Data from: The first half of tetrapod evolution, sampling proxies, and fossil record quality

The first half of tetrapod evolution witnessed substantial diversification of the clade and several major turnovers and mass extinctions. In the time since their origin, more than 380 Myr ago, to the beginning of the Middle Jurassic 175 Myr ago, tetrapods apparently diversified fitfully, reaching their highest level in the Middle Permian, and showing major diversity declines in the late Moscovian, Early Permian, Wordian, lower Wuchiapingian, end-Permian, lower Anisian, lower Ladinian, Late Triassic (lower Norian to upper Rhaetian), end-Triassic, and Early Jurassic (upper Sinemurian, lower Pliensbachian). Of these diversity drops, only the end-Permian and end-Triassic correspond to recognised mass extinctions, and the late Moscovian and early Norian drops to other previously identified environmental crises. The remainder could be real extinction or turnover events, or partially artefacts of biased sampling. There are strong correlations between formation counts and tetrapod palaeodiversity, suggesting a sampling component in the raw data, but the covariation is not uniform through the whole time span, being poor from Devonian to Middle Permian, and better from Late Permian to Early Jurassic. There is limited evidence for covariation between the tetrapod palaeodiversity time series and other putative sampling metrics, such as specimen completeness, numbers of publications, map areas, gap-bounded sedimentary units, rock volumes, formations, and fossil collections. Modelling by multiple correlations shows that formation count is generally the best explanatory model, either on its own, or combined with other ‘sampling’ time series. However, it is not clear that formation count is independent of the palaeodiversity time series, because rises and falls in both signals could reflect variations in original diversity or in preservation or in sampling.

四足类（tetrapod）演化的前半段见证了该演化支（clade）的显著辐射演化，以及多次重大更替事件与集群灭绝（mass extinctions）。自其于3.8亿多年前起源，至1.75亿年前的中侏罗世初期，四足类的多样性演化呈现间歇性特征：于中二叠世达到峰值，并在莫斯科阶晚期、早二叠世、沃德期、吴家坪阶早期、二叠纪末、安尼阶早期、拉丁阶早期、三叠纪晚期（诺利阶早期至瑞替阶晚期）、三叠纪末以及早侏罗世（锡内穆阶晚期、普林斯巴阶早期）出现显著的多样性下降。
在这些多样性下降事件中，仅二叠纪末与三叠纪末事件属于已被确认的集群灭绝，莫斯科阶晚期与诺利阶早期的下降则对应此前已识别的其他环境危机。其余下降事件可能是真实的灭绝或更替事件，也可能部分是采样偏差（biased sampling）所导致的假象。
岩层组（formation）数量与四足类古生物多样性（palaeodiversity）之间存在显著相关性，这表明原始数据中存在采样效应，但该协变关系在整个时间跨度内并不均匀：泥盆纪至中二叠世期间相关性较弱，而晚二叠世至早侏罗世期间相关性更佳。
四足类古生物多样性时间序列与其他推定的采样指标（如标本完整度、发表文献数量、制图区域面积、以间断为界的沉积单元、岩石体积、岩层组数量以及化石收藏量）之间的协变证据有限。
多元相关性建模结果显示，岩层组数量通常是最佳的解释模型，无论是单独使用还是与其他“采样”时间序列结合使用。然而，目前尚不清楚岩层组数量是否独立于古生物多样性时间序列，因为两者信号的升降既可能反映原始多样性的变化，也可能反映保存作用或采样作用的变化。