INTERPRETATION OF A DEVONIAN ENCRUSTED BRACHIOPOD ASSEMBLAGE: SOLON MEMBER, CEDAR VALLEY FORMATION, IOWA

Encrusting organisms offer a unique perspective in the fossil record because of the preservation of spatial and competitive relationships. Encrusters are preserved in life position, allowing interpretation of ecological interactions between encrusters and their hosts. However, techniques for sampling encrusted assemblages vary between fine, specific samples that are collected bed by bed, to coarse samples that can encompass an entire outcrop. Fine-scale sampling facilitates in-depth interpretation of ecological patterns. Encrusted brachiopods were collected from the Solon Member of the Cedar Valley Formation at Robins Quarry in Iowa. Four fine-scale samples were collected from single horizons within a 20 cm stratigraphic interval and a 10 square meter area. Several coarse samples were collected from the same area for comparison of general trends.

Both sampling techniques show the same general trend for encruster abundance and valve preference. However, the abundance of brachiopod taxa differs and the coarse samples do not show the average of the ecological patterns observed in the fine-scale samples. Between the fine-scale samples, variations occur in the proportion of encrusted brachiopods and the amount of encrustation within each taxon. Encrustation gradually decreases through time, with the stratigraphically lowest sample having 38% of the brachiopods being encrusted, and the youngest sample showing 24% encrustation. The most common taxa of brachiopods were Independatrypa, Pseudatrypa, and Seratrypa. The most common encruster was Spirorbis (53-71% of the observed encrusters) followed by an encrusting bryozoan (14-23%) and Hederella (8-14%). Brachiopod diversity remains relatively stable through time; however the number of encrusting genera more than doubles from the lowest stratigraphic sample to the third sample. The fourth and youngest sample decreases to an average diversity. Competition among encrusters could account for the trends observed.