Ontogeny, Intraspecific Variation, and Systematics of the Late Cambrian Trilobite <i>Dikelocephalus</i>

Abstract

Biometric analyses of well-localized specimens of the trilobite <i>Dikelocephalus</i> from the St. Lawrence Formation (Upper Cambrian), northern Mississippi Valley, suggest that all specimens belong to a single, highly variable morphospecies, <i>D. minnesotensis</i>. A complex pattern of ontogenetically-related and ontogeny-independent variation produced a mosaic of morphotypes, which show greater diversity than previously recorded within trilobite species. There is considerable variation within collections made from single beds. Variations of characters among collections are mosaic, and are clinal in some cases. Patterns of variation within <i>Dikelocephalus</i> cannot be related to lithofacies occurrence. There are no obvious temporal variations in <i>D. minnesotensis</i> within the St. Lawrence Formation, but some <i>Dikelocephalus</i> from the underlying Tunnel City Group may belong to a different taxon. The validity of this early taxon is questionable due to a lack of available material. The mosaic pattern of variation in <i>Dikelocephalus</i> mimics that documented at higher taxonomic levels in primitive libristomate trilobites, and helps explain difficulties in providing a workable taxonomy of primitive trilobites. Results caution proposition of evolutionary scenarios that do not take account of intraspecific variation. The recovery of dorsal shields of <i>Dikelocephalus</i> permits the first detailed reconstruction of the entire exoskeleton. The systematics of the genus is revised and twenty-five species are suppressed as junior synonyms of <i>D. minnesotensis</i>.