PUMP UP THE VOLUME: PARALLEL BODY SIZE INCREASES IN CAMBRIAN-DEVONIAN BRACHIOPOD LINEAGES

The Cambrian through Devonian was an interval of significant evolutionary and ecological changes in the marine benthos. Body size is widely thought to have increased during this interval across marine groups, but the timing, magnitude, and extent of this increase have been poorly documented. This talk documents size trends in brachiopods from deep-subtidal, soft-substrate assemblages during the Cambrian through Devonian. Brachiopods represent an ideal group for examining these changes because of their frequency in marine biotas and excellent preservation.

Body size of nearly 400 brachiopod genera was measured along standard body axes (anteroposterior, transverse, and dorsoventral) from monographic plates. Body volume, a more direct proxy for body mass, was then estimated using an empirical regression of body volume. Results are similar using other morphometric measurements.

Mean brachiopod genus volume increases substantially, gradually, and continuously during the study interval, from approximately 0.06 milliliters during the Middle Cambrian to approximately 2 milliliters during the Late Devonian. There is a similar increase in the volume of the smallest brachiopod genera. These characteristics fulfill the criteria for Cope's rule and are consistent with a driven trend, wherein lineages over time are forced away from small size. Furthermore, the actively driven increase is taxonomically and ecologically widespread, occurring in unison in both the generally mobile, burrowing linguliformeans and sedentary, epifaunal rhynchonelliformeans. Among eight rhynchonelliformean orders documented with adequate data, seven exhibit similar increases, although not all are as clearly driven. Only the Athyridida do not display a marked increase, remaining either at a constant volume or diminishing slightly.

The magnitude, prolonged duration, and taxonomically widespread nature of these increases suggest that size increase is a significant component of the evolutionary and ecological changes during the Early and Middle Paleozoic. Combined with evidence of similar size increases in other benthic groups in this habitat, the cause of this increase appears increasingly linked to synoptic changes in ecosystem structure, including greater nutrient availability and more complex trophic interactions.