COMPARISON OF BRACHIOPOD GENERA FROM THE SILURIAN ROCKWOOD FORMATION IN TIFTONIA, HAMILTON COUNTY, TN

The Silurian Rockwood Formation exposed in Tiftonia, Tennessee has a remarkably rich invertebrate fauna as compared to other Rockwood outcrops in the region. At this locality near Chattanooga, there is an abundance of fossils representing diverse ecosystem preserved in < 15 m of subtidal, storm-dominated strata deposited in a foreland-basin setting. The community includes brachiopods, tabulate and solitary rugose corals, bryozoans, echinoderms (represented by crinoid/blastoid columnals), trilobites, trace fossils, gastropods, bivalves and orthocone nautiloids.

Here, we present a more detailed and quantitative description of a subset of the brachiopod population. We determine a functional morphospace (measuring length, width and thickness) of the 9 brachiopod genera that are statistically relevant and identify the narrow stratigraphic horizons of 3 brachiopod genera within the Tiftonia strata. Plots are made of length vs. width to correlate individual taxa within the overall brachiopod population. A linear trend was observed from the collected data that enables us to identify the functional morphospace for the local brachiopod community. The ratio of length to width never exceeded 1.0, with one genus, Dalmanella, reaching a ratio of 0.941. The average for the 262 specimens used in the study was 0.801, indicating that all genera of brachiopods present at Tiftonia have a greater width than length. This allows us to describe what is the most advantageous functional morphospace for the local population.

Several genera are positively correlated to horizon or substrate type. Leptaena is usually found in the shale units, while Eoplectodonta and Spirigerina are usually part of reestablished communities on top of storm beds. The uppermost portion of the outcrop is also where the majority of the rugose corals are found, indicating either a change in grain size and/or water depth.