DIKELOCEPHALUS REVISITED: SPECIES DISCRIMINATION AND METHODOLOGICAL PROGRESS

Species recognition in paleontology is important yet commonly difficult. My 1994 monograph on the trilobite Dikelocephalus, based partly on “traditional” morphometric analysis of collections made from individual beds, formally synonymized 26 species from the late Cambrian St. Lawrence Formation into a single species, D. minnesotensis. Although variation among St. Lawrence Formation Dikelocephalus was marked I was unable to reliably discriminate subgroups within the entire sample, partly because levels of intra-collectional variation were notably high. Variable degrees of sclerite compression likely also obscured original differences in shape. Re-analysis of my specimen suite along with the type suite of the specimens used to establish the original species complex using 2D geometric morphometric methods, offers improved resolution of morphological groups within St. Lawrence Formation Dikelocephalus. More importantly, it also permits relating the occurrence of these morphotypes to the high precision stratigraphic framework for these rocks provided by Tony Runkel and his colleagues. Formal revision of the taxonomy will require the revival of some suppressed species names and the proposition of new ones. Despite the improved ability to discriminate species within Dikelocephalus, the genus in its type area remains remarkable for the degree of intracollectional variation, which contrasts with that in many other trilobites. More broadly, the 1994 study was part of an ongoing trend toward more comprehensive specimen illustration and description when formally recognizing species in trilobite paleontology.