MORPHOLOGICAL VARIATION, ONTOGENY, AND ORIGIN OF ZACANTHOIDID TRILOBITES: A GEOMETRIC MORPHOMETRIC ANALYSIS OF THE EARLY CAMBRIAN GENUS ZACANTHOPSIS

Zacanthoidid trilobites are a distinctive and common component of early and middle Cambrian faunas. The clade is relatively diverse with approximately 130 species across 26 genera. Although zacanthoidid trilobites have a long history of taxonomic study, the origin and early evolutionary history of the clade is still poorly understood. To address this problem, new and existing collections of the early zacanthoidid genus Zacanthopsis are studied. Collections consist of well-preserved, undistorted, silicified cranidia from six tightly constrained stratigraphic intervals within the Combined Metals Member of the Pioche Formation (upper Dyeran Stage, traditional "lower" Cambrian) of Nevada. Geometric morphometric analyses, utilizing both landmark and semilandmark data, are used to assess ontogenetic, geographic, and stratigraphic variation in the collections. Three species of Zacanthopsis (two new) are represented. Phylogenetic analysis demonstrates that the species form a clade, and it is probable that they constitute an anagenetic lineage. Partitioning of phenotypic variation into interspecific disparity and intraspecific ontogenetic, geographic, and stratigraphic components provides a baseline that assists in the interpretation of phenotypic differences between specimens in other, less well-sampled, zacanthoidids. The phylogenetic and morphological data presented herein improve our understanding of how development was evolutionarily modified during the early Cambrian radiation of zacanthoidid trilobites, and offers insight into how development can constrain phenotypic evolution in general.