The subtle divide: a new method for identifying chop marks on long bone diaphyses using fracture morphology

Introduction: Chop marks provide critical evidence for reconstructing butchery practices, yet their identification criteria are underdeveloped and rely heavily on the presence of sharp force trauma. Sharp force trauma does not occur with every chop, and on long bone shafts in particular, chops can resemble blunt force trauma.

Methods: This study addresses this issue through controlled experiments comparing chop and bash impacts on Sus scrofa femoral diaphyses. Using an Instron 9440 Drop Tower System, impacts were delivered with copper, bronze, iron, and steel axes, as well as a rounded steel hammer, under standardized energy and orientation. Fracture pattern analysis including surface modifications, and impact area shape were recorded for each impact.

Results: Statistical analyses, including Fisher's exact tests, hierarchical clustering, and logistic regression, identified straight impact location shape, non-comminuted fractures, and the presence of SFT as the strongest predictors of chopping.

Discussion: By establishing diagnostic criteria that remain visible even in the absence of SFT, this study provides a robust framework for distinguishing chopping from bashing in archaeological and forensic assemblages, improving our ability to reconstruct tool use, carcass processing, and human behavior from fragmented bone.

1 Introduction

Chop marks provide key evidence for reconstructing butchery practices and tool use, yet their identification in archaeological bone assemblages remains poorly understood. This paper applies experimental fracture mechanics to link impact mechanics and bone response, refining how these marks are recognized in assemblages. Food acquisition and processing lie at the intersection of biological need and cultural expression, linking subsistence with innovation and technology. Archaeological traces of butchery provide one of the most informative proxies for investigating past behavior, revealing how communities exploited animal resources, organized subsistence activities, and used different kinds of tools (Pawłowska et al., 2014; Okaluk and Greenfield, 2022; Trentacoste et al., 2021; Domínguez-Rodrigo et al., 2021; Seetah, 2006). Among the various traces left on bone, chop marks are particularly significant. They reflect deliberate, forceful actions carried out with heavy edged tools and can offer insight into carcass processing methods, tool use, and task organization. However, identifying these marks on archaeological bone is not always straightforward, especially when they do not present the classic traits associated with sharp force trauma (SFT) (Okaluk et al., 2025).

Chop marks have long been regarded as the most straightforward butchery marks to identify in zooarchaeology (Gifford-Gonzalez, 2018). Their characteristic flat cut surfaces, more accurately described as SFT (Tamminen et al., 2023; Okaluk and Greenfield, 2022), and highly diagnostic V-shapes can appear particularly conspicuous in assemblages (Olsen, 1988; Walker and Long, 1977). Yet these obvious criteria represent only a small subset of chop marks. SFT does not occur with every impact, since its formation depends on factors such as ax material, bone shape, and angle of strike (Okaluk and Greenfield, 2022; Okaluk et al., 2025; Peace et al., 2020). Additionally, incomplete chops that preserve a full V-shaped indentation are rare, reflecting failed or chance blows that do not fracture the bone completely (Okaluk and Greenfield, 2022). Successful chops produce complete breaks that divide the mark and essentially erase the negative tool impression (kerf) critical for slice-mark type analyses. With only half of the mark preserved, chop signatures are often ambiguous and much more difficult to identify, particularly on long bone diaphyses.

Experimental studies have shown that when an ax strikes the diaphysis of an unsupported long bone, clear traces of sharp force trauma are uncommon (Okaluk et al., 2025). Because of the mechanical properties of long bone shafts, impacts typically generate bending fractures rather than clean cuts, and the resulting traces can resemble non-diagnostic fragmentation. Forensic research has noted similar challenges, with chop wounds on long bone shafts often appearing irregular, crushed, or fractured rather than producing SFT, making them difficult to interpret (de Gruchy and Rogers, 2002; Humphrey and Hutchinson, 2001; Lynn and Fairgrieve, 2009; Peace et al., 2020). Despite the interpretive importance of these traces, zooarchaeological research has rarely addressed how to distinguish chop impacts that lack SFT from other kinds of bone breakage, particularly percussion marks created during marrow extraction.

This raises a crucial question: is it possible to identify chop marks on long bone shafts when sharp force trauma is absent? If so, how do these signatures differ from the percussion marks typically associated with marrow extraction? To address this problem, this study presents an experimental comparison of chop and hammer impacts on Sus scrofa femoral shafts. The results provide new identification criteria for recognizing chop marks on long bone diaphyses, expanding our ability to distinguish anthropogenic modifications that have previously been unidentified.

Accurately distinguishing between chopping and bashing matters because these signatures are routinely used to reconstruct butchery practices, carcass processing, and subsistence strategies (Lyman, 1987; Maltby, 2007; Seetah, 2005, 2007, 2008). If identification relies only on clear SFT, then instances where chopping did not produce such traces are effectively invisible in the archaeological record. This omission risks underestimating the prevalence of chopping, mischaracterizing the tools used, and distorting reconstructions of carcass treatment. In short, if we want to talk meaningfully about butchery practices, we need to be able to identify all forms of butchery, not just those that leave the most conspicuous marks. The present study addresses this need by establishing criteria for distinguishing chop marks from bashes on long bone shafts, even in the absence of SFT.

2 Background

Despite their long-recognized diagnostic potential, chop marks remain among the least systematically studied forms of butchery evidence in zooarchaeology. Recent work has begun to address this gap (Okaluk and Greenfield, 2022; Okaluk et al., 2025), demonstrating that long bone shafts are mechanically predisposed to fail through bending and fracture propagation, limiting the expression of sharp force trauma. Yet methods for identifying chop marks continue to lag behind the more established frameworks for percussion damage associated with marrow extraction (Galán et al., 2009; Pickering and Egeland, 2006; Blasco et al., 2014). Standardized descriptive protocols exist for notches, pits, and fracture morphologies created by hammerstone percussion, but equivalent criteria for chopping have been slower to develop. This imbalance has hindered efforts to distinguish between chopping and bashing on long bone shafts, particularly when SFT is absent.

Early experimental approaches to bone fracture in zooarchaeology sought to identify diagnostic features of human modification but were limited by uncontrolled methods and small sample sizes. Morlan (1984) emphasized the need for experimentally derived criteria linking fracture morphology to causative processes, yet acknowledged that relationships between mechanical loading and fracture form remained poorly understood. Davis (1985) advanced this work through controlled experiments on antelope long bones, showing that fracture orientation and morphology vary systematically with element shape, weathering, and loading regime. Dynamic and torsional stresses produced distinct fracture patterns, underscoring the mechanical complexity of bone failure. These studies laid the foundation for a more mechanistic understanding of bone fracture but did not incorporate standardized energy or detailed comparative analysis of different impact types. The present study builds on this framework by experimentally comparing chopping and bashing under controlled energy and angle conditions, quantifying diagnostic fracture traits through statistical modeling.

These foundational studies established that bone fracture is governed by both mechanical loading and element morphology, providing essential context for later debates about how to distinguish different forms of bone breakage. Building on this foundation, subsequent zooarchaeological work began to address the overlap between percussion and chopping signatures. White (1992) observed that sharp-edged hammerstones can create marks that closely resemble chop signatures, noting that such similarities likely contributed to the underestimation of percussion damage in archaeological assemblages. Pickering and Egeland (2006) expanded this through systematic experiments that documented the variability of percussion traces. They demonstrated that while percussion notches are diagnostic when present, their formation is inconsistent, and many blows produce ambiguous fracture morphologies. Together, these studies established a methodological foundation for percussion analysis and underscored the equifinality between percussive and chopping signatures.

Subsequent experimental research on chopping has revealed comparable variability. Okaluk and Greenfield (2022) showed that ax impacts produce a broad range of morphologies, with the presence of SFT strongly dependent on ax material. Although metal axes are more likely to generate SFT than stone, all tool types can produce ambiguous traces such as crushing or pitting along fracture margins. Clear V-shaped chops are rare, since most successful blows result in complete fractures that divide the mark and erase the negative tool impression. As a result, many chop marks do not exhibit classic sharp force traits, complicating their identification on long bone shafts.

Attempts to distinguish chopping from bashing directly have been limited. Heinrich (2014) used both edges of a steel hand ax to produce chop and bash marks but provided only broad qualitative distinctions, often relying on prior knowledge of which ax surface was used during experimentation. His study illustrated the difficulty of establishing consistent diagnostic traits that can be applied without experimental context. More systematic approaches have instead focused on percussion. Blasco et al. (2014) proposed a widely adopted framework for fracture outlines, edges, angles, and diagnostic surface modifications such as notches and flakes. Although their experiments did not include chopping, they noted that some percussion features could resemble chop marks, highlighting the need for clear differentiating criteria.

Forensic studies provide complementary insights into chopping variability. Research on cleavers, machetes, and axes has shown that chopping signatures often deviate from idealized V-shaped kerfs. Humphrey and Hutchinson (2001) documented that cleavers produced relatively smooth kerfs, while axes and machetes frequently created irregular, crushed, or flaked fractures. de Gruchy and Rogers (2002) observed similar irregularities, including oblong or triangular lesions, and emphasized the difficulty of differentiating these from other forms of trauma. Lynn and Fairgrieve (2009) reported spiral and oblique fractures, irregular flaking, and chattering from ax and hatchet impacts, further demonstrating variability. More recent experimental work has reinforced this pattern, showing that hacking and chopping impacts frequently fail to produce classic sharp force trauma and can overlap extensively with blunt force signatures depending on impact angle and loading conditions (Peace et al., 2020). McGehee et al. (2023) examined kerf characteristics created by different chopping implements, focusing on measurable wall and base attributes. Their experimental design prioritized shallow, incomplete marks suitable for measurement, however, and is therefore not directly comparable to studies examining complete fracture morphology under higher-energy impacts.

Taken together, both zooarchaeological and forensic research demonstrates that chop marks on long bone shafts frequently lack clear SFT and overlap with percussion signatures in ways that have not been systematically resolved. Existing approaches either focus on percussion, emphasize incomplete chops, or rely on criteria not suited to archaeological assemblages. The present study builds on this foundation by testing whether chop and bash impacts can be differentiated through fracture morphology and impact area characteristics alone, providing a methodological framework that extends beyond SFT.

3 Methods

3.1 Materials

Experimental impacts were carried out on subadult femoral shafts of Sus scrofa (n = 31). Specimens were obtained defleshed from local abattoirs, kept frozen until the day of testing, then thawed in water and used immediately.

All strikes, both chops (n = 20) and bashes (n = 11), were performed using the Instron 9440 Drop Tower System, which ensured controlled and repeatable delivery of force across all trials (Figure 1A).

Figure 1

Figure 1. Experimental impact setup and tools: (A) Instron 9440 Drop Tower System with the impact arm and base highlighted; (B) reproduction axes used for chop impacts (copper, bronze, iron, and modern steel); (C) femur placed unrestricted on a flat wooden board on the base beneath the impact arm prior to testing; (D) rounded stainless steel blunt impactor used for percussion impacts.

Chop impacts were delivered using reproduction axes made of copper, bronze, iron, and modern steel (Figure 1B). These replicas were based on generalized archaeological forms and metallurgical compositions from the southern Levant rather than a single specific artifact (Ashkenazi et al., 2016; Erez et al., 2016; Gottlieb, 2010). The pure copper, bronze (5.5% tin), and hand-forged bloomery iron axes reflect technological developments spanning from the Chalcolithic to the Iron Age, capturing key shifts in material properties and manufacturing techniques that would have influenced chopping performance. Ax dimensions are provided in Table 1 for documentation. Blade width was not treated as an analytical variable, as experimental work demonstrates that fracture mechanics, rather than blade width, govern sharp force trauma expression on long bone shafts under controlled impact conditions (Okaluk et al., 2025). The use of generalized forms provides a representative sample of ancient technological variability while avoiding the idiosyncrasies of individual artifacts. Ancient axes were not standardized and display considerable variation in both form and material (Shalev et al., 2014), so this approach allows for controlled comparison across broad technological categories. A modern steel ax was also included for comparative purposes, offering a reference point for a highly durable cutting edge. Each ax was drilled and fitted with bespoke attachments to allow secure mounting to the impact machine arm.

Table 1

Table 1. General dimensional characteristics of reproduction axes used in the experiment.

Bash impacts were delivered using the rounded stainless steel hammer supplied with the Instron 9440 Drop Tower System (Figure 1C). The hammer measured 1.5 cm in diameter (Figure 1D). Using a smooth stainless steel hammer as the percussive impactor ensured that bash and chop trials could be compared under identical and repeatable force and placement conditions. Archaeological hammerstones vary widely in form and surface texture, and their irregularity introduces heterogeneous contact geometries and pressure distributions at the point of impact, which can alter local stress concentrations and fracture initiation even under similar loading regimes. This variability was intentionally controlled by using a standardized blunt impactor in order to minimize within-class variance in the blunt condition and isolate fracture-level responses to blunt vs. edged impacts. The steel hammer thus served as a controlled, reproducible proxy for a rounded blunt impactor, prioritizing comparability and repeatability across trials rather than replicating the full range of archaeological hammerstone morphologies.

3.2 Procedure

Thawed femora were placed unrestricted and unsupported on a flat wooden board beneath the Instron 9440 Drop Tower impact arm, with the bone resting directly on the board and no restraints applied to either end. Alignment of the impact point was visually checked prior to each strike. Impacts were delivered by releasing the drop tower carriage from a fixed height, allowing the impact arm with the mounted tool to fall vertically and strike the bone in a single percussive blow. The machine controlled for speed, mass, direction, and force, and recorded velocity at the moment of impact.

Each bone was subjected to a single impact delivered to the posterior face of the diaphysis at a 90° angle along the transverse plane. Impacts were delivered at 30 J of energy, corresponding to an 8.2 kg mass and an impact velocity of 2.7 m/s. Impact energy was standardized at 30 joules as this was the minimum level determined through pretesting to achieve complete fractures in femora without causing catastrophic fragmentation, ensuring realistic and comparable chopping conditions. Specimens that did not fracture, or that fractured only partially, were classified as incomplete.

Following impact, bones were boiled and treated with detergent as a degreasing agent to remove residual fats. Fragments were then re-fitted with white glue to reconstruct specimens for analysis. Each specimen was examined under raking light, and all fracture characteristics were recorded by the first author according to the variables and definitions outlined below.

3.3 Variables and definitions

Each impact was coded for impact location shape (straight or irregular), gross fracture type (e.g., transverse, oblique, butterfly, comminuted), bone surface modifications (outer conchoidal flake, percussion notch, pecking, none), fracture origin (anterior, posterior, lateral, medial), and the presence or absence of SFT.

These variables were selected to capture both surface-level and internal fracture characteristics relevant to differentiating chopping and bashing. Impact location shape records the geometry of the fracture edge at the point of impact. Gross fracture type documents overall break morphology, which reflects how force propagated through the bone (Figure 2). Bone surface modifications refer specifically to anthropogenic alterations to the cortical surface, including SFT, percussion notches, and conchoidal flakes, following common zooarchaeological usage (Blasco et al., 2014; James and Thompson, 2015; Pickering and Egeland, 2006; White, 1992). Fracture origin records the location where failure initiated on the bone surface, providing insight into the loading mechanics of each impact. Definitions follow established zooarchaeological and biomechanical terminology, with clarifications provided below.

Figure 2

Figure 2. Schematic of gross fracture types produced in experimental sample.

Butterfly fracture and butterfly type

• Butterfly fractures are characterized by a triangular wedge fragment bounded by two oblique fracture planes, typically forming on the compressive side of the bone under bending forces (Figure 2). In this study, normal butterfly fractures refer to wedges with their base on the compressive side and apex directed toward the tensile side, consistent with standard descriptions. Reverse butterfly fractures refer to wedges that occur on the tensile side, with their base on the tensile cortex and apex directed toward the compressive side. Reverse butterflies are less commonly described, and their formation is not fully understood, but they have been documented in both experimental and forensic studies (Reber and Simmons, 2015). Here, the distinction is recorded descriptively and used as a categorical variable, without inferring specific loading mechanisms.

Fracture mechanics (contextual definitions)

• Fracture origin–the point on the bone surface where failure initiated under applied stress. In bending fractures, the origin typically occurs on the tensile (convex) side of the shaft, where microcracks first form and propagate across the cortex. In compression fractures, the origin is found on the compressive (concave) side where cortical collapse initiates fracture propagation. Identifying fracture origin provides insight into the loading conditions that produced the break (Christensen et al., 2021; Zephro and Galloway, 2014; Wedel and Galloway, 2014).

• Compression fracture-a fracture produced when bone fails under compressive stress, typically initiating on the compressive side and propagating along planes of maximum stress (Reber and Simmons, 2015; Wedel and Galloway, 2014). See Figure 3.

• Bending fracture-a fracture produced when force is applied perpendicular to the long axis of a bone, creating tensile stress on the convex surface and compressive stress on the concave surface. Failure usually initiates on the tensile side and propagates across the shaft (Reber and Simmons, 2015; Wedel and Galloway, 2014). See Figure 3.

Figure 3

Figure 3. Schematic illustration of mechanical stressors acting on bone prior to and during fracture. These loading modes represent the conditions that lead to specific crack initiation and fracture propagation patterns.

Impact area morphology

• Straight–linear fracture edge at the place of impact (Figure 4A).

• Irregular–irregular fracture edge at the place of impact (Figure 4B).

Figure 4

Figure 4. Impact area morphology on the experimental sample (A) straight impact location; (B) irregular impact location.

Bone surface modifications

• Pecking–localized pitting or crushed indentations occurring along fracture edges or impact margins. These features are morphologically similar to what is described as chattering in the forensic literature but differ in that they form outside of kerf walls and are produced during percussive impact and fracture propagation rather than during cutting motion (Figure 5A).

• Outer conchoidal flake–a flake scar with distinct rippling, detached from the exterior cortical surface as a result of impact (see Figure 5B) (White, 1992; Fisher, 1995).

• Percussion notch–a localized feature at the impact location where pressure from impact detaches a bone flake. This produces a scalloped edge on the exterior outline and a conchoidal scar on the interior (Binford, 1981; Capaldo and Blumenschine, 1994; Fisher, 1995; Blasco et al., 2014; White, 1992) and is due to compression forces. See Figure 5C.

Figure 5

Figure 5. Bone surface modifications on the experimental sample: (A) pecking along the fracture line; (B) outer conchoidal flake scar; (C) scalloped edges of unconfirmed percussion notch.

Sharp force trauma (SFT)

• SFT refers to injury to bone or soft tissue produced by a sharp or sharpened edge, including knives, machetes, cleavers, axes, or other edged tools (Figure 6). It is characterized by relatively smooth cut surfaces, linear kerfs, and striations created as the edge penetrates and slices through tissue (Bartelink et al., 2001; Braun et al., 2022). In zooarchaeology, SFT describes the flat cut surface produced when the edge of a heavy tool penetrates bone. Because chops combine a sharp edge with blunt force, they may display classic sharp force traits, blunt force traits, or both (Okaluk et al., 2025). This can include smooth cut surfaces alongside crushing or flaking (Okaluk and Greenfield, 2022).

Figure 6

Figure 6. Sharp force trauma (flat cut surface) on Bone 49 indicated by red circle.

Fractographic features

To locate impact areas and interpret fracture propagation, internal fracture surface features were examined following forensic fractography approaches (Christensen et al., 2021, 2018). Dynamic bending fractures typically exhibit three distinct zones (Figure 7).

• Mirror-A smooth, often glossy area around the fracture origin. It forms during slow, stable crack growth and helps pinpoint the impact location on the opposite cortex.

• Mist-A slightly roughened transitional zone surrounding the mirror, marking the shift from stable to unstable crack growth.

• Hackle-The outer zone, with coarse, feathered ridges that form as the crack accelerates. Hackle lines radiate away from the mirror zone and indicate fracture direction.

Figure 7

Figure 7. Fractographic features of a bending fracture on Bone 55, illustrating the impact location along with mirror, mist, and hackle zones.

3.4 Statistical analysis

Analyses focused on identifying which variables were most predictive of chop vs. bash impacts and evaluating how these variables performed in combination. Three complementary approaches were used.

First, unsupervised divisive hierarchical clustering using Gower distance was performed to explore underlying structure in the dataset and assess whether impacts produced by different ax materials (copper, bronze, iron, modern steel) could be grouped for analysis.

Second, variable-level associations were tested using two-tailed Fisher's Exact Tests. Exact p values were calculated for 2 × 2 tables. Variables were then ranked by p value to guide model building.

Finally, multivariate logistic regression models were constructed by adding variables stepwise in order of their Fisher rankings using a step-wise forward-selection WAIC algorithm. Model performance was compared using the Widely Applicable Information Criterion (WAIC) and Leave-One-Out cross-validation (LOO), both of which balance predictive accuracy with model complexity. Variables were modeled additively without interaction terms.

All analyses were conducted in R using standard statistical and modeling packages, including “brms” for Bayesian logistic regression (Bürkner, 2017), which allowed for model comparison using WAIC and LOO criteria.

4 Results

4.1 Clustering analysis and tool grouping

A divisive hierarchical clustering using Gower distance was conducted to assess whether impacts created by different tool types could be meaningfully grouped for subsequent analyses. This unsupervised method grouped observations according to their fracture characteristics without using chop or bash labels as a target. Comparing cluster compactness reveals that separation into five clusters is optimal (Figure 8A). At this resolution, the results revealed a clear separation between bashing and chopping impacts (Figure 8B). All steel hammer impacts formed a single, distinct cluster that was completely separated from the clusters containing the four ax materials (copper, bronze, iron, and modern steel axes). Although some internal sub-clustering among ax types was observed, particularly between modern steel and bronze construction, and iron and copper construction, unsupervised clustering provides support for the idea that bash impacts can be clearly distinguished from other types of impact. While inter-ax differences are addressed elsewhere (Okaluk et al., 2025), pooling them in the analyses that follow can further explicate their differences without the loss of statistical power associated with fragmenting the classes, and while reflecting their overall similarity in fracture morphology. The unsupervised clustering thus aligns with the statistical modeling by indicating that chopping and bashing impacts are systematically distinct. For full data see Table 2.

Figure 8

Figure 8. Divisive clustering results based on Gower distances: (A) Elbow plot showing the within-cluster sum of squares (SS) plotted against the number of clusters, with a clear inflection around five clusters indicating an optimal balance between cluster compactness and model simplicity; (B) divisive hierarchical clustering dendrogram illustrating the dissimilarity structure among tool materials (bronze, iron, copper, steel, and modern steel), supporting the separation of chopping and bashing impacts.

Table 2

Table 2. Complete dataset of experimental impacts on Sus scrofa femora, detailing tool material, impact type, gross fracture morphology, impact location shape, surface modifications, fracture origin, and the presence or absence of sharp force trauma for each chop and bash event.

4.2 Variable-level associations: fisher's exact tests

Two-tailed Fisher's Exact Tests were used to identify individual variables that differed significantly between chop and bash impacts. Several variables were strongly associated with impact type, while others showed weaker or non-significant relationships but nonetheless contributed to predictive performance in combination (Table 3). The most significant differences were observed in impact location shape, the presence SFT, and fracture type. Straight impact location shapes were significantly associated with chops rather than bashes (p = 0.0201), and SFT occurred exclusively in chop impacts (p = 0.0331). Fracture type also differed significantly between impact types (p = 0.0416), with comminuted fractures occurring more frequently in bashes, whereas transverse and oblique fracture bases were more common in chops. Several additional variables showed weaker associations: the absence of impact marks approached significance (p = 0.0659), and butterfly type showed a less pronounced but suggestive pattern (p = 0.151). Other variables, such as pecking (p = 0.2409), transverse fracture types considered alone (p = 0.2755), fracture origin categories, and flake scars (p = 0.631, the lowest predictive score), did not show significant individual differences. Nonetheless, all variables were retained for multivariate modeling because even those with weak individual associations may contribute predictive information when combined with others.

Table 3

Table 3. Two-tailed Fisher's exact p-value results for variables mentioned in-text.

4.3 Logistic regression modeling

Logistic regression models were used to evaluate how combinations of variables predict the probability that an impact was produced by chopping. Variables were added stepwise in order of their Fisher test rankings, and model performance was assessed using WAIC and LOO scores to balance predictive power with model simplicity.

Across all models, three variables consistently emerged as the strongest predictors of chop impacts: straight impact location shape, the presence of SFT, and fracture type. Pecking and butterfly type contributed additional, though weaker, predictive information. Regression coefficients are reported on the logit scale, which represents the log odds of a chop impact when each feature is present.

Straight impact location shape had a coefficient of 0.82, corresponding to approximately 2.3 times higher odds of a chop impact relative to irregular impact locations. The presence of SFT had a coefficient of 0.98, equivalent to 2.7 times higher odds of a chop impact compared to its absence. Comminuted fractures had a negative coefficient of −0.96, corresponding to about 0.38 times the odds of a chop relative to other fracture types. Butterfly type (normal) had a negative coefficient of −0.74, indicating a weaker but consistent trend, while pecking had a positive coefficient of 0.63, reflecting a modest increase in the likelihood of a chop when present.

Importantly, the model could still predict chop impacts with high probability even when SFT was absent. For example, in a case with a straight impact area and pecking, but without SFT, a butterfly fracture, or comminution, the model would still predict the impact to be a chop. This model predicted these variables to be a chop with an 81.7% probability. This demonstrates that SFT is a useful indicator when present, but it is not required for chop identification.

The best-performing model correctly classified the majority of impacts, identifying 18 of 20 chop impacts and 7 of 11 bash impacts correctly, with an overall accuracy of 80.6% and a sensitivity of 90% for chop impacts (Table 4). Several models produced similar WAIC and LOO scores, indicating that the results are stable across alternative model structures.

Table 4

Table 4. Classification performance for the best-performing field logistic regression model including SFT.

5 Discussion

5.1 Key predictive patterns

The results highlight both clear tendencies and meaningful areas of overlap between chopping and bashing impacts. Across all statistical approaches, including Fisher's exact tests, hierarchical clustering, and logistic regression modeling, three variables consistently emerged as the strongest predictors of chop impacts: straight impact location shape, the presence of SFT, and non-comminuted fracture bases. Straight impact location shape showed the strongest positive association with chopping, reflecting the concentrated linear loading applied by an ax blade compared to the more diffuse force of a hammer. The presence of SFT increased the likelihood of a chop classification, although the presence of SFT was not vital for identification. Comminuted fracture bases were negatively associated with chopping, aligning with expectations that blunt impacts are more likely to disperse force broadly and produce shattering.

Logistic regression models that incorporated both direct morphological features and absence features performed slightly better in-sample according to WAIC and LOO scores. This reflects the controlled experimental context, where absence features such as incomplete fractures or unmodified surfaces can meaningfully signal chop impacts. When these features were excluded, the same core set of predictors emerged, and their relative rankings remained stable. This suggests that the relationships between key fracture characteristics and impact type are robust across different observational contexts.

Hierarchical clustering provided further support for this distinction. It revealed a clear separation between the hammer and the axes, while showing little differentiation among the four ax materials. This indicates that the primary differences reflect the mechanics of chopping vs. bashing, rather than metallurgical variation between ax types. Grouping all ax materials together was therefore justified for the main analysis.

Taken together, these patterns provide a coherent framework for distinguishing between chopping and bashing on long bone shafts. Rather than relying on any single diagnostic criterion, the combination of impact area shape, fracture type, and, when present, SFT provides a statistically supported basis for identifying chop impacts. At the same time, the presence of overlapping traits such as bashes occasionally producing straight fractures or chops lacking surface modification emphasizes that these distinctions are probabilistic rather than absolute, and that some individual specimens may remain ambiguous when considered in isolation. Effective interpretation depends on evaluating multiple variables in concert and considering the mechanical and taphonomic context of each assemblage. See Figure 9 for a schematic analytical workflow illustrating how these criteria can be applied in practice.

Figure 9

Figure 9. Analytical workflow for distinguishing chopping and bashing impacts on long bone shafts. The schematic outlines the recommended sequence of observations, beginning with assessment of dynamic fracture behavior, followed by impact area localization using fractographic cues, and evaluation of impact location shape, gross fracture pattern, and surface modifications. The framework emphasizes probabilistic classification based on multiple converging criteria rather than reliance on a single diagnostic feature.

5.2 Fracture characteristics

Fracture morphology provides some of the clearest distinctions between chopping and bashing on long bone shafts. Among the variables tested, impact location shape and gross fracture type were especially informative. Straight impact locations were strongly associated with chopping, reflecting the concentrated, linear loading of an ax blade. Bashes typically produced irregular impact locations, consistent with the diffuse force of a rounded hammer surface. This pattern matches previous percussion experiments, where irregular outlines are typical of hammerstone impacts (Pickering and Egeland, 2006; Galán et al., 2009; Blasco et al., 2014), and experimental chopping studies that show the linear geometry of ax edges produces distinct straight zones (Okaluk and Greenfield, 2022). Gross fracture type also differentiated the two categories. Comminuted fractures were significantly more common in bashes, reflecting broader force dispersal, whereas chops more frequently produced transverse or oblique fractures.

Fractographic features provide a critical tool for locating impact areas and interpreting fracture mechanics. Mirror, mist, and hackle zones are key indicators of how bending fractures propagate. In bending, the fracture initiates on the tensile surface, then travels across the bone toward the compressive side. This means that the point of crack initiation is opposite the actual impact location. By examining the internal fracture surface, analysts can trace the mirror, mist, and hackle zones backward to locate the impact zone on the opposite cortex. This approach is especially valuable when external surface modifications are absent or ambiguous. These fractographic features form during mid-high-energy dynamic loading events, such as blows or strikes, and are not expected in low-energy taphonomic breaks like trampling or sediment pressure (Christensen et al., 2018, 2021). Thus, these features provide the first indication that the fracture may be anthropogenic and should be examined further.

Incorporating fractography strengthens classification by providing a reliable reference point for evaluating impact location shape and fracture morphology. Surface modifications such as percussion notches, flakes, or pecking were infrequent in this study (pecking in 35%, flakes in 45%, a single confirmed notch), echoing previous findings that these features form inconsistently and vary with tool type and number of blows (Blasco et al., 2014; Galán et al., 2009; Pickering and Egeland, 2006). Because surface traces are unreliable on their own, impact identification should center on internal fracture morphology, with surface features treated as supplementary evidence. These patterns were derived from diaphyseal loading, where bending dominates, and should not be generalized to non-diaphyseal contexts without further testing.

5.3 Bone surface modifications and equifinality

Zooarchaeological and forensic studies have traditionally focused on surface modifications such as notches, flake scars, and pitting to identify percussive activity, but they have rarely examined how these features function in distinguishing chopping from bashing in a systematic way. Percussion notches, in particular, have often been treated as hallmark indicators of hammerstone activity, characterized by scalloped cortical margins and the detachment of an inner flake with conchoidal features (Binford, 1981; White, 1992; Fisher, 1995; Pickering and Egeland, 2006; Galán et al., 2009; Domínguez-Rodrigo et al., 2009; Blasco et al., 2014). Flake scars and pitting have similarly been associated with localized impact forces, and their presence has frequently been used to infer intentional tool use (Blasco et al., 2014; White, 1992; Okaluk and Greenfield, 2022). In this study, however, surface modifications were not statistically significant predictors of impact type. This does not reflect a lack of diagnostic potential, but rather their inconsistent occurrence, even under controlled experimental conditions. Pecking was observed in roughly 35% of impacts, flake scars in 45%, and only a single confirmed percussion notch was recorded, produced by a bash. These frequencies mirror patterns observed in experimental percussion studies, which show that notches and related features form unpredictably and vary with tool type, carcass size, and number of blows (Pickering and Egeland, 2006; Galán et al., 2009; Blasco et al., 2014). Because these features occur irregularly, many chop and bash impacts leave no surface modification, even though every impact necessarily produces a fracture and an impact zone. Over-reliance on surface modifications would therefore lead to significant underrepresentation of butchery, since their absence does not imply that no impact occurred.

A deep, incomplete chop mark created by the copper ax (Bone 77) illustrates the interpretive challenges of surface modifications (Figure 10). Unlike most chops in the sample, which produced bending fractures with distinct morphologies, this impact failed in compression and did not propagate through the bone. The blow left a pronounced V-shaped indentation with scalloped cortical margins along a straight impact edge. These scallops resemble percussion notches but were classified as unconfirmed because no interior flake detachment was visible as the inner surface was not exposed. Morphologically, however, they closely mimic hammerstone percussion notches. Had the fracture propagated, these features could have easily been misidentified as hammerstone percussion marks, demonstrating that although uncommon, chop impacts can sometimes resemble bashes when they fail in compression. Crucially, the scallops occurred along a straight impact edge, meaning that the two diagnostic criteria, impact location shape and surface modification, can appear together but be misread if considered separately. In this case, impact location shape provides the clearer indicator of impact type, matching the statistical findings that straight impact edges are strong predictors of chopping. These observations show that surface modifications have diagnostic value when present but should be interpreted alongside fracture morphology and impact zone identification. This combined approach reduces equifinality and improves classification confidence.

Figure 10

Figure 10. Impact area on Bone 77 exhibiting scalloped edges and unconfirmed percussion notches.

5.4 Bridging zooarchaeological and forensic approaches

Although the analytical framework developed here was designed for zooarchaeological contexts, it also draws on and contributes to methods established in forensic fracture analysis. Both fields confront similar challenges in distinguishing chop impacts from blunt-force trauma, particularly when sharp force traits are absent. Forensic research has long acknowledged that the ax trauma often blurs the line between sharp and blunt force trauma and presents significant challenges for weapon identification (Humphrey and Hutchinson, 2001; de Gruchy and Rogers, 2002; Lynn and Fairgrieve, 2009; McGehee et al., 2023). This overlap in diagnostic challenges reflects the shared mechanical principles that govern how bone responds to dynamic, percussive loading. The results of this study show that consistent, fracture-based criteria can help distinguish between chopping and blunt-force impacts. This alignment underscores the value of shared experimental and analytical approaches in advancing both archaeological and forensic interpretations of bone trauma.

6 Conclusion

This study is the first to statistically demonstrate and quantify morphological differences between chop and bash marks on medium mammal long bones, and to establish diagnostic criteria for identifying chop marks that do not express sharp force trauma. Until now, the absence of clear sharp force traits has meant that many chop marks on long bone shafts could not be reliably recognized, leaving a significant gap in our ability to detect and interpret chopping activity. By systematically linking impact location shape, fracture type, and surface modifications through experimental testing and statistical modeling, this study extends the analytical toolkit available for butchery analysis in both zooarchaeological and forensic contexts. Crucially, the comparative experimental design that set chopping and bashing side by side was fundamental for isolating these diagnostic variables, showing that what has often been treated as ambiguous can, in fact, be systematically differentiated.

Practically, analysts should begin by assessing whether the fracture reflects dynamic loading, since this confirms that the break was produced by a percussive or chopping event rather than a static or post-depositional process. Once dynamic loading is established, the next step is to locate the impact area using fractographic cues. Impact location shape and gross fracture type should then be evaluated as primary indicators, with surface modifications treated as supporting evidence. Because pecking, flake scars, and percussion notches occur inconsistently, reliance on these features alone will underrepresent butchery. Every impact creates a fracture and an impact zone, therefore fracture-based criteria provide a consistently available foundation for classification.

These criteria build on existing approaches to percussion damage but represent a significant refinement for chopping, offering the first reliable framework for identifying chop marks on long bones in the absence of sharp force trauma. Chopping and bashing share some overlap, yet they diverge in measurable and interpretable ways. Straight impact areas capture the concentrated loading of an ax edge, comminution reflects broader force dispersion typical of blunt strikes, and SFT remains a strong indicator when present. Used together, these variables support confident classification in many cases while acknowledging uncertainty where signatures are mixed or altered. Centering fracture morphology and impact area identification bridges the gap between elegant surface mark typologies and the practical realities of assemblage analysis.

Ultimately, this study demonstrates that chop marks are not always defined by dramatic sharp force traits. Instead, they often leave subtler but systematic signatures that, when carefully assessed, can be recognized and interpreted with confidence. This represents a major step forward in our ability to detect chopping activity and, by extension, to reconstruct the behaviors and practices that shaped bone assemblages. Expanding the methodological toolkit in this way allows for richer and more accurate interpretations of butchery, tool use, and past human behavior.

7 Limitations

This study examines experimentally produced chopping and bashing impacts under controlled conditions, which enables clear assessment of diagnostic criteria but necessarily simplifies the variability encountered in archaeological assemblages. All impacts were delivered to fresh bone using standardized energy and orientation, whereas archaeological fractures reflect a wider range of force levels, tool angles, bone conditions, and post-depositional histories that may influence the expression and preservation of diagnostic features.

The standardized blunt impactor used here does not capture the full range of morphological variability present in archaeological hammerstones. Irregular hammerstone shapes and surfaces are likely to introduce greater variability in impact expression, representing an important avenue for future research.

Although straight impact edges emerged as a strong indicator of chopping, it remains theoretically possible that other processes could occasionally produce similar morphologies. While no known natural taphonomic processes, such as trampling, generate fractures with comparable dynamic loading signatures, this possibility cannot be entirely excluded.

Finally, this study focuses on impacts produced by metal axes. Stone ax impacts were not examined, and the criteria developed here may not apply directly to such tools.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Ethics statement

Ethical approval was not required for the study involving animals in accordance with the local legislation and institutional requirements because the bones used in this study were purchased as waste material from butcher shops. No animals were killed or harmed for this study.

Author contributions

TO: Writing – original draft, Investigation, Formal analysis, Data curation, Visualization, Funding acquisition, Project administration, Methodology, Conceptualization, Writing – review & editing. JD: Writing – original draft, Formal analysis. HC: Writing – review & editing, Resources. IS: Supervision, Writing – review & editing, Resources, Funding acquisition.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This research was funded by Ariel University, which also provided laboratory facilities and equipment. Additional support was provided by the Social Sciences and Humanities Research Council of Canada (SSHRC; Grant No. 752-2023-0072).

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The author IS declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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