THE STRATIGRAPHIC CONTEXT OF THE LATE CAMBRIAN TRILOBITE WALCOTTASPIS VANHORNEI FROM THE UPPER MISSISSIPPI VALLEY, AND ITS MORPHOLOGICAL VARIATION

The sequence stratigraphic architecture of the northern Mississippi Valley Cambrian provides a valuable context in which to view and interpret patterns of phenotypic variation among the late Paleozoic dikelocephaline trilobites well known in the region. Walcottaspis vanhornei is a large but presently poorly described dikelocephaline trilobite that occurs within the heterolithic facies of the St. Lawrence Formation, interpreted to represent the early part of a falling stage systems tract. At present this species is known only from a single parasequence in the St. Lawrence Formation, occurring within a narrow arcuate belt that parallels the paleoshoreline and that reflects a rapidly shifting zone of sediment accumulation related to the basinward migration of the prograding shoreface. As such, the record of W. vanhornei suggests a short-lived appearance in the fossil record, spanning perhaps less than 100,000 years based on its parasequence occurrence. In nominal, ordinal and metric characters, the cranidium of W. vanhornei strongly resembles the well-known dikelocephaline Dikelocephalus minnesotensis, which is widespread within the heterolithic facies of the St. Lawrence Formation with local occurrence spanning perhaps 1.5 million years and encompassing the temporal range of W. vanhornei. The two species also reached comparably large adult sizes. The pygidia of the two species are, however, markedly distinct. Geometric morphometrics reveals that phenotypic variation among W. vanhornei cranidia resembles that in D. minnesotensis and includes a comparable ontogenetic component, although ontogeny explains only a small proportion of total shape variance. Pygidia of W. vanhornei also show ontogenetic variation but are sufficiently different in shape from those of D. minnesotensis as to make direct comparison difficult. Phylogenetic analysis of the dikelocephalines is on-going, but if D. minnesotensis is confirmed as the sister taxon of W. vanhornei, then its fleeting occurrence within the early part of a falling stage systems tract may be informative about its evolutionary mode.