FINE-SCALE VERSUS COARSE SAMPLING EFFECTS ON DEVONIAN ENCRUSTED BRACHIOPOD ASSEMBLAGES: CEDAR VALLEY FORMATION, IOWA

Epibionts directly preserve ecological data in the fossil record such as spatial and competitive relationships. However, samples are often collected at different stratigraphic scales, from fine, bed-by-bed sampling to coarse, entire-outcrop samples. To investigate this difference, Givetian brachiopods were collected from the Cedar Valley Formation at Robins Quarry in Iowa. Four fine-scale samples were taken from different single horizons within a 20cm stratigraphic interval. A fifth coarse sample was at some distance from the first four, encompassed approximately a 2m stratigraphic interval, nesting the fine-scale interval.

The fine-scale samples contain a gradual decrease through time in the proportion of encrusted brachiopods. Encrusted brachiopods initially account for 39% of the stratigraphically lowest sample, but as time progresses, the percent drops to 26. The most common brachiopod taxa in the fine-scale samples are Independatrypa (44-48% of total brachiopod assemblage; 33-59% encrusted); Pseudoatrypa (13-22%; 7-13% encrusted); and Seratrypa (13-20%; 15-34% encrusted). Fine-scale samples vary in the number of encrusting organisms or colonies per host (average 1.3 to 2.4 encrusters per encrusted brachiopod), but host preferences and encruster taxon abundances are similar between samples. The most common encrusters are Spirorbis (53-71% of the total epibiont assemblage), encrusting bryozoan (14-23%), and Hederella (8-14%).

Encruster taxon abundances between the fine-scale samples and the coarse sample are generally similar. Instead, the coarse sample varies notably from the fine-scale sample in a higher proportion of encrusted brachiopods (46%) and the high abundance of Schizophoria and Spinatrypa, which are rare to absent in the fine-scale samples. Independatrypa (25% of the assemblage; 51% encrusted) remains the most abundant brachiopod, although much less common than in any of the fine-scale samples, followed by Schizophoria (24%; 69% encrusted) and Spinatrypa (16%; 56% encrusted). Although the larger, coarse-scale sample does retain general abundance trends in encrusting organisms, proportions of brachiopod taxa differ greatly and encruster relationships with potential substrates, and ecological-scale patterns are not averaged in the coarse sample.