EVALUATING THE EARLY CAMBRIAN ORIGINS AND INTER-CLADE RELATIONSHIPS OF CORYNEXOCHINE TRILOBITES; EXAMINATION OF THE GENUS ZACANTHOPSIS

Unraveling the origins and inter-clade relationships of major trilobites groups from the early Cambrian has proven problematic due to the lack of well-defined synapomorphic character states, similarity and convergence of morphologies, and often limited and/or damaged specimens with significant temporal and geographic gaps. Corynexochine trilobites origins are rooted in the early Cambrian, but many of the distinguishing morphologies are not well developed until the middle Cambrian. The clade is recognized by a distinctive glabella shape, large pygidia, and a hypostome that is fused to the rostral plate. However, recent studies demonstrate that some early corynexochine trilobites exhibited a libristomate hypostome condition in mature specimens, specifically in the early Cambrian in the zacanthoidid genus Zacanthopsis. This raises the questions; what morphologies define true “corynexochine” trilobites? How can characters that shift during ontogeny be incorporated into analyses of early trilobite evolutionary histories?

The Zacanthoididae is a diverse (at least 26 genera and approximately 130 species) clade with distinctive morphology, and is traditionally grouped with other corynexochine trilobites. However, its relationship with other corynexochine clades is poorly resolved. To better understand corynexochine origins and inter-clade relationships, zacanthoidid trilobites from the genus Zacanthopsis are examined. Zacanthopsis species appear to share corynexochine characteristics while maintaining pleisomorphic conditions typical of redlichioid trilobites, making the genus an ideal candidate for assessing corynexochine origins. The evolutionary history of the genus is assessed using two methodologies: 1) A phylogenetic analysis for the genus Zacanthopsis to better understand its relationship with other zacanthoidid trilobites and early corynexochines. 2) Morphometric analyses incorporating landmark and semi-landmark data to assess ontogeny, species fidelity, and temporal/geographic variation within Zacanthopis. This analysis also includes new material from North American Laurentia and assesses the biostratigraphic utility of Zacanthopsis species.