Traces of missing encrusters: borings reveal sclerobiont taphonomy in the Upper Ordovician (Katian) of the Cincinnati region, USA

ABSTRACT

The abundant shells and hardgrounds in the Cincinnatian Group (Upper Ordovician, Katian) of the upper midwestern United States were commonly encrusted and bored by a variety of organisms. Numerous studies of these sclerobiont communities have provided valuable data for models of ecological succession, symbiosis, space and food resource competition, and taphonomy. An underlying assumption of this work is that most of the skeletal encrusters have remained in place, firmly attached to their hard substrates. This is especially the case with the most common encrusters, thick trepostome bryozoan and cystoporate skeletons, on their most common substrates, flat strophomenide brachiopods. We present evidence here, though, that these bryozoans were often dislodged from their brachiopod hosts, leaving no evidence of their attachment other than horizontal borings in semi-relief from organisms that excavated tunnels (Trypanites and Palaeosabella) at the interface of the brachiopod shell and attaching bryozoan. Similar borings are found on the bases of dislodged bryozoans and in bryoimmured mollusc external moulds. These borings along the bryozoan attachment surfaces caution us that there are significant numbers of missing skeletal encrusters on these hard substrates.

KEYWORDS:

• Ordovician
• sclerobionts
• taphonomy
• encrusters
• bryozoans
• borings

Acknowledgments

College of Wooster students Russell Kohrs and Coleman Fitch helped collect some of the specimens used in this study, as did College of Wooster geological technician Nick Wiesenberg. Dan Cox and Andrew Trimby (Amgueddfa Cymru) prepared the thin sections. We thank Andrej Ernst, Michał Zatoń, and an anonymous reviewer for their insightful comments and corrections to the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).