Using Individually Dated Bivalve and Brachiopod Shells to Reconstruct Epibiont Colonization and Taphonomy Over Millennial Timescales

Taphonomic deployment experiments provide insight to encrustation over months or years, but a detailed understanding of epibiont colonization patterns and fossil preservation over centuries to millennia require temporal control from dating techniques. Individual shells dated by radiocarbon-calibrated amino-acid racemization rates provide this temporal control for taphonomic and paleoecological studies. Specimens of Semele and Bouchardia collected from 10 and 30 meters depth in Ubatuba Bay (SP, Brazil) provide age, depth, size and substrate control for the evaluation of encrustation on a shallow, subtropical siliciclastic shelf over the past 8000 years. Infaunal bivalves are less frequently encrusted than epifaunal brachiopods, but 33% of bivalves at 30 m and 89% of bivalves from 10 m were exposed to epibiont colonization at the surface, requiring posthumous exhumation. Encrustation of both bivalve and brachiopod shells are largely similar as a function of size, with colonization by serpulids and cheilostome bryozoans, foraminifera, barnacles, and other taxa. Host valve surface area does not change significantly over time, and both bivalves and brachiopods possess similar valve surface areas at each site. Site-specific factors such as depth, primary productivity, or bioturbation rates must be invoked to produce observed differences in encrustation between sites after specimens are limited to the same size or age range. There is no evidence of long term epibiont accretion on older valves based on abundance and richness trends over time, although an increase in heavily colonized shells in the past 500 years may be the result of coastal eutrophication. As the composition of epibiont faunas is volatile over shorter timescales, the perils and virtues of time-averaging which govern their hosts apply to them as well.