Projectile impact fractures and launching mechanisms: Results of a controlled ballistic experiment using replica Levallois points

Identifying the use of stone-tipped projectile weapons in prehistory is important for understanding hominin strategic behavior and cognitive capacities. Such identifications are based on 'diagnostic impact fractures' (DIFs), assumed to form as a result of collisions between the tips and organic materials in the prey body. However, demonstrating weapon use requires documenting an impact speed and/or kinetic energy beyond those likely to occur accidentally or as a by-product of other tasks. We present a new experiment aimed at investigating the influence of speed on impact fracture formation in controlled conditions. Using an air-gun, we fired 234 nearly identical spears tipped with copies of a Levallois point cast in soda-lime glass into a composite target made of polyurethane bone-like plates, ballistic gelatin, and leather. The impact speed ranged from ≈7 to ≈30m/s and the impact angle (IA) varied in increments of 15°, from 90° to -45°. We show that realistic DIFs can be produced under these controlled conditions. The frequency of longitudinal tip macrofractures is directly proportional to the impact speed but inversely proportional to the IA. The relationship between the tip fracture type and the type of damage left on the target explains the contact conditions for the formation of different DIFs. No relationship between either initiation or termination type and speed could be established. Therefore, we conclude that 'step-terminating bending fractures' should not be considered diagnostic of weapon use without further supporting evidence. Further, although fracture length increases with speed when IA is held constant, a great deal of overlap exists between trials with different IAs. Given the expected high variance in IA in real hunting situations, large longitudinal macrofractures on the tips of archaeologically recovered lithics should not automatically be interpreted as resulting from the use of high-speed projectiles. We discuss the study's implications for the differentiation of prehistoric weapon-delivery systems, especially regarding recognizing stonetipped weapon use by Neandertals.