Towards an understanding of cosmopolitanism in deep time: a case study of ammonoids from the middle Permian to the Middle Triassic

Cosmopolitanism occurred recurrently during the geological past, especially after mass extinctions, but the underlying mechanisms remain poorly known. Three theoretical models, not mutually exclusive, can lead to cosmopolitanism: (1) selective extinction in endemic taxa, (2) endemic taxa becoming cosmopolitan after the extinction and (3) an increase in the number of newly originated cosmopolitan taxa after extinction. We analyzed an updated occurrence dataset including 831 middle Permian to Middle Triassic ammonoid genera and used two network methods to distinguish major episodes of ammonoid cosmopolitanism during this time interval. Then, we tested the three proposed models in these case studies. Our results confirm that at least two remarkable cosmopolitanism events after the Permian–Triassic and late Smithian (Early Triassic) extinctions, respectively. Partitioned analyses of survivors and newcomers revealed that the immediate cosmopolitanism event (Griesbachian) after the Permian–Tr...地质历史时期，广布性（cosmopolitanism）现象反复出现，尤以大灭绝事件之后为甚，但其背后的作用机制仍未得到充分阐明。目前存在三种互不排斥的理论模型可解释广布性的形成：（1）特有类群（endemic taxa）的选择性灭绝；（2）大灭绝后特有类群演变为广布类群；（3）大灭绝后新生广布类群的数量增加。本研究整合了一套更新后的出现数据集（occurrence dataset），涵盖831个中二叠世至中三叠世的菊石（ammonoid）属级分类单元，并采用两种网络分析方法，厘清了该时段内菊石广布性的主要演化事件。随后，针对上述三个理论模型开展案例验证。研究结果证实，二叠纪—三叠纪（Permian–Triassic）灭绝事件与晚斯密斯亚期（早三叠世，late Smithian (Early Triassic)）灭绝事件之后，分别发生了至少两起显著的广布性事件。对残存类群与新生类群的分区分析（partitioned analyses）显示，二叠纪—三叠纪灭绝后即刻发生的广布性事件（格里斯巴赫期，Griesbachian）……