Ontogeny and geographic variation of a new species of the corynexochine trilobite Zacanthopsis (Dyeran, Cambrian)

Assessment of ontogenetic and geographic variation can have substantial influence on species delimitation and thereby on perceived patterns of species-level morphological variation and diversity in space and time. Here we describe the ontogeny and intraspecific variation of the early Cambrian trilobite, Zacanthopsis palmeri n. sp., based on silicified material from east-central Nevada, USA. Zacanthopsis palmeri is the oldest documented Cambrian corynexochine to shift from possessing a fused rostal-hypostomal plate to a functional hypostomal suture in mature specimens during ontogeny. Six geographically distinct samples of mature Z. palmeri from a single silicified limestone bed traceable over tens of kilometers in east-central Nevada permit exploration of geographic variation within this species using geometric morphometric methods. No one sample encompasses all of the shape variation expressed by Z. palmeri and several geographically segregated samples show some degree of morphological separation in pairwise comparison. Nonetheless, these samples are not qualitatively or quantitatively different from one another when all samples are taken into account. The degree of variation within Z. palmeri is similar in magnitude to the differences between other species in the genus known from much less material.