AbstractWe examine the morphological variation of a Paleozoic pterineid during a time of relative ecological and taxonomic stability in the Middle Devonian Appalachian Basin in central and eastern New York. We discuss the taxonomic status of the Middle Devonian bivalveActinopteria boydi(Conrad, 1842) and quantify the variability of its shell disk as well as the width and angle of the auricles and sulci of this otherwise character-poor bivalve species using geometric morphometric techniques employing Cartesian landmarks. We compare variants from three stratigraphic levels (Skaneateles, Ludlowville, and Moscow formations) and from different habitats characterized by lithofacies.The phenotypic variation observed in our data does not amount to an overall directional shift in morphology, i.e., they constitute reversible changes of morphology in a single variable taxon. Our study finds small-scale variation in morphology that represents evidence for ecophenotypic variation through ~3–4 Myr. Differences in substrate coupled with water energy seem to impact this taxon’s morphology. Although no clearly separated groups can be observed, material from muddy facies develops variants with, on average, rounder and broader shell disks than are found in material from silty facies. This morphology could have increased the flow rate of water channeled over the posterior shell portion thereby improving filtration rate, which is especially beneficial in environments with low water energy.