Behavioral stereotypy and some ecological consequences of entrance-shaft placement of the domichnium Sanctum laurentiensis in Ordovician trepostomate Bryozoa

Abstract

The ichnofossil Sanctum laurentiensis from the Late Ordovician (Cincinnatian) of Laurentia is a domichnium tunneled into live colonies of ramose and frondose trepostome Bryozoa. A preferred entrance location was chosen by the trace-making endoskeletozoan. The organism chose to tunnel in positions near growing tips of branches where one of two conditions prevailed. At stereotypical sites, here considered “determinate”, domicile entrances were made where a trepostome branch was in the growth process of bifurcation; sites considered “indeterminate” were chosen where one blade or branch was impinging on another, or on some other nearby substrate, thus interfering with normal colony growth in a random, or non-ontogenetic, manner. This stereotypy is a rare example of demonstrable entrance-siting behavior by an unknown invertebrate organism. A skeleto-structural condition was present at both determinate and indeterminate locations on ramose trepostome colonies. Stereotypical entrance-site choices included a location where bryozoan freewall membrane was compromised or thinned and exozone was incompletely developed. Sanctum producers preferred incomplete exozone in order to access the endozone for excavation to form a domichnium. Middle and Late Ordovician diversification of ramose bryozoan forms stimulated escalation of bioeroder and symbiont taxa in living zoaria thus encouraging further trepostome evolution.

Keywords:

• Stereotypical behavior
• domichnium entrance placement
• Sanctum laurentiensis
• Ordovician bryozoan endoskeletozoans
• trepostome diversification
• escalation

Acknowledgments

The St. Lawrence University Geology Department stimulated this research during the course of twenty-five field excursions in the Cincinnati, Ohio, region between 1981 and 2011. R. Trammel, D. Meyer, the Trammel Fossil Park of the City of Sharonville, Ohio, and the Dry Dredgers who help maintain it, have advanced this work by creating a paleontology-friendly location for teaching and research. St. Lawrence University students of Invertebrate Paleontology are thanked for their assiduous collecting of bryozoan material over the years. D. Waugh stimulated discussion of zoarial morphologies and trace impacts. T. Bouchard prepared thin sections of S. laurentiensis and made some of the images used herein. R. Cuffey discussed bryozoan diaphragms with the author on multiple occasions. D. Waugh, T. Smrecak and M. Burton-Kelly assisted with collection of material. Preliminary versions of this paper were reviewed by D. A. Waugh whose critique was helpful in clarifying the relationships documented here. The final document was improved by the careful attention of reviewers O. Vinn and M. Wisshak and the co-editors M. Gingras and L. Buatois for Ichnos.

Disclosure statement

No potential conflict of interest was reported by the authors.