Harnessing stratigraphic bias at the section scale: conodont diversity in the Homerian (Silurian) of the Midland Platform, England

Fossil abundance and diversity in geological successions are subject to bias arising from shifting depositional and diagenetic environments, resulting in variable rates of fossil accumulation and preservation. In simulations, this bias can be constrained based on sequence-stratigraphic architecture. Nonetheless, a practical quantitative method of incorporating the contribution of sequence-stratigraphic architecture in community palaeoecology and diversity analyses derived from individual successions is missing. As a model of faunal turnover affected by the stratigraphic bias, we use the ‘Mulde event’, a postulated mid-Silurian interval of elevated conodont turnover, which coincides with global eustatic sea-level changes and which has been based on regionally constrained observations. We test whether conodont turnover is highest at the boundary corresponding to the ‘event’ and post-‘event’ interval against the alternative that conodont turnover reflects habitat tracking and peaks at facies shifts. Based on the previously documented, parasequence-level stratigraphic framework of sections in the northern and central part of the Midland Platform, the relative controls of sequence-stratigraphic architecture, time and depositional environment over conodont distribution are evaluated using permutational multivariate analysis of variance. The depositional environment controls the largest part of variability in conodont assemblage composition, whereas the postulated ‘Mulde event’, or genuine temporal change in conodont diversity, cannot be detected. Depending on the binning of the stratigraphic succession, contrasting diversity and turnover patterns can be produced. The simple approach proposed here, emulating partitioning of β diversity into spatial and temporal components, may help to constrain the stratigraphic bias, even at the scale of an individual section.

地层序列中的化石丰度与多样性易受偏差影响，该偏差源于沉积与成岩环境的变迁，进而导致化石富集与保存速率存在差异。在模拟研究中，可基于层序地层构型（sequence-stratigraphic architecture）对该偏差进行约束。然而，目前仍缺乏一种切实可行的定量方法，将层序地层构型的贡献纳入基于单个地层序列的群落古生态学与多样性分析之中。作为受地层偏差影响的动物群更替研究模型，我们选取“穆尔德事件（Mulde event）”作为研究对象：该事件被假定为志留纪中期牙形石（conodont）更替速率升高的时段，其发生与全球海平面升降变化相吻合，相关结论基于区域限定性观测数据得出。我们检验牙形石更替速率是否在对应“事件”及其后时段的边界处达到峰值，同时对立假说认为牙形石更替反映生境追踪行为，其峰值出现在岩相偏移处。基于此前已发表的密德兰陆棚（Midland Platform）北部与中部地区剖面的准层序级（parasequence-level）地层框架，我们采用置换多元方差分析（permutational multivariate analysis of variance）评估层序地层构型、时间与沉积环境对牙形石分布的相对控制作用。结果显示，沉积环境对牙形石组合组成的变异解释度最高，而此前假定的“穆尔德事件”，或是牙形石多样性的真实时间变化，均未被检测到。根据地层序列的分箱方式不同，可得到截然不同的多样性与更替模式。本文提出的简易方法借鉴了将β多样性（β diversity）划分为空间与时间组分的思路，即便针对单个地层剖面，也有助于约束地层偏差的影响。