Bivariate scatter plot of absolute abundance of specimens plotted against drilling frequency.

<p>A significant negative correlation is observed, with drillholes being less frequent in samples that came from more fossiliferous units. Inset plot shows that the pattern holds when data are plotted by horizon (with values averaged across samples for Horizons 1 and 2), although first differences are not significantly correlated. Symbols, <i>r</i> = Spearman correlation coefficient; <i>r_D</i> = Spearman correlation coefficient based on first differences; <i>p</i> = a two-tailed probability of <i>r</i> = 0; <i>p_D</i> = a two-tailed probability of <i>r_D</i> = 0.