Fig. 1.

Increasing size trend in Cambrian–Devonian brachiopod genera from deep-subtidal, soft-substrate habitats. Time scale from ref. 21 with 11-Myr bins used in the Paleobiology Database. Each data point is the observed body volume for a single genus plotted at its bin midpoint age; several points overlap. Simultaneously increasing minimum, mean, and maximum sizes through most durations are consistent with Cope's rule. SE bars around means are 1 SD from the distribution of 2,000 bootstrap replicates. Trend in median sizes (data not shown) is nearly identical to mean trends. Ma, megaannum (millions of years ago).