EVOLUTIONARY TEMPO AND MODE IN CAMBRIAN UPPER MISSISSIPPI VALLEY DIKELOCEPHALID TRILOBITES I: DIKELOCEPHALUS

Late Cambrian strata of the upper Mississippi Valley provide opportunity to explore evolutionary tempo and mode in an ancient nearshore setting within a well-represented trilobite clade, the Dikelocephalidae. In the most common dikelocephalid genus, Dikelocephalus, specimens within the latest stages of a transgressive systems tract are morphometrically distinct from those overlying them in the subsequent falling stage systems tract, but the transition between these forms is opaque due to the absent sedimentary record associated with maximum flooding. Morphological variation during the early falling stage systems tract Dikelocephalus conforms to Darwin’s expectations about evolutionary patterns within common and widespread taxa. Dikelocephalus cranidial and pygidial shapes define a continuous morphological landscape with four common morphotypes emergent, two of which are unique to a particular bed at one locality only. Collectively these may represent 4 species, 2 dimorphic species, or more complex patterns of polymorphy. Evident continuous and subtle morphological variation among these Dikelocephalus accords with the high temporal fidelity of the early falling stage deposits, and these upper Mississippi Valley Dikelocephalus are morphologically consistent with contemporaries found in other inner detrital and carbonate belt settings on the Laurentian craton. Morphological variation among these Dikelocephalus is that expected in a widespread, populous and enduring lineage. Although the four early falling stage morphotypes can be interpreted in several ways, variation among these Dikelocephalus shows no significant temporal trend throughout the approximately 1 Ma of its regional representation. Dikelocephalus provides an example of net morphological stasis within a notably morphologically variable lineage showing complex patterns of variation likely to include microevolutionary change. Further, Dikelocephalus provides a baseline to compare with closely related genera in order to assess the evolutionary tempo and mode of dikelocephalid evolution within the upper Mississippi Valley during the late Cambrian.