ASSESSING GEOGRAPHIC VARIATION IN FOSSIL TAXA: RESULTS FROM MORPHOMETRIC ANALYSES OF EARLY CAMBRIAN TRILOBITES

Geographic variation, particularly of widespread species, complicates taxonomic practices in the fossil record with cascading effects on both stratigraphic correlation and perceived patterns of evolution. Four geographically distinct and minimally time-averaged samples of mature Zacanthopsis from a single limestone bed traceable over tens of kilometers provide an opportunity to identify geographic variation within an Early Cambrian trilobite species. After accounting for conflation of results from taphonomic and ontogenetic influence, geometric morphometric analyses of landmark data show significant differences between some of the samples. By analogy to modern marine arthropods, paleo-distances between localities were not great enough to inhibit gene flow; therefore this variation may be attributable to ecophenotypy. Comparisons with related species suggest that between-population variation is smaller than between-species variation. These results lend confidence to the taxonomy of this clade based on a “time-slice” sampling approach but do not assess the robustness of species delimitation over the history of the genus. Because phenotypic variation is the raw material on which selection operates, geographic variation has been claimed to promote speciation. This claim is investigated by mapping geographic variation onto a hypothesis of phylogenetic relationship among these early animals placed in stratigraphic context.