COLONISTS OF A "LOST WORLD": QUANTITATIVE ANALYSIS OF BRACHIOPOD ENCRUSTATION ON THE SUBTROPICAL SHELF OF THE SOUTHEAST BRAZILIAN BIGHT

The continental shelf of the Southeast Brazilian Bight harbors a benthic fauna akin to "The Lost World" of Arthur Conan Doyle's classic story, home to an abundance of creatures which once dominated the faunas of a bygone era: in this case, articulate brachiopods. Quantitative analysis of the encrusting fauna of these brachiopods allows for direct comparison with modern bivalves, while comparison with analogous fossil communities can be used assess the development of encrustation through evolutionary time.

Encrustation frequencies for each brachiopod species collected from the outer shelf vary across an order of magnitude, despite similarities in both valve and sample size, and correspond primarily to brachiopod valve ornamentation. Mean encrustation frequencies for the dominant species Bouchardia rosea increase from outer shelf values of 2% to 85% per site on the inner shelf. However, we observed smaller-scale, site-to-site variations in encrustation within both inner and outer shelf sites which do not correlate directly with depth or geography. This implies that at finer scales, encrustation is controlled by other factors (potentially productivity).

The encrusting fauna of the inner shelf is similar to those reported for modern bivalves, dominated by bryozoans and calcareous worm tubes (serpulids and spirorbids), with minor and variable roles played by bivalves, barnacles, foraminifera, algae and other taxa. Encrustation is primarily post-mortem and consistent with cryptic habits, as valve interiors are preferentially encrusted. Even at coarse taxonomic resolution, diversity increases logarithmically with valve area, confirming suggestions that epizoans follow ecological principles akin to the island-area effect. Patterns and frequencies of encrustation are similar to those of Paleozoic brachiopods despite radical change in epizoan faunas, and imply the presence of similar ecological controls on encrusting taxa through much of the Phanerozoic.