groove shape Internal microstriations Microstriation trajectory Location of microstriations Differentiates trampling from retouched ﬂake Shoulder effect Flaking on shoulder Differentiates trampling from both Groove trajectory

X2 ¼ 3.6048; DF ¼ 2; p ¼ 0.1649

1

0.5487

a X2 used where comparisons are of a distribution, all other p-values are Fisher's exact comparisons of 2 2 table proportions (e.g. only two variables were present); shaded values indicate signiﬁcance below the a ¼ 0.05 level.

Please cite this article in press as: Thompson, J.C., et al., Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia, Journal of Human Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.013

J.C. Thompson et al. / Journal of Human Evolution xxx (2015) 1e24

19

Figure 9. Results of resampling (1,000 permutations) for mark length (left) and area (right). Vertical dashed line shows the probability of drawing a mark sample with the same sizes as the DIK-55 specimens from the mark sizes represented in the circle collections.

collections had extremely low incidences of internal microstriations. This may be the result of post-depositional processes that erase tiny microfeatures in shallow marks, such as weathering, smoothing, or even simply the fossilization process (Behrensmeyer et al., 1986; Olsen and Shipman, 1988; Domínguez-Rodrigo et al., 2009). However, both trampling (Domínguez-Rodrigo et al., 2009), and crocodile damage have now speciﬁcally been noted to produce marks with microstriations (Baquedano et al., 2012). Carnivores may also produce them when they modify bones with grit in their mouths. Taken together, it now seems that the simple presence of microstriations can be caused by a number of taphonomic variables, but that attributes of microstriations such as their trajectory, placement, number, or even depth may be shown to better separate some of these processes. Further experimental work that includes more quantitative means of recording difﬁcult variables, such as depth, microstriation attributes, and mark crosssection, will be required to better understand this phenomenon and apply it to the fossil record (Bello and Soligo, 2008; Bello et al., 2009; Newman, 2015). This work must be done in light of the fact that fossils with long histories of fossilization and exposure may not preserve ﬁne features such as microstriations and shoulder ﬂaking with sufﬁcient ﬁdelity at all sites for different localities to be comparable to one another, or to the experimental datasets. Microabrasion is a process that has been reported to occur exclusively from trampling in comparison to butchery (DomínguezRodrigo et al., 2009). However, in spite of samples taken across a range of depositional contexts at Dikika, and in spite of their surﬁcial contexts, the incidence of microabrasion was rare overall at 4%. Only tooth and trample marks comprised samples sufﬁcient to examine their speciﬁc co-occurrence with microabrasion. Out of 119 specimens with microabrasion, 35 (29%) also had at least one HC tooth mark, while 44 (37%) also had at least one HC trample mark (Fisher's p ¼ 0.2708). Thus, at Dikika, microabrasion is rare but when it does occur it may occur equally in association with either tooth or trample marks. Analyzed another way, signiﬁcantly more HC trample-marked specimens (75%; n ¼ 59) had microabrasion than did HC tooth-marked specimens (28%; n ¼ 127) (Fisher's p < 0.0001). This supports Domínguez-Rodrigo et al.’s (2009) observation that trampled bones are more likely to also exhibit microabrasion, which has been reported in instances as high as 100% in experimental trampling samples. However, it has been reported to occur on 98% of experimental cut-marked bones that have been moderately trampled, and at Dikika the incidence of microabrasion is much higher on specimens that have marks (46% of marked specimens) than it is in the overall sample. This suggests that if specimens are exposed long enough to acquire any surface modiﬁcations, then they are also exposed long enough to acquire moderate abrasion that can occur even after minimal trampling periods (Domínguez-Rodrigo et al., 2009). Therefore, the presence/

absence of trampling marks does predict the presence of microabrasion, but microabrasion is not a good predictor for what sorts of other mark morphologies will be found on the same specimen. 5.2. Taphonomic histories at Dikika Because butchery is a reductive process, and especially because activities such as hammerstone percussion result in fragmentation of bones, the rarity of evidence for butchery damage on assemblages collected for paleontological purposes may be interpreted in one of several ways: 1) hominin butchery activities were rare across the Pliocene paleolandscape, and the DIK-55 specimens were a fortunate discovery; 2) hominin butchery activities were absent, and the marks on the DIK-55 specimens represent a highly unusual case of equiﬁnality between butchery marks and another process or processes; 3) because samples selected for paleontology typically do not include fragmented large-mammal midshafts and ribs e which a large body of actualistic research has shown are some of the most likely specimens to retain butchery damage e there is a false appearance of rarity of this behavior; 4) the habitats sampled by paleontological studies are not those where hominin butchery tended to occur, or 5) an insufﬁcient number of fossils overall has been examined meaningfully to assess the relative incidence of butchery damage or other damage types. The taphonomic data from the circle collections provide the ﬁrst step in evaluating each of these possibilities, and allowing reconstruction of taphonomic processes from surface modiﬁcation and other lines of evidence at multiple localities within the DRP area. Paleoenvironmental and paleoecological interpretations of the hominin-bearing deposits of the Hadar Basin have shown a mosaic of habitats on the landscapes inhabited by australopiths (Bobe et al., 2007; Reed, 2008). Taxonomic abundances of the sieved samples from the DRP area reﬂect this, but also offer an opportunity to resolve ﬁner details about the spatial distribution of these mosaic environments. Although lake and stream deposits preserve fossils well, australopith behavior might be expected to be concentrated at localities that experienced periods of exposure, rather than places where faunal representation indicates more permanently submerged conditions. When all fauna were separated into those that could be identiﬁed as aquatic or non-aquatic, aquatic specimens by NISP dominate all assemblages (Fig. 10). However, there are signiﬁcant differences in the representation of aquatic fauna, with the localities of DIK-42 and DIK-43 being particularly rich in taxa such as ﬁsh, aquatic turtles, crocodiles, and hippopotamuses (X2 ¼ 59.916; DF ¼ 6; p < 0.0001). The percentage of aquatic fauna across the seven localities is negatively correlated with the percentage of fragments in weathering stage 3 or higher (Rs ¼ 0.8214; p ¼ 0.0356), which supports the inference that localities with high numbers of aquatic fauna were inundated and

Please cite this article in press as: Thompson, J.C., et al., Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia, Journal of Human Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.013

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Figure 11. Boxplots showing fragment size class distributions at the different localities. Refer to Table 6 for size class categories. Circles and asterisks represent outliers.

Figure 10. Aquatic fauna representation between localities: (a) with ﬁsh; and (b) without ﬁsh, since ﬁsh NISP is likely inﬂated because of extensive fragmentation. Numbers in bars represent numbers of fragments at each locality.

therefore not exposed to signiﬁcant episodes of subaerial weathering even over the time-averaged scales represented by the collections. Bones within sandy sheetwash units may have undergone some weathering before deposition into the lake, but most bones at localities dominated by lacustrine deposits were not exposed to signiﬁcant episodes of subaerial weathering (Table 2). Given the spatial proximity of most localities to one another, each section likely represents a series of depositional environments that were adjacent to one another over time. The deposition of the fossils within them would have also been governed by similar processes (e.g. alternating episodes of lacustrine, sheetwash, mudﬂat, and channel deposition), but these episodes are represented in different proportions by each section. Thus, examining fossils that had eroded from these sections provides a time-averaged sample of these different proportions of habitats e a scale that lies between the largest landscape scale and the smallest microhabitat scale that would require excavation of individual facies and result in a smaller overall fossil sample. Within the aquatic fauna category, ﬁsh are a special case. They represent between 21 and 56% of all specimens by NISP across the different localities, and are differentially distributed between them (X2 ¼ 197.16; DF ¼ 6; p < 0.0001). However, their abundances may obscure other taxonomic patterning. This is because they can be more easily identiﬁed from small fragments than non-aquatic taxa.

They are also small e no individual ﬁsh fragment in any collection was larger than 5 cm. Small fragments may travel farther during post-depositional ﬂuvial transport (Behrensmeyer, 1988; Pante and Blumenschine, 2010), so assemblages from localities with a lot of ﬁsh may represent autochthonous accumulations of ﬁsh in lacustrine deposits or allochthonous concentrations of small fragments dominated by ﬁsh that were transported from elsewhere. The sedimentological data suggest that the former possibility is more likely, and the fragment size and edge rounding data provide further support for this. The fragment size class data show that the majority of specimens from the circle collections are smaller than about 3 cm, and the skeletal element abundance data show that most are fragmented and non-diagnostic. This is typical of a sieved assemblage in comparison to non-sieved collections (Shaffer and Sanchez, 1994; Bush et al., 2007). Where sieving occurred, the degree of winnowing through ﬂuvial transport can be assessed; where fragment sizes are uniformly distributed, winnowing may be suspected (Behrensmeyer, 1988). Pante and Blumenschine (2010) showed that in experimental ﬂume conditions approximately 73% of all fragments smaller than 2 cm are transported, and thus have a high probability of being winnowed away from their original locations and redeposited elsewhere. All localities except the Basal Member collection at DIK-58 exhibit a range of fragment size classes (Fig. 11). These size class differences are not the result of recent fragmentation, because localities such as DIK-58 and DIK-42 have some of the most uniform fragment size classes but some of the lowest incidences of fossils with recent breaks. Sedimentologically, DIK-58 is also quite distinct, being dominated by sediments indicative of high-energy ephemeral streams. Determining the degree to which fossils from different localities represent autochthonous or allochthonous accumulations provides insight into when and how in their taphonomic histories bone modiﬁcations likely accumulated on their surfaces. Regardless of the time-averaging effect of surface collecting, we have found that

Please cite this article in press as: Thompson, J.C., et al., Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia, Journal of Human Evolution (2015), http://dx.doi.org/10.1016/j.jhevol.2015.06.013

Predominately autochthonous; Abundant aquatic habitats during deposition; little fragmentation from recent exposure Autochthonous; Few aquatic habitats during deposition; some fragmentation from recent exposure Autochthonous; Abundant aquatic habitats during deposition; some fragmentation from recent exposure Autochthonous; Dominated by aquatic habitats during deposition; substantial fragmentation from recent exposure

DIK-41

Autochthonous; Abundant aquatic habitats during deposition; substantial fragmentation from recent exposure Interpretation

Edge Rounding

Autochthonous; Abundant aquatic habitats during deposition; some fragmentation from recent exposure

More than half, some severe Predominately allochthonous; size-sorted and with input from many depositional contexts; some recent fragmentation

Moderate Substantial (bimodal) Concentrated in Class 2

Low Moderate Concentrated between Class 1 & 7 About half, some severe Moderate Substantial Concentrated between Class 1 & 9 Some, none severe Moderate Very little Concentrated between Class 1 & 6 Very little High Very little Concentrated between Class 1 & 4 Almost none Moderate Very little Concentrated between Class 1 & 3 Almost none High Very little Concentrated between Class 1 & 4 Very little

High DIK-49

Very high

DIK-43

Moderate

DIK-48

Very low

Moderate

DIK-50 Very high Moderate

DIK-42

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Aquatic signature (excluding ﬁsh) Recent Breaks Weathering Stage Fragment Sizes

Table 12 Overall assessment of the time-averaged peri-nutritive and post-nutritive taphonomic histories of each locality.

the sum results of the varied depositional environments at each locality are different, and they can be categorized along a continuum where some experienced a majority of fossil input from aquatic environments to localities that experienced a majority of fossil input from terrestrial environments. An overall assessment of each locality is given in Table 12. After establishing how representative each assemblage is of the dominant depositional and post-depositional processes that occurred at that locality, it becomes possible to examine the spatial distributions of bone-modifying agents that were active on different parts of the paleolandscape, and which left marks on the bone surfaces. Many of the processes associated with these agents (e.g. carnivore gnawing, crocodile modiﬁcation, microbial bioerosion, and butchery) typically occur when the bones are in a nutritive state, although other processes (e.g. trampling, root etching, insect invasion, and chemical dissolution) may occur later in a bone's post-depositional taphonomic history. Characterization of the main processes that modiﬁed bones within the sediments represented at each locality provides a starting point for future work, where speciﬁc facies of interest can be targeted and taphonomic variability might be examined through excavation at the microhabitat scale. It is useful to establish where and under what conditions bone surface modiﬁcations are expected to be preserved, as some taphonomic processes may erase or modify existing bone surface marks, such as processes that cause rounding and weathering (Thompson, 2005). Rounding class did not correlate with the number of preserved marks (Rs ¼ 0.374; p ¼ 0.5564), which is consistent with the experimental ﬁnding that modiﬁcations such as cut, percussion, and tooth marks do preserve on bones that have been transported within a ﬂuvial system (Pante and Blumenschine, 2010). However, on rounded specimens, post-depositional modiﬁcations such as marks with trampling morphology do occur more commonly (9%; n ¼ 8) than do modiﬁcations such as tooth marks (3%; n ¼ 12). A Fisher's exact test shows these two proportions to be signiﬁcantly different (p ¼ 0.0487), which suggests that the timing of interactions between biotic and abiotic processes can be extracted at a ﬁner scale across the landscape. There is a signiﬁcant positive correlation between the fragment size class of a specimen and the number of marks on its surface (Rs ¼ 0.2120; p ¼ 0.0007), which has been found to be the case with both experimental and archaeological assemblages (Abe et al., 2002; Thompson, 2008). Weathering stage was also signiﬁcantly, albeit weakly, associated with the number of marks (Rs ¼ 0.1396; p ¼ 0.0272), supporting the inference that longer periods of exposure open opportunities for modiﬁcations to accumulate on bone surfaces. Marks are therefore more likely to occur on larger fragments, and when they do they should represent a palimpsest of modiﬁcations from a range of agents. Thus, assigning all marks on a specimen to a single process will overly simplify and ultimately confound our understanding of past taphonomic processes. With respect to the issue of equiﬁnality, extensive experimentation has shown that in any given population of bone surface marks there are always some that overlap in morphology with marks made by other processes. Therefore, large experimental and fossil populations of marked bones are desirable for interpretation and analysis. These are not always available, which is particularly problematic for purported butchery marks that pre-date 2.0 Ma. Apart from the two DIK-55 specimens, the overall published sample of candidate butchered bones prior to ca. 2.0 Ma consists of only three specimens from Bouri (de Heinzelin et al., 1999), nine specimens (two of which conjoin) from Gona (Domínguez-Rodrigo et al., 2005), a specimen from Hadar (Kimbel et al., 1996), and two specimens from Lokalalei 1 (Kibunjia, 1994). Of these, only two

DIK-58

J.C. Thompson et al. / Journal of Human Evolution xxx (2015) 1e24

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specimens from Bouri, four from Gona (via SEM), and the two from Dikika have been illustrated. Domínguez-Rodrigo et al. (2005) note that there are a few other sites pre-dating 2.0 Ma that preserve in situ association of stone tools and fossils, but that these sites do not have a demonstrated link between the two through bone surface modiﬁcations. At sites such as Excavation 1 at Kanjera South, Kenya, it is noted that bone surface preservation is good and cut marks may be discerned in the future (Plummer et al., 1999), while at Lokalalei 2C (West Turkana, Kenya), bone surfaces are poorly preserved (Roche et al., 1999). The only reported hominin modiﬁcation from this site is brief mention of one cut mark on a small-medium mammal fragment found on the surface (Delagnes and Roche, 2005). This rarity of butchery-marked fossils in itself may be informative about the behavior and ecology of later Australopithecus and early Homo, but it is impossible to make this evaluation without systematic study of fully-collected fossil assemblages dating to the time period between ca. 3.5e2.0 Ma. Therefore, we advocate here a multi-pronged approach to interpretation of early hominin behavior that includes systematic taphonomic analysis of fossils from deposits that do not necessarily contain ﬂaked stone tools. This shift in methodology has the potential to address controversial issues, add a new aspect to paleoenvironmental and microhabitat reconstruction, and result in new discoveries on fossils that may have been previously overlooked. 6. Conclusions Combined anatomical, archaeological, ecological and stable isotope data provide the paleobiological, and paleoecological context of early hominin subsistence between ca. 3.4e2.0 Ma. None of these lines of evidence militates against incorporation of large ungulate tissues into the early hominin diet prior to the emergence of Homo by 2.8 Ma (Villmoare et al., 2015), or in conjunction with the earliest ﬂaked stone artifacts (Harmand et al., 2015). It is known that in the terminal Pliocene, hominins were ﬂaking stone (Harmand et al., 2015). It is not known how or when this behavior originated (Panger et al., 2002; Mercader et al., 2007), but it is known that by 2.6 Ma they had acquired an understanding of raw material selection and fracture mechanics (Stout et al., 2005, 2010) Therefore, to understand the progression of key hominin behaviors, including stone-tool assisted meat-eating, deposits that do not contain Oldowan tools should be studied with the same care that is given to deposits with them. Descriptions of the abundances, fragmentation patterns, and surface modiﬁcations of different types of fossils and their bone surface modiﬁcations provide detail about the environments and ecosystems of which australopiths were a part, and will assist in targeting localities for ﬁner-scale investigation of the microhabitats that comprise them. The results of a comprehensive taphonomic study of fossils from the DRP are further relevant for interpretation of taphonomic signatures on fossils from nearby fossil-bearing research areas such as Hadar, Gona, Woranso-Mille, and LediGeraru. Our study showed that fossils recovered using protocols 1 and 2 from across the DRP area have variably preserved surfaces, with damage that includes both shallow and deep modiﬁcations e the latter frequently exhibiting characteristics resembling experimental crocodile activity. However, no surface modiﬁcations except those on the original two DIK-55 specimens occur that would have been interpreted as butchery damage using the criteria that were applied to initial diagnosis of those marks. Fossils from seven drysieved surface collections provide a more systematic assessment of the peri-depositional and post-depositional taphonomic processes in operation in the area around DIK-55 and elsewhere in the DRP.

These samples represent fossil input during the sedimentary conditions found in the overlying geological sections, and thus can be broadly characterized in terms of the taphonomic processes that dominated at each locality. They also allow testing of the hypothesis that the DIK-55 marks were created through incidental trampling damage. Domínguez-Rodrigo et al. (2010, 2011, 2012) invoked four main lines of argument to assert that the DIK-55 marks were created by trampling: 1) parsimony and uniformitarianism associated with trampling damage, because it is a widespread and naturallyoccurring process, whilst butchery damage is rarely documented prior to 2.0 Ma; 2) the fact that the DIK-55 specimens may have derived from a sandy sediment; 3) the presence of microabrasion on the DIK-55 marks; and 4) the superﬁcial similarity between experimental trample marks and some of the DIK-55 marks. Our study has shown with a large sample of fossils that the DIK-55 marks appear both morphologically different, different in their qualitative variables, and substantially larger than the background population of marks in the DRP area where sedimentary processes and potential conditions for trampling are the same. Moreover, they differ in many of these respects from the experimental trampling datasets presented by Domínguez-Rodrigo et al. (2009). Claims of butchery marks on large mammal bones in deposits where stone tools are rare or absent can be contextualized by analysis of the surfaces of substantial numbers of other fossils found in the same deposits, rather than relying solely on the morphologies of individual marks. This has now been done for the DIK-55 specimens, as part of a larger sample of fossils from Dikika. If the DIK-55 marks were caused by trampling, then we would expect to ﬁnd other deep, linear marks that resemble cut marks, or at least ﬁnd that the DIK-55 marks fall within the range of sizes and qualitative characteristics exhibited by linear marks on other specimens from the same sedimentary packages e and we do not. Thus, we consider that there is little support for the ﬁrst argument by Domínguez-Rodrigo et al. (2010, 2011, 2012) for trampling as the causal agent behind the DIK-55 marks because it would invoke a very special set of trampling conditions at that locality that have no direct analog within the current range of experimental trampling datasets. Because the ecological role played by Australopithecus-grade hominins is little known, a major problem of using modern analogs to reconstruct their behavior is predicting and then detecting potential signatures of that behavior across the landscape. The methodology proposed here is one that will enable characterization of the taphonomic processes in operation across the paleolandscapes on which australopiths were active, including statistically robust assessments of bone surface modiﬁcations that have the potential to provide new information about the paleoecology and behavior of Pliocene hominins. Acknowledgements We thank the Authority for Research and Conservation of Cultural Heritage in Ethiopia for permission to conduct ﬁeld work and their generous access to the fossils curated in their facility, working space, and access to a microscope with a camera attachment. We thank curators Tomas Getachew and Yared Assefa for their help. The Max Planck Institute for Evolutionary Anthropology provided a portable microscope for the study. Moges Mekonnen assisted in transport of fossils between the collections and the workspace. Bill Kimbel facilitated lodging for JT in Addis Ababa for the duration of the study. Funding to conduct ﬁeld and lab work was provided by Margaret and Will Hearst. We also wish to thank Sarah Elton, J. Tyler Faith, and three anonymous reviewers for their constructive critique of our original submission.

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Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia

Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia

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