60-7 Rhynchotrema dentatum and the Danger in Detail; Does it Deserve a Sub-Species?

Session: 2YC and 4YCU Geoscience Student Research Poster Showcase

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Abstract:

The Brachiopod Rhynchotrema dentatum was initially described by Hall and is relatively abundant in the Ordovician Whitewater and Arnheim formations of Ohio, Indiana, and Kentucky.  Foerste (1909) established the subspecies Rhynchotrema dentatum arnheimensis, with the differences between the primary Whitewater species and the Arnheim sub-species being larger size, lesser convexity of the valves, and frequent reflexing towards the anterolateral angles in the Arnheim. However, this distinction has not yet been validated using modern morphometric techniques. To test the validity of this distinction, specimens from both formations were collected and subjected to character based morphometric analysis. A total of 96 measurable specimens were collected from the Whitewater formation, and 139 from the Arnheim formation. Within the smaller sample pool, measurements of 18 continuous characters were taken on each specimen taken with digital calipers with an uncertainty range of +/- .01 cm and normalized by dividing each measurement by valve length. Following this, the hypothesis that the Arnheim and Whitewater specimens represent distinct subspecies was tested using a suite of analyses including a Principal Components Analysis (PCA), cluster analysis, and a Permutational Multivariate Analysis Of Variance (PERMANOVA) in the R programming environment. Although the PERMANOVA results indicate that the Whitewater and Arnheim groups are statistically distinct, only 7% of the variation could be attributed to formation. The statistical distinction between these groups does not appear to be due to differences in centroid location, but rather to differences between within group dispersion. This is shown visually in the plot of PCA results analysis which demonstrate that the Arnheim and Whitewater groups show considerable overlap in morphospace, but that the Arnheim group occupies a larger region of morphospace than the Whitewater group. Cluster analysis confirms the lack of clear separation between the Arnheim and Whitewater specimens. The greater dispersion observed within the Arnheim group as compared to the Whitewater group is attributed to the greater geographic coverage of the Arnheim sample. Arnheim specimens came from several outcrops whereas the Whitewater specimens came from a single outcrop. Based on the results of our analyses, we conclude that the Arnheim and Whitewater specimens of R. dentatum are likely not distinct subspecies and the statistical difference observed is due to the greater geographic extent of the Arnheim collection.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6896

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