Description and interpretation ofSanctum laurentiensis, new ichnogenus and ichnospecies, a domichnium mined into Late Ordovician (Cincinnatian) ramose bryozoan colonies

Abstract

During the Late Ordovician, arborescent and frondose trepostome, and cystoporate Bryozoa were frequently bored in a manner distinctly different fromTrypanites, Palaeosabella, andVermiforichnus, domichnia that are commonly associated with these organisms. Maysvillian and Richmondian bryozoan taxa on the Cincinnati Arch were particularly infested by an unidentified organism that used the interior of bryozoan branches as its domicile. The domichnial traceSanctum laurentiensisis newly described. Dwelling openings range in size from 1.1 mm to 3.2 mm and are located singly on surfaces of colony branches, commonly in a somewhat protected position. The circular opening leads through the exozone into an elongate or saccate chamber representing a variably shaped excavation of the bryozoan endozone. Traces ranging from 3.0 mm to 8.8 mm wide and 9.7 mm to 53 mm long are documented. Thin sections demonstrate that chambers were unlined and had irregular interior walls resulting from organismal mining of zooecial tubes.Cavity makers likely were multiple individuals of amphipod-like crustaceans (Arthropoda) that fed outside their domicile. Bryozoan colonies were occupied while upright, either entirely or partially live, or in some cases dead and overgrown by other bryozoans. Avoidance of predation and the ability to dwell and feed in a higher tier than that of infaunal tracemakers at the sediment-water interface were potential benefits of this domichnium. Presence of cavities reduced the strength of host branches, thus having a profound effect on colony morphology and growth over its lifetime. Unlike the many epizoans that used trepostomes as substrates with little long-term affect,Sanctum laurentiensissignificantly impacted its bryozoan host.