MAPPING A SMALL DEVONIAN WORLD: A GIS ANALYSIS OF THE DISTRIBUTION OF EPIZOANS ON BRACHIOPOD SHELLS

Geographic Information Systems (GIS) are typically used to analyze spatial data on a large scale. This study uses GIS to examine the origin, growth, and distribution of a variety of epizoans on 609 specimens of the Middle Devonian spririferid brachiopod Mucrospirifer thedfordensis in order to better understand their paleoecology. Methodology and results are compared with a study of the coeval but larger Paraspirifer (Kesling et al., 1980).

Epizoan locations were observed under a microscope, plotted on a base image of a Mucrospirifer shell, and then digitized in ArcGIS for spatial analysis. Epizoan point distributions were analyzed using quadrat analysis and kernel density maps were produced. Visualizations created by aggregating points within regular spatial sectors allowed comparison with Kesling et al. (1980). Epizoan polygon density was mapped by rasterizing the count of overlapping polygons. To determine the growth direction of colonies, polygon centroids were compared to origin points.

The distribution of fossil epizoans on mature brachiopod hosts are the result of a complex array of stochastic and intrinsic factors. Epizoans may be randomly distributed or prefer to live along the commissure (to gain assistance from or compete with the filter-feeding of the host) or at the highest point on the shell (to avoid the sediment-water interface, maximize current access, or avoid competition with the host). In addition, the ability (if any) of the epizoan larvae to choose a location on the shell, the relative survivorship and/or vitality of the individual or colony, and the size and orientation of the host as well as its time of death relative to epizoan colonization all play roles. GIS is the ideal methodology for testing hypotheses about these complex systems.

The results of the GIS analysis indicate that the tabulate Aulopora preferred originating and growing along the commissure of the host. Crinoid holdfasts were also most common near the commissure. The problematic coiled “Spirorbis” was widely distributed with a slight preference for the fold and plications near the commissure. The most common epizoan, the bryozoan Hederella, had the most complex distribution with origins most common along the fold, maximum colony density on the central part of the brachial valve, and a preferred growth direction toward that area.