INTERPRETING ENCRUSTATION DESPITE TAPHONOMIC OVERPRINT: EPIZOANS ON BRACHIOPODS FROM THE DEVONIAN CERRO GORDO MEMBER, LIME CREEK SHALE, ROCKFORD, IA

Fossil encrusting organisms are useful for examining in-situ relationships in the fossil record.  However, when the host life status at the time of encrustation is indeterminate and/or when taphonomic bias is suspected to decrease the preserved encrusting community, then analysis of relationships becomes problematic.  Fortunately, remaining data can be useful for determining host taphonomy at time of encrustation, preferred neighboring encrusters, and potential larval preferences of settling encrusting organisms.

In the uppermost beds of the Cerro Gordo shale at Rockford, Iowa, encrusting organisms were present on nearly half of the brachiopods collected.  Encrusted brachiopods bore approximately 1.7 epizoans per specimen; this total, however, is largely an artifact of highly abundant Theodossia spiriferide brachiopods.  Theodossia is 50% of total sampled brachiopod specimens; almost 85% of total encrusted brachiopods are Theodossia, and of Theodossia individuals alone, nearly 85% are encrusted.

Hederella is rare, often only a trace on the brachiopod shell, and Ascodictyon is absent in this study, although traces of Ascodictyon are noted on a few specimens from other Rockford collections.  Craniid brachiopods also leave only traces in host brachiopod ornament.  These data suggest a taphonomic bias against preservation of epibionts in this locality.  In addition, a radiating dendritic attachment scar was seen on many host brachiopods, but the majority of hosts were abraded and disarticulated, suggesting post-mortem encrustation of these specimens.

Hederella rarely co-occurred with congenerics and conspecifics, in general avoiding all other hederellids.  Other epizoans did not show strong preferences for neighbors, and apparently were tolerant of conspecific and other encrusters.

Based on presence-absence data of encrustation of brachiopod taxa, encrusting organisms largely avoided highly plicate brachiopods, preferring those with finer surface texture (mainly Theodossia and Schizophoria) and those with spines (particularly Spinatrypa). Because choice of substrate must be made at metamorphosis of the encrusting organism, and based on the fact that many Modern encrusting organisms have some, albeit limited, control over settlement, these data may reflect larval substrate preferences.