Raman and optical microscopy of bone micro-residues on cobbles from the Cerutti mastodon site

Abstract

Cobbles from the Cerutti Mastodon (CM) site have impact marks and usewear suggesting that mastodon bones were placed on stone anvils and struck with stone hammers to produce two concentrations of broken bones and stones. Critics have suggested that the stones may have broken by rolling down slopes rather than in situ at the two concentrations. Our analysis of two cobbles (pegmatite CM-254 and andesite CM-281) identifies bone micro-residues that are not evenly distributed over the cobbles, and are unlikely to have been transferred from sediment or from passive contact with adjacent macro-bones. Bone micro-residues on cobble CM-254 were recovered from surfaces associated with usewear, but were absent from the naturally broken surface found in direct contact with a mastodon rib. In addition, bone micro-residues on cobble CM-281 were recovered from upward facing locations with impact marks and other usewear; but were absent on the downward facing surface. Bone micro-residues are absent in sediment away from the bone concentrations. These new data support the argument that the associated concentration of broken stones and mastodon bones is in situ, and that bones in this concentration were likely broken by the pegmatite cobble (comprising CM-254 and other fragments), when it struck mastodon bones placed on the andesite cobble CM-281. These findings add to the totality of evidence that supports human agency rather than geological processes as the driver responsible for the CM taphonomic pattern.