Model of a Biotic Hard Substrate Community: Paleoecology of Large Trepostome Bryozoans from the Upper Ordovician (Katian) of the Cincinnati Region, USA — The Independent Study project of Kate Runciman (’22)

Editor’s Note: Independent Study (IS) at The College of Wooster is a three-course series required of every student before graduation. Earth Sciences students typically begin in the second semester of their junior years with project identification, literature review, and a thesis essentially setting out the hypotheses and parameters of the work. Most students do fieldwork or lab work to collect data, and then spend their senior years finishing extensive Senior I.S. theses. Kate Runciman (’22) was the first in our department to submit her thesis (#7 in the College!). Kate had three advisors on her project: Mark Wilson (me!), Shelley Judge, and Caroline Buttler of the National Museum Wales. The following is her thesis abstract —

The calcite skeletons of trepostome bryozoan colonies from the Upper Ordovician (Katian) of the Cincinnati region record the diverse interactions and growth responses these colonies experienced. Trepostome specimens from three Cincinnatian strata; the Bellevue Member, the Bull Fork Formation, and the Whitewater Formation, were studied within this project. These three strata were deposited in a shallow epicontinental sea environment that was located in the southern subtropics, approximately 20-23°S at the time of deposition. The focus of this project was the paleoecology of large trepostome bryozoans, which was studied by examining bryozoan growth patterns, trace fossils, and sedimentation. Microscopic examination of these features was conducted by sectioning colonies and making acetate peels and thin sections. Through this examination many trace fossils were found, with Trypanites borings being the most common. These borings often contained calcite “ghosts” and appear to have been excavated mechanically by a worm such as a sipunculan or phoronid. A subset of the observed borings prompted growth reactions in their host bryozoan, indicating that these borings progressed through a live portion of the colony. Growth reactions served to seal the cavity and regain feeding surfaces by: (1) Zooids surrounding the cavity growing upwards and angling inwards, creating a “tent” with the cavity closed off; (2) Zooids growing laterally over the cavity opening, sealing it off with a flat “roof”; or (3) Zooids budding down into the cavity then angling upwards, filling in all open space and resuming a feeding surface above. Other features observed in the trepostomes studied include calcite tubes, which are interpreted as fossil cornulitids; a tube and holdfast, interpreted as a sphenothallid; and prismatic calcite features, which are interpreted as the remains of aragonitic shells. All colonies and trace fossils included in this study were infilled with one or more of: sparry calcite cement, dolomite rhombs, biosparite, micrite, prismatic calcite, and phosphate. This range of infilling materials suggests that infilling processes were episodic. The episodic nature of these processes allowed for the preservation of ghosts and occasionally geopetal structures. Internal surfaces were observed that indicated regions of self-overgrowth in the colony. These self-overgrowths were commonly associated with brown bodies. Work continues to combine insights provided by the trace fossils, growth responses, and infill observed in the Cincinnatian trepostomes to interpret the ecology and life modes of these bryozoans.

The image at the top is a thin-section longitudinal view of one of Kate’s bryozoans. It shows a bioclaustration of a soft-bodied encruster, most likely a sphenothallid. Location C/W-152.

This longitudinally-cut thin-section shows bryozoan zooecia with terminal diaphragms below a discontinuity and self-overgrowth. The terminating chambers contain remnants of the original soft-bodied bryozoan zooids (or polypides) known as “brown bodies“. Location C/W-153 (Bull Fork Formation near Maysville, Mason County, Kentucky; 38º 35.111′ N, 083º 42.094′ W).

And here’s Kate with an especially large trepostome bryozoan colony. Congratulations on a successful project!

About Mark Wilson

Mark Wilson is a Professor of Geology at The College of Wooster. He specializes in invertebrate paleontology, carbonate sedimentology, and stratigraphy. He also is an expert on pseudoscience, especially creationism.
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