Stratigraphic, Sedimentary, and Paleontological Topics
In attempting to name species of Oepikina and Strophomena in a survey of Middle Ordovician faunas (Mifflin Member of the Platteville Formation) in Wisconsin, Illinois, Iowa, and Minnesota, we found that supposedly diagnostic characters of the various species, particularly characters describing the profile of the valves, appeared to intergrade, and that combinations of characters that were thought to have been consistently associated appeared sometimes to vary independently. We postulated that most of the Mifflin strophomenids represented species populations of either Strophomena plattinensis Fenton or of Oepikina minnesotensis (Winchell), and that each of these species displayed a certain amount of variability, probably environmentally controlled, in form. We investigated patterns of morphological variability within and among local populations in an attempt to map gradients of variability and to determine their probable relation to paleoecological parameters.
For pedicle and brachial valves of each species, a principal components analysis was performed on measurements of size, shape, inflation, and symmetry. The first principal component accounted for at least 40 percent of the variation in each of the four correlation matrices, and the first two together for more than 60 percent of the variation. Inflation characters loaded strongly on the first axis, whereas symmetry characters had the highest loading on the second axis.
Clinal maps, prepared for each valve type by plotting the mean values of one inflation character and one symmetry character in each of 16 sampling areas, showed isophene patterns that generally paralleled probable topographic features of the shallow sea floor. The inferred nearshore populations were generally flatter and less symmetrical than those from deeper water. Examination of patterns of intrapopulation variability showed that the level of variability, especially for the inflation character, was greater in the onshore populations than in the offshore ones. Greater morphological variability in shallow-water populations could reflect adaptation to increased levels of spatial heterogeneity, most likely resulting from nearshore substratum patchiness.