Extended Data Fig. 2: Comparison of community structure metrics.

a–d, Left, δ¹³C bulk and stable isotopes as in Fig. 1a. Dark green, benthic³²; light green, bulk. Downcore plots of Σ_CV (a), Bray–Curtis dissimilarity (b), Simpson’s index (d; grey dashed lines; black line indicates the 150-kyr moving average) and the variance (in 150-kyr windows) in the Simpson’s index (c). Vertical grey lines in a and b show the level of background inferred from rank order plots of these data. All four metrics (Σ_CV, Bray–Curtis dissimilarity, Simpson’s index and variance in the Simpson’s index) show a distinction of volatility between early Danian regime 1 (n = 137 data points) and regime 2 (the rest of the record, n = 861 data points). For example, the Wilcoxon rank-sum (W) value for the Simpson’s record was W = 46,646; P < 0.001 on first differences with 95% confidence limits of −0.013, −0.006. A W value of zero would support a null hypothesis. The test was two-sided. The Simpson’s index shows a diversity minimum in the earliest Danian and then a rapid increase and steady long-term trend towards more diverse, more even communities, but with high variability in the early Danian. This fluctuation in the Simpson’s index, as recorded by the variance of the record (c) shows similar patterns to Bray–Curtis dissimilarity and Σ_CV with high variance in the early Danian before dropping down. The variance in the Simpson’s index also shows high background fluctuations and a sustained increase in amplitude of fluctuations around the isotope shift in the Palaeocene Carbon Isotope Maximum, reflecting oligotroph diversification, which the Simpson’s index shows strongly due to its higher sensitivity to rare taxa. In effect, metric sensitivity to the richness in taxa and rare taxa increased from a to c (from abundance variance to diversity variance). Note, the Simpson’s index can only be calculated on full assemblage data and therefore the record extends only from 66 to 55.5 Myr ago.