BORING MORPHOLOGY OF A NEWLY DESCRIBED DOMICHNIUM IN ORDOVICIAN, INSITU, BRANCHING BRYOZOAN COLONIES

During the Late Ordovician, arborescent and frondose trepostomate, and cystoporate Bryozoa were frequently bored in a manner distinctly different from the several domichnia previously associated with these organisms. Maysvillian taxa on the Cincinnati Arch were particularly infested by an unidentified organism that used the interior of bryozoan branches as its domicile. In the substantial literature of Paleozoic borings these traces may have been grouped with species of Vermiforichnus and Trypanites in some works due to their single opening, but they are quite distinctive from these genera internally.

Dwelling openings range in size at least from 1.1 mm to 3.2 mm and are located singly on colony branch surfaces, often in somewhat protected positions. The circular opening leads through skeletal exozone into an elongate or saccate chamber which occupies an excavated portion of the endozone of a bryozoan host skeleton. Chambers, or cavities, were variable in shape dependent upon size and shape of the endozone of the bryozoan being occupied. Traces ranging from 3.0 mm to 8.8 mm wide and 9.7 mm to 53 mm long have been documented. Larger sizes are probable because a majority of the bryozoan samples we have studied have broken branches that truncate the traces. Thin sections demonstrate that chambers were unlined and had irregular interior walls resulting entirely from organismal mining of zooecia.

Cavity makers likely were annelids or arthropods. The single opening suggests that feeding was done outside the chamber either by extending a filtration/food-capture organ or by the entire organism emerging to browse, predate or filter feed. In the latter case there may have been more than one organism dwelling in a cavity. Bryozoan colonies were occupied while upright, either living or at least partly dead, and often overgrown by other bryozoans. Borers likely saw two benefits, protection from predation and ability to feed in a higher tier where competition was reduced. Presence of cavities reduced the strength of branches in most instances, thus having a profound effect on colony morphology and growth. Unlike the many epizoans that used trepostomates as substrates with little long-term effect, this newly-recognized trace maker significantly impacted bryozoan ecology.