VARIABILITY BETWEEN COMMUNITIES OF ENCRUSTING ORGANISMS: GIVETIAN, MICHIGAN AND OHIO, USA

Encrusting organisms are important recorders of ecosystem dynamics in the fossil record, such as spatial interactions and biodiversity. In the Middle Devonian paleoecosystems of the Michigan Basin and the adjacent region, epizoans were common members of many paleoecosystems, frequently encrusting large brachiopods. This study investigates the variability of encrusters on “stropheodontoid” brachiopods from localities in the Silica and Ferron Point Formations. In general, encrustation rates were high at all localities, often exceeding 75% of large(shell width > 2 cm) brachiopod specimens encrusted. Most brachiopods were articulated but many were abraded and showed other signs of wear, suggesting time-averaging. Ventral (pedicle) valves were encrusted more frequently than dorsal (brachial) valves at all localities. At a few localities, several bryozoans and Spirorbis encrusted brachial valves more frequently than would be expected under a null model based on the distribution of all encrusters; however, this pattern of preferred brachial valve encrustation does not hold up when compared across localities. In most encruster communities, colonies of bryozoans and corals outnumbered solitary organisms, but where solitary organisms were dominant, Spirorbis was the most abundant community member. Several hederellids were consistently abundant encrusters at all localities, whereas other common encrusters such as Aulopora, Spirorbis, craniid brachiopods, and several bryozoans vary greatly in abundance between samples.

Bryozoans typically co-occurred with conspecifics, with bryozoans of other encrusting morphologies (e.g., hederellids with sheet-like encrusters), or with non-bryozoans, but often avoided bryozoans of similar morphology. Spirorbis, Aulopora, and craniid brachiopods also were often found in the company of other encrusters and commonly occurred with con-specifics.

These results suggest that, although a few general patterns can be recovered (such as preferential encrustation of pedicle valves), no single locality or sample can be used to represent the patterns of encrustation for a given stratigraphic unit; assessment of lateral variation at all scales is essential.