Shifting speciation mode and biogeographic patterns during the Late Ordovician (Sandbian-Katian) in Laurentian brachiopods (Atrypida, Anazygidae)

The Late Ordovician was a period of substantial environmental change. The co-evolution of Earth systems changes involving tectonic uplift, eustasy, and biotic changes are recorded in the Late Ordovician in Laurentia. In this study, species of the brachiopod family Anazygidae within the genera Zygospira and Catazyga were analyzed to assess dispersal pathways among eight sedimentary basins. Stochastic analysis applied to a Bayesian phylogeny (Anazygidae_tree.tre) through BioGeoBEARS facilitated estimation of speciation events including episodes of vicariance and basin-to-basin dispersal events (Anazygidae_geoareas.data) within the clade. Three algorithms were implemented and compared (BioGeoBEARS_code.R). Dispersal-extinction-cladogenesis (DEC) + jump dispersal parameter was the best supported model. For suggestions, advice, or requests related to the code and how to re-run our results, please contact the author. 

Results (b_AICc_table.csv) indicate that Anazygidae originated from an ancestral vicariance event, and most subsequent speciation events occurred via dispersal (total of 14 distinct episodes). Vicariance events are constrained to Mohawkian (Sandbian 2 to Katian 1) time and correlate to tectonic events. Dispersal events dominated during Cincinnatian (Katian 1–4) time and promoted increased diversification of anazygid species. Dispersal events are related to epicontinental circulation patterns and regional sea level changes and can be linked to connectivity and isolation pulses between the mid-continent and southern basins. Dispersal events exhibit a directional trend from the mid-continent toward the southern-most basins in Laurentia. Long-distance dispersal can be explained by jump dispersal events, with marginal-Laurentian islands acting as “stepping-stones” for intracratonic species. Anazygid speciation declined in the Richmondian (Katian 4). Zygospira and Catazyga became extinct in Laurentia as the epicontinental seas drained and climate cooled during the late Katian.

奥陶纪晚期（Late Ordovician）是一段发生显著环境变化的地质时期。涉及构造抬升、海平面升降与生物更替的地球系统协同演化信号，保存在劳伦大陆（Laurentia）的奥陶纪晚期沉积记录中。本研究针对腕足动物无褶贝科（Anazygidae）下褶脊贝属（Zygospira）与卡特贝属（Catazyga）的物种展开分析，以厘清8个沉积盆地间的生物扩散路径。研究通过BioGeoBEARS工具对贝叶斯系统发育树（Bayesian phylogeny，对应文件Anazygidae_tree.tre）开展随机分析，得以估算该演化支内的物种形成事件，包括隔离分化（vicariance）事件与盆地间扩散事件，相关地理分区数据存储于Anazygidae_geoareas.data文件中。本研究共实施并对比了三种算法，相关代码存储于BioGeoBEARS_code.R文件中。其中，扩散-灭绝-分支演化（dispersal-extinction-cladogenesis，DEC）+跳跃扩散参数模型得到了最高的支持度。

若需获取代码相关建议或复现本研究结果，请联系本文作者。

结果文件b_AICc_table.csv显示，无褶贝科起源于一次祖先类群的隔离分化事件，后续绝大多数物种形成事件均通过扩散作用完成（共计14次独立事件）。隔离分化事件仅局限于莫霍克期（Sandbian 2至Katian 1），并与区域构造事件存在显著相关性。辛辛那提期（Katian 1–4）以扩散事件为主导，推动了无褶贝科物种的快速辐射演化。扩散事件与陆表海环流模式及区域海平面变化密切相关，可对应劳伦大陆中部盆地与南部盆地间的连通与隔离脉冲过程。整体扩散呈现出从大陆中部向劳伦大陆最南部盆地的定向趋势。长距离扩散可通过跳跃扩散事件解释，劳伦大陆边缘的岛屿充当了"踏脚石"为克拉通内部物种提供扩散通道。里奇蒙期（Katian 4）期间，无褶贝科的物种形成速率显著下降。晚卡蒂安期（late Katian）陆表海逐渐退缩、气候持续变冷，最终导致褶脊贝属与卡特贝属在劳伦大陆彻底灭绝。