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.

在生物地理学领域中，相似性距离衰减（similarity distance decay, SDD）关系指群落间的组成相似性随地理距离增大而降低的规律。尽管该关系是生物地理学中应用最为广泛的规律之一，但现有文献综述显示：（1）SDD同时受空间范围与样本量的影响；（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格局的影响仍不明确；（4）当前SDD研究多以现代（主要为陆生）生物类群为研究对象，极少从该视角探讨化石类群。本研究借助该关系，针对显著生物地理分区背景下的西特提斯洋及邻区早侏罗世（普林斯巴阶早期，约190.8 Ma至187.6 Ma）菊石类群开展长距离扩散研究。结果表明，这些菊石的长距离扩散与壳体大小、形态无关，而是与其所属生物地理分区的环境特征密切相关。这暗示其长距离扩散主要由孵化后幼体阶段的被动浮游漂移所驱动。此外，研究还发现SDD关系并非总能适用于表征生物地理学结构的存在性。为评估不同尺度效应下SDD分析的敏感性，我们在多种空间、采样及系统发育尺度下开展了SDD分析。该多尺度研究结果显示，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。