Qualitative observations on the mactrid bivalve Mulinia suggest this genus often varies notably in morphology among fossil sites and across present day localities. However, there are no cited attempts to quantify the magnitude of that variability. To decipher the role of spatio-temporal and environmental gradients, the morphospace of Mulinia was evaluated using 202 samples of M. congesta from 17 fossil localities along the latitudinal gradient from Virginia to Florida. Materials were derived from bulk samples in museum and dissertation collections. All samples were chosen from Pliocene formations that are roughly co-eval temporally to the Yorktown Formation, Virginia. We also analyzed 76 specimens of M. coloradoensis (Gulf of California, Recent) and M. lateralis (Florida, Pleistocene) for purpose of comparison.

Each valve was digitally imaged and x-y coordinates for fifteen landmarks and pseudolandmarks were acquired. Procrustes method was used to simultaneously fit the landmark points and derive shape coordinates that are invariant in respect to variations in size, rotation, and position of specimens. Tangent coordinates were used to derive principle component ordination to compare the overall shape differences between shells from different regions, and size was estimated using shell length and centroid size.

When samples are grouped by geography, the morphospace occupied by M. congesta varies significantly among the regions. Whereas geography affects Mulinia morphospace, there is no apparent latitudinal trend. Relative comparison of the environmental versus geographic gradients shows that within a geographic locality variation remains low, even as environmental conditions change over time. In contrast, geographic variation among samples collected from temporally and environmentally comparable units is much larger. Finally, the amount of variation seen between geographic regions is comparable to the amount of variation seen between the three Mulinia species. This study suggests that geographic variability in morphology of Mulinia exceeds notably environmental variability and may be comparable in level to interspecific variability within the genus. Geographic gradients may thus exert influence on morphology that is comparable to morphological effects of speciation.