A GIS ANALYSIS OF THE DISTRIBUTION OF EPIZOANS ON MUCROSPIRIFER THEDFORDENSIS (BRACHIOPODA, SPIRIFERIDA) FROM THE MIDDLE DEVONIAN OF ONTARIO

Epizoans are animals that grow on other organisms for a variety of reasons. In marine settings with muddy substrates, it is common to find larger benthic animals encrusted with small filter feeding animals such as bryozoans and corals. These animals grow on the shells for a variety of reasons including: to maximize their distance from anoxic or turbid water near the sediment-water interface (SWI); to have a hard substrate to anchor themselves upon; and/or to augment their filter feeding with the currents created by the animals they grow on. The distribution of these epizoans on the host animal can give important clues as to which of these behaviors is most critical.

Brachiopods were common hosts to epizoans during the Paleozoic. The abundant Middle Devonian brachiopod Mucrospirifer thedfordensis was used in this study. A total of 205 specimens from the Widder Formation (Hamilton Group) of Thedford Ontario were analyzed. The most common epizoans were the cyclostomate bryozoan Hederella, the tabulate coral Aulopora, and the problematic “Spirorbis” (a microconchian, which may represent a worm).

GIS analysis (ArcGIS) was used to determine the density distributions of these epizoans. A lat-long grid was created over an image of a Mucrospirifer brachial valve. Polygons were created for the location of each epizoan and where polygons overlapped they would be added to produce density maps superimposed on the brachiopod shell and lat-long system.

Results indicate that Hederalla (n=105) positioned itself preferentially near the highest point on the Mucrospirifer shell. No specimens were recorded near the shell opening or at lower points on the shell opposite the opening. It is clear that Hederella was located in a manner that maximized its height above the SWI for either oxygen or turbidity reasons. “Spirorbis” (n=87) was more evenly distributed across the brachial valve, but had a clear preference for the area near the commissure, suggesting that currents created by the brachiopod were an important factor in their positioning. Finally, Aulopora specimens (n=11) have a more complicated distribution suggesting that colonies may have established near the commissure, but then expanded to a more random distribution. Since “Spirorbis” and Hederella preferred different locations on the shell, they commonly occur together (n=31).