Data from: Breaking down the lithification bias: the effect of preferential sampling of larger specimens on the estimate of species richness, evenness, and average specimen size

Lithification, the transition of unconsolidated sediments to fully indurated rocks, can potentially bias estimates of species richness, evenness, and body size distribution derived from 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 assemblage. This phenomenon is known as the lithification bias. While the bias itself has been demonstrated empirically, much less attention has been paid to its causes. Proposed causes include taphonomic processes (e.g., destruction of small specimens during early diagenesis) or methodological differences (e.g., sieving vs. counting specimens on outcrops, bedding surfaces, or mechanically split surfaces). Here we investigate the potential effects of preferential intersection that could also result in a methodologically related bias: the preferential sampling of larger specimens relative to smaller ones when fossils are counted on rock surfaces. We used an analog model to simulate preferential intersection (fossil collection via splitting fossiliferous rock) and compare the results to a random draw model that approximates the effects of sieving. The model was parameterized using nine different combinations of species abundance and species size distributions. The results show that, with rare exceptions, species richness is 5–23% lower, evenness 5-25% lower, and average specimen size 24–150% higher in preferential intersection than in random draw simulations. We conclude that preferential intersection can impose a significant bias independent of other mechanisms (e.g., preferential destruction of smaller specimens during diagenetic or sampling processes), that the magnitude of this bias is partially dependent on the species abundance and size distributions, and that this bias alone does not fully account for empirically observed lithification bias on species richness (i.e., other sources of bias are also at work).

成岩作用（Lithification）指未固结沉积物向完全固结岩石的转化过程，其可能会对基于化石组合得到的物种丰富度、物种均匀度以及体型分布的估算结果产生偏倚。即便采自同一化石组合，取自完全固结岩石的化石采集样本也始终表现出更低的物种丰富度、更低的物种均匀度，且标本体型分布偏向更大个体，这一现象被称为成岩作用偏倚（lithification bias）。尽管该偏倚已被实证证实，但学界对其成因的关注却相对匮乏。已提出的成因包括埋藏学过程（taphonomic processes，例如早期成岩作用阶段小型标本的破坏），或是方法学差异，例如在露头、层理面或机械劈露面直接计数标本与采用筛分法采集标本之间的方法差异。本文探讨了另一种可能引发方法学相关偏倚的优先截获效应（preferential intersection）：当在岩石表面计数化石时，相较于小型标本，大型标本更易被优先采样。我们采用类比模型模拟优先截获过程，即通过劈裂含化石岩石进行化石采集，并将模拟结果与近似筛分法效果的随机抽取模型进行对比。该模型基于9种不同的物种丰度与物种体型分布组合完成参数化设置。结果显示，除极少数例外情况外，优先截获模拟中的物种丰富度较随机抽取模拟低5%~23%，物种均匀度低5%~25%，平均标本体型则高24%~150%。我们得出如下结论：优先截获可独立于其他机制，例如成岩作用或采样过程中对小型标本的优先破坏，带来显著偏倚；该偏倚的强度部分取决于物种丰度与体型分布；且仅靠该偏倚无法完全解释实证观测到的成岩作用偏倚对物种丰富度的影响，即仍存在其他偏倚来源。