Novack-Gottshall, P.M. and M.A. Lanier. 2008. Scale-dependence of Cope’s rule in body size evolution of Paleozoic brachiopods. Proceedings of the National Academy of Sciences (U.S.A.) 105: 5430-5434 (.pdf reprint)
Presented at Southeastern Section of Geological Society of America at Savannah, GA (29–30 March 2007)
Abstract:
The average body size of brachiopods from a single habitat type increased gradually by more than two orders of magnitude during their initial Cambrian–Devonian radiation. This increase occurred nearly in parallel across all major brachiopod clades (classes and orders) and is consistent with Cope’s rule: the tendency for size to increase over geological time. The increase is not observed within small, constituent clades (represented here by families), which underwent random, unbiased size changes. This scale-dependence is caused by the preferential origination of new families possessing initially larger body sizes. However, this increased family body size does not confer advantages in terms of greater geological duration or genus richness over families possessing smaller body sizes. We suggest that the combination of size-biased origination of families and parallel size increases among major, more inclusive brachiopod clades from a single habitat type is best explained by long-term, secular environmental changes during the Paleozoic that provided opportunities for body size increases associated with major morphological evolution.
Fig. 1. Increasing size trend in Cambrian–Devonian brachiopod genera from deep-subtidal, soft-substrate habitats. Time scale from Gradstein et al. (2005) with 11-Myr bins used in the Paleobiology Database. Each data point is the observed body volume for a single brachiopod 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).