Since many of the major groups of trilobites were defined in a pre-cladistic context, it is perhaps not surprising that several of these groups are paraphyletic, e.g., “Ptychopariida.” This has partly served as an impediment to large scale phylogenetic studies aimed at working out ordinal relationships. “Ptychopariida” is a pivotal group that gives rise to the great majority of post-Cambrian trilobites (Eldredge, 1977; Fortey and Chatterton, 1988; Edgecombe, 1992; Fortey and Owens, 1997; Fortey, 2001; Jell and Adrain, 2003). At this time “Ptychopariida” largely comprises a waste basket taxon of small Middle and Upper Cambrian trilobites, often disarticulated, that includes the most common trilobite, and among the most prolific of all fossils, Elrathia kingii (see image).