File name: Nicholson_et_al_manual_splits_9Myr_splits_ESM Title: Time binsDescription: File containing temporal bin information for use in Shareholder Quorum Subsampling analyses from Latitudinal diversity gradients in Mesozoic non-marine turtles

The latitudinal biodiversity gradient (LBG)–the pattern of increasing taxonomic richness with decreasing latitude–is prevalent in the structure of the modern biota. However, some freshwater taxa show peak richness at mid-latitudes; for example, extant Testudines (turtles, terrapins and tortoises) exhibit their greatest diversity at 25° N, a pattern sometimes attributed to recent bursts of climatically mediated species diversification. Here, we test whether this pattern also characterizes the Mesozoic distribution of turtles, to determine whether it was established during either their initial diversification or as a more modern phenomenon. Using global occurrence data for non-marine testudinate genera, we find that subsampled richness peaks at palaeolatitudes of 15 – 30° N in the Jurassic, 30 – 45° N through the Cretaceous to the Campanian, and from 30° to 60° N in the Maastrichtian. The absence of a significant diversity peak in southern latitudes is consistent with results from climatic models and turtle niche modelling that demonstrate a dearth of suitable turtle habitat in Gondwana during the Jurassic and Late Cretaceous. Our analyses confirm that the modern testudinate LBG has a deep-time origin and further demonstrate that LBGs are not always expressed as a smooth, equator-to-pole distribution.

纬度多样性梯度（latitudinal biodiversity gradient, LBG）——即分类单元丰富度随纬度降低而升高的分布模式——广泛存在于现代生物群的结构特征中。然而，部分淡水类群的丰富度峰值出现在中纬度区域；以现生龟鳖目（Testudines）为例，其多样性最高值出现于北纬25°处，该模式有时被归因于气候介导的近期物种爆发式分化。本研究旨在验证这一模式是否同样契合龟类的中生代分布特征，以此明确该梯度究竟形成于龟类的初始分化阶段，还是更晚近的演化时期。本研究利用非海生龟鳖目属级的全球化石产出数据开展分析，结果显示：经采样校正的丰富度峰值在侏罗纪时期出现于古纬度15°~30° N区域，白垩纪至坎潘期则位于30°~45° N区域，而马斯特里赫特期的峰值范围为30°~60° N。南半球未出现显著的多样性峰值，这一结果与气候模型及龟类生态位模型的模拟结论一致——后者显示，侏罗纪与晚白垩世时期的冈瓦纳大陆缺乏适宜龟类生存的栖息地。本研究证实，现生龟鳖类的现代纬度多样性梯度具有深远的深时起源，同时也证明纬度多样性梯度并非总是表现为从赤道到两极的平滑递减分布。