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，以及数次重大的类群更替与大灭绝（mass extinctions）事件。自其起源于距今超过3.8亿年（即380百万年前）以来，至1.75亿年前的中侏罗世初期，四足类的多样性演化整体呈现间歇性辐射的特征：其多样性峰值出现在中二叠世，而在莫斯科阶晚期、早二叠世、沃德期、吴家坪阶下部、二叠纪末、安尼阶下部、拉丁阶下部、晚三叠世（诺利阶下部至瑞替阶上部）、三叠纪末以及早侏罗世（辛缪尔阶上部、普林斯巴赫阶下部）均出现了显著的多样性下降。
在上述多样性下降事件中，仅二叠纪末与三叠纪末的下降对应已被公认的大灭绝事件，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。其余的多样性下降可能代表真实的灭绝或类群更替事件，也可能部分源于采样偏差（biased sampling）造成的人为假象。
地层单元数（formation counts）与四足类古多样性（palaeodiversity）之间存在显著的相关性，这表明原始数据中存在采样效应，但这种共变（covariation）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。
目前关于四足类古多样性时间序列与其他推定采样指标（如标本完整度、文献发表数量、古地理图覆盖面积、有间断界限的沉积单元、岩石体积、地层单元数以及化石馆藏量）之间共变关系的证据较为有限。
多重相关性建模结果显示，地层单元数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，目前尚不清楚地层单元数是否独立于古多样性时间序列，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强度的波动。