This post was originally written to appear on Rob Gargett’s blog http://www.thesubversivearchaeologist.com/2012/11/part-two-of-there-ya-go-again-putative.html, but was considered too lengthy by his server and so will be posted here and linked across. Please read the original posting at the Subversive Archaeologist for the context of this reply.
Rob has clearly dedicated a significant amount of time to deciphering the details of the latest claim for an early ancestry of spear technologies in southern Africa, and has done a good job of attempting to de-tangle the devil in the details of this work. I will respond in my personal capacity to the impact fracture and trampling work that he has issue with and leave the remaining critiques up to others as they feel fit to respond.
‘What’s the DIF?’ has become a big question in the discourse on hunting technologies over the past 28 years, partly because macrofracture analysis provides a time efficient means of pre-sorting lithic and bone assemblages for signs of wear damage associated with impact activities. Following macrofracture analysis, and other morphometric observations on tools (e.g. TCSA, TCSP), with more time-consuming and energy intensive analyses such as micro-wear and micro-residue, allows analysts to reach probabilistic sample sizes and to make population-level inferences about the use of tools as weapon components. It is important to keep in mind, however, that impact fractures are not diagnostic criteria on their own. The question of what DIFs actually are, and how they form in relation to post-depositional process, is what spurred the series of ongoing trampling and knapping experiments, using “home made [?]” tools, that Rob refers to in this posting (Pargeter 2011, and for other examples see Sano 2009, Pargeter & Bradfield 2012). Although Wilkins et al cite more than just “one paper” (they also refer to Sano 2009) for post-depositional macrofracture damage, I will comment mostly from the perspective taken in the Pargeter (2011) experiments.
First, Rob refers to step termination (ing) fractures and burin removals as the two DIF types used in macrofracture analysis. Referring back to the original experimental literature on the subject (Fischer et al. 1984) one notes that the key fracture types designated as diagnostic were: step terminating bending fractures, unifacial spin-off fractures > 6 mm (Fischer et al allowed for spin-offs as small as 1mm) and bifacial spin-off fractures. Burination fractures (or impact burinations) were only added later to the DIF category by Barton & Bergman (1982), Bergman & Newcomer (1983) and Lombard (2005). The Pargeter (2011) table of trampling results Rob has pasted into his post has 50 % of the diagnostic fracture types crossed out, leaving only step and burination fractures. Spin-off fractures, absent in the Pargeter experiments and almost all trampling experiments thus far (c.f. Pargeter & Bradfield 2012 and Sano 2009), are in fact thought to be more diagnostic of impact than either of the fractures singled out by Rob. If these been included, Rob might have gone on to say that what Wilkins et al did not find on any of their convergent flakes were spin-off fractures. Had they done so, the case for dismissing post-depositional trampling, not bio-turbation or any other post-depositional process, would have been significantly increased. Rob goes on to state that “those that have been crossed out [spin-off fractures in the table] are not considered DIFs, per se”. In fact, per se, step terminating fractures and impact burinations, the fractures Rob chooses to leave uncrosses, are considered the least diagnostic impact fracture types, owing to their consistent appearance in these and the Sano (2009) trampling experiments. The “four [count 'em, 4] flakes…found with step terminating fractures” versus the 0 spin-off fractures are a further indication of this. Impact burinations are particularly ambiguous fracture types as they occur on tool margins, are sometimes confused with intentional burinations (but see Lombard 2005) and do not “only occur at the tip”, but have also been recorded on proximal ends of tools as a result of rebound forces from hafts.
Because of the ambiguous nature of impact fractures (i.e. that they can occur on any sufficiently brittle edge of a flake when trampled) it is important to pay attention to tool morphology when conducting macrofracture analyses. Rob is correct in stating that step terminations and impact burinations are not “DIFs unless they occur on pointy stones”, anyone who has even been confronted by a wounded buffalo will know that a “pointy stone” is a far more valuable aid than a circular scraper. Moreover, “Amorphous flakes” (and I am not exactly sure what Rob meant by this, but I will assume flakes that are not retouched, or are shaped in different ways to typical ethnographic / ethno-historic weapon tips) feature prominently in the history of macrofracture analysis. The segments from Klasies River Mouth that Rob includes pictures of were long considered to be amorphous weapon components in southern Africa, and for this reason their function was long assumed. Replication experiments assessing their functionality as weapon tips hafted in a variety of configurations (Pargeter 2007) and the subsequent macrofracture analysis of these pieces (Lombard & Pargeter 2008) showed that these amorphous tools function well as hunting weapon components. Subsequent micro-wear and micro-residue analyses (Lombard 2007, 2008, 2011) on segments from the same site and time period has shown that out initial conclusions based on the hunting experiments were valid. Moreover, the original spear experiments referred to in the Wilkins et al paper (from Lombard et al. 2004) were based on unretouched tools, which at the time were not considered to be “true projectile points”. Flakes do break and morphologies change throughout the lifetime of an artefact, but unless reconstruction of these original morphologies can be justified, macrofractures need to be analyzed with the functional morphologies of hunting weapons in mind.
Third, “we know that substrate density will have a bearing on damage from trampling-for example, between sand and bedrock”. Rob is correct in being cautious about this component of any trampling experiment. Since at least 1998, with the McBrearty et al. Tools Underfoot trampling experiments, we have known this. While this variable was not the explicit focus of the Pargeter 2011 experiments, it was controlled for in these and subsequent experiments using goats as trampling agents (Pargeter & Bradfield 2012). The results of the goat trampling experiments showed that smaller animals do not significantly alter the frequency of macrofracture damage on stone tools, but do, as expected, have less of an impact on edge damage occurrences than humans or cattle.
Lastly, being able to determine whether a tool was used for hunting or to distinguish between spears and arrows based solely on macrofracture scars is a fallacy, and one that recent publications on the subject have endeavored to make clear (e.g. Pargeter & Bradfield 2012). Our current state of knowledge of macrofracture formation is not such that we can tell whether a certain fracture formed as a result of a certain weapon delivery system, or to even to securely deduce, from these traces alone, the hunting function of stone artefacts. As mentioned already in this reply, macrofracture analysis is a pre-sorting tool that allows researchers to save time in sorting large artefact samples for subsequent analyses that can include micro-wear or micro-residue analysis. Clearly, as Rob indicates, there is a lot of work to do regarding the formation of macrofracture traces and their relationship to contextual factors such as substrate composition and post-depositional formation processes. Thankfully we are working on these questions and will continue to produce results for the scientific community to evaluate.
Access to the Pargeter papers referred to in this reply can be found at: http://sunysb.academia.edu/JustinPargeter