DILUTION, DECLINE, OR DIFFERENT ENERGETIC STRUCTURE? BODY SIZE, TROPHIC LEVEL, AND THE ECOLOGICAL ROLE OF POST-CAMBRIAN TRILOBITES

The Post-Cambrian ecological history of trilobites has been a contentious subject. Though framed in community paleoecological terms, the debate has largely been argued with compilations of diversity data. The ecological meaning of such data is difficult to interpret -numerous modern examples show that the relationship between abundance, biomass and diversity can vary considerably even within a single clade. Body size and trophic level are two related factors which have been shown to strongly affect the nature of this relationship. Trilobite body sizes spanned six orders of magnitude and they are thought to have occupied at least 2 trophic levels, and both of these variables should be accounted for when examining the ecological trajectory of trilobites. We analyze a database of >400 whole-macrofauna collections with taxon counts from the Ordovician of Laurentia. Collections are restricted to mixed carbonate-clastic depositional systems, though they represent a variety of tectonic environments. Body size estimates were obtained through direct measurement of individuals in our collections and in museum collections. We used a recently proposed model based primarily on hypostome position to assign genera to 2 broad trophic groups: particle/detritus feeders and predators/scavengers. While the relative abundance of trilobites declines sharply between the Lower and Late Ordovician, average body size increases over the same interval. Thus, the energetic significance of trilobites as a whole is roughly constant through this interval and is decoupled from the decline in abundance. This stasis masks considerable trophic turnover, however: genera interpreted to be predators/scavengers show a slight increase in average biomass in most environments, while typically smaller and more numerous particle/detritus feeders are progressively restricted to offshore and slope environments. We propose that this reflects differential responses to the diversification of filter and suspension-feeding taxa in onshore environments, which indirectly restricted food availability to particle/detritus-feeding trilobites while increasing opportunities for predators and scavengers. This model has implications for observed trends in trilobite morphological diversity, community structure, and taxonomic turnover.