Paper No. 6
Presentation Time: 3:20 PM
THROWING GENICULATION A CURVE. A QUANTITATIVE APPROACH TO SHELL FORM IN RAFINESQUINA ALTERNATA
The abundant Upper Ordovician brachiopod, Rafinesquina alternata, is highly variable in shape, and it has been suggested that this variation correlates with attributes of the brachiopod's environment, such as wave energy, or fouling potential. It may also inform us about the brachiopod's life orientation. In particular, geniculation rate, the incidence of shells with a pronounced change in curvature during ontogeny, has been suggested as either a correlate of shallow or deep water by various authors using different methodologies. This study examines shell curvature quantitatively to determine if arcuate and geniculate forms are indeed distinct and whether shell curvature correlates with facies. R. alternata collections from limestone-dominated, shale-dominated, and mixed facies were collected along the Cincinnatian Arch in Kentucky, Ohio, and Indiana. A computer program written to analyze changes in shell curvature was used to determine the point and magnitude of maximum change and differences in curvature before and after the maximum point. Preliminary results of fifty specimens from mixed limestone-shale facies, Brooksville North, found no evidence for a bimodal distribution of maximum curvature, as would be expected if geniculation were real. All R. alternata have a maximum change in curvature 2-3 cm from the umbo, and have less inflated curvature after this point. These observations suggest that R. alternata, after reaching 2-3 cm in size, change growth strategies. This change may represent a decrease in growth rate, perhaps due to the onset of reproduction. We are testing this hypothesis by examining growth increments within the shell.