Bigger and Deeper: Discordant Size Trends in Epifaunal and Infaunal Jurassic Bivalves

Secular trends in size are vital to our understanding of long-term patterns of biomass, predation, and ecosystem structure. Nevertheless, marine invertebrates, even those with an excellent fossil record, lack a detailed temporal framework for size trends. Preliminary analyses of a database of individual volume measurements of Jurassic bivalves demonstrate consistent differences in size trends between epifaunal and infaunal genera. The maximum size of infaunal bivalves increases by two orders of magnitude between the Pliensbachian and Callovian stages. During the same interval epifaunal bivalves decrease in maximum size by at least an order of magnitude. Size-frequency distributions (SFDs) of individuals within genera also indicate a subtle, yet consistent discordance. Skewness coefficients for infaunal genera are normally-distributed in the early Jurassic and become more left-skewed by late Jurassic. This shift toward more left-skewed SFDs occurs in two phases: between Early and Middle Jurassic for shallow infaunal genera, and between Middle and Late Jurassic for deep infaunal genera. Conversely, skewness coefficients of epifaunal genera increase in variance through the interval but remain, on average, normally-distributed.

The Jurassic is an interval of profound evolutionary and ecological diversification in the marine realm. Particularly well-documented is the increased infaunalization of the bivalve fauna perhaps in response to the incipient increase in durophagous predation during the Mesozoic Marine Revolution. A change in shell-volume as a result of this ecological restructuring would not be entirely unexpected, but this has never been explicitly tested. If these results withstand further scrutiny and data collection, the discordant size patterns between infaunal and epifaunal bivalve genera could be a heretofore unrecognized consequence of the Mesozoic Marine Revolution, one of the major events in the history of life.