UC Riverside

Pygidial shape variation of the trilobite morphospecies Dikelocephalus minnesotensis (late Cambrian, northern Mississippi Valley) is assessed using landmark-based geometric morphometrics including the use of semilandmarks. Relative warp analyses of pygidial shape show that although pygidial morphospace occupation by D. minnesotensis from the St. Lawrence Formation shows some regionalization by collection, variance within collections is notable and overlap in shape occurs among almost all collections. Morphological characters that dominate the structure of pygidial shape variation include the base of the posterolateral spine, proportions of the border, and form of the pleural ribs. Between collections and through time, a continuous gradient of morphotypes suggests a mosaic pattern of variation. Ontogenetic variation plays little role in shape variance both within individual collections and in the sample as a whole. The designation of St. Lawrence Formation D. minnesotensis as a single, highly variable morphospecies stands when analyzed with landmark (and semilandmark) based geometric morphometrics. These results support the view that St. Lawrence Formation Dikelocephalus were highly variable in form, and this might relate to environmental variability in this nearshore setting. More generally, results may also exemplify Rosa’s rule of elevated morphological variation early in clade history. Further, significant morphological distinctions between Dikelocephalus specimens from the underlying Tunnel City Group is consistent with a recent sequence stratigraphic model of the northern Mississippi Valley that suggests both a temporal gap and environmental shift between the deposition of these two units.