Data from: Gauging scale effects and biogeographical signals in similarity distance decay analyses: an Early Jurassic ammonite case study

In biogeography, the similarity distance decay (SDD) relationship refers to the decrease in compositional similarity between communities with geographical distance. Although representing one of the most widely used relationships in biogeography, a review of the literature reveals that: (1) SDD is influenced by both spatial extent and sample size; (2) the potential effect of the phylogenetic level has yet to be tested; (3) the effect of a marked biogeographical structuring upon SDD patterns is largely unknown; and (4) the SDD relationship is usually explored with modern, mainly terrestrial organisms, whereas fossil taxa are seldom used in that perspective. Using this relationship, we explore the long-distance dispersal of the Early Jurassic (early Pliensbachian, c. 190.8 Ma to 187.6 Ma) ammonites of the western Tethys and adjacent areas, in a context of marked provincialism. We show that the long-distance dispersal of these ammonites is not related to shell size and shape, but rather to the environmental characteristics of the province to which they belong. This suggests that their long-distance dispersal may have been essentially driven by passive planktonic drift during early juvenile, post-hatching stages. Furthermore, it seems that the SDD relationship is not always an appropriate method to characterize the existence of a biogeographical structuring. We conducted SDD analyses at various spatial, sampling and phylogenetic scales in order to evaluate their sensitivity to scale effects. This multi-scale approach indicates that the sampling scale may influence SDD rates in an unpredictable way and that the phylogenetic level has a major impact on SDD patterns.

生物地理学中，相似性距离衰减（Similarity Distance Decay，以下简称SDD）关系指的是群落组成相似性随地理距离增加而降低的现象。尽管该关系是生物地理学中应用最为广泛的关系之一，但现有文献综述显示：（1）SDD同时受空间幅度与样本量的影响；（2）系统发育层级的潜在效应尚未得到验证；（3）显著生物地理学结构对SDD格局的影响目前仍尚未明确；（4）SDD关系的研究通常以现代（主要为陆生）生物类群为对象，而该视角下极少使用化石类群。借助该关系，我们针对特提斯洋西部及周边区域的早侏罗世（普林斯巴阶早期，约1.908亿至1.876亿年前）菊石开展了长距离扩散研究，研究背景为显著的生物分区现象。研究结果显示，这些菊石的长距离扩散与其壳体大小、形态无关，而是与其所属生物分区的环境特征密切相关。这表明它们的长距离扩散可能主要是在孵化后的幼体早期阶段，通过被动浮游漂移实现的。此外，研究表明SDD关系并非总能作为表征生物地理学结构存在的合适方法。为评估SDD分析对尺度效应的敏感性，我们在不同空间尺度、采样尺度与系统发育尺度下开展了SDD分析。该多尺度分析结果显示，采样尺度可能以不可预测的方式影响SDD速率，而系统发育层级则对SDD格局具有显著影响。