SIZE AND SHAPE VARIATION IN HEBERTELLA (ORTHIDA, BRACHIOPODA) FROM THE RICHMOND GROUP OF THE CINCINNATI ARCH (UPPER ORDOVICIAN)

Understanding morphological variation is vital to a deeper knowledge of how evolution works, as well as having practical applications in other fields of inquiry, e.g. stratigraphy. Brachiopods, which have a relatively simple morphology and are often preserved complete or nearly complete, are particularly useful for studies of size and shape variation. With this in mind, stratigraphically-ordered, bulk sampled collections of the orthidine brachiopod Hebertella from the upper Ordovician Richmond Group of Southeast Indiana were made, with the majority of samples from the Liberty Formation. Hebertella is a large, abundant, easily identified brachiopod with a stratigraphic range spanning most of the Cincinnatian. Morphology was analyzed using distance measures on twelve characters and a combination of univariate, bivariate, and multivariate methods.

Size distributions for Hebertella samples change dramatically over the study interval, and more subtle changes in shape are also present. The stratigraphic pattern of size and shape changes is consistent over a large geographic area. These changes divide the Hebertella samples into four stratigraphic intervals: 1) samples from the Glyptorthis epibole, 2) a so-called “large” interval when Hebertella is the sole orthid, 3) an interval of co-occurrence with Plaesiomys, and 4) samples from the Upper Whitewater. The results show that the largest size changes are not associated with the largest shape changes, probably a result of the relative linearity of orthid ontogeny.

Despite large changes in maximum size between the intervals, small specimens are present in all samples, and most intervals span a range of lithologies and taphonomic regimes, suggesting that these changes reflect an underlying biological reality rather than simply being taphonomic artifacts. While the observed stratigraphic pattern of size and shape change seems strongly correlated with the presence/absence of other orthid brachiopods, it is not clear that this is directly responsible. This study adds to our knowledge of the dynamics of size and shape variation in brachiopods, and also provides an added tool for correlation of widely spaced outcrops.