ASSEMBLAGE-WIDE, BODY-SIZE TRENDS IN CAMBRIAN-DEVONIAN MARINE BIOTAS: WHAT DO WE LEAVE OUT BY EMPHASIZING INDIVIDUAL LINEAGES?

Although trends in increasing body-size are widely reported for individual lineages, such increases should also have important ecological consequences for other community members. Important hypotheses include: Do prey or predators tend to increase in size first? Is the diversification of epibionts a consequence of size increase in suitable hosts? Do increases in nutrient availability in an environment facilitate larger sizes for all species? These wider implications are poorly explored.

This talk documents assemblage-wide size trends in well-preserved samples from deep-subtidal, soft-substrate assemblages during the Cambrian through Devonian. Body size of type specimens was measured along standard body axes from monographic plates. Body volume, a more direct proxy for body mass, was then estimated using an empirical regression of body volume based on known volume of taxonomically diverse invertebrate fossils spanning five orders of volumetric magnitude and a wide range of morphological and ecological forms. Results are similar using other morphometric measurements.

Body volume of individual species increases significantly from the Cambrian to the Late Ordovician, leveling thereafter. This trend is robust to various sampling treatments and when using abundance data: Species in samples, individuals in samples, and sample-wide biomass (product of species-specific abundance and body volume) all increase in body volume. The trend is also independent of sample species richness. The trend is more pronounced using abundance data, and may be associated with increasing population density. The overall range in body volume within assemblages increases as well, with an increasing maximum and decreasing minimum.

The increase in body volume coincides with the timing of the Ordovician radiation, and is linked, in part, to concomitant and protracted body-volume increases in major diversifying clades, especially rhynchonelliform brachiopods. Trilobites, linguliformeans, and non-crinoid echinoderms also display similar increases throughout the study interval, while molluscan classes only increase during the Ordovician, stabilizing, or decreasing somewhat, subsequently. The broad nature of these increases is further evidence for important changes in the ecological architecture of Lower and Middle Paleozoic communities.