MODELLING THE LITHIFICATION BIAS: UNDERSTANDING ITS CAUSES AND ITS IMPLICATIONS

A series of papers published in recent years has demonstrated that lithification, the transition from unconsolidated sediments to fully indurated rocks, can impose a bias on the apparent diversity, evenness, and body size distribution of fossil assemblages. Fossil collections made from well indurated rocks consistently exhibit lower species richness, lower evenness, and a specimen size distribution skewed towards larger specimens relative to collections made from unconsolidated sediments, even when collections are drawn from the same paleocommunity. This bias remains even after correction for sample size. Whereas the bias itself has been demonstrated empirically, what causes it is less clear. Proposed mechanisms include destruction of small individuals during early diagenesis, destruction during mechanical breaking of well indurated rocks, or differences in collection methodology between unconsolidated sediments and indurated rock. Identifying the cause(s) of the lithification bias is significant because it is likely to have important implications for how, and whether, this bias can be corrected for. We used a simple computer model of fossil collection via cracking out vs sieving to analyze how differences in fossil sampling methodology between well indurated rocks and unconsolidated sediments can impart a bias on species richness, sample evenness, and the body size distribution. Our results demonstrate that for at least some collections the lithification bias can be explained entirely by methodological effects. The results also suggest that this type of modelling might provide a simple tool by which data collected from unconsolidated sediments and well indurated rocks could be made more comparable, but this possibility needs further investigation.