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).
Evolution and Systematics
Understanding the relationships of the dominant post-Cambrian groups of trilobite to their older Cambrian sister taxa remains a major unresolved question in trilobite evolution. This problem has been termed “cryptogenesis”. At present this question has been largely unexamined due in part to poor sampling across the critical time of diversification. Worker bias (i.e., specialists working only on Cambrian or only on Ordovician trilobites) has also hampered our understanding of the origins of the dominant post-Cambrian groups (Edgecombe, 1992; Fortey, 2001).
Edgecombe (1992) commented in detail on “cryptogenesis” and ptychopariid paraphyly. In particular, its existence exaggerates the perceived amount of taxonomic turnover at the Cambrian-Ordovician boundary because a number of de novo Ordovician trilobite orders have been established without considering the context of their origins; further, the “Ptychopariida” was presumed to disappear at the end of the Cambrian (Edgecombe, 1992).
There have been some attempts to unravel phylogenetic structure within the group. In particular, Fortey and Chatterton (1988), which appears to represent the first published study on trilobites using computer analysis of character data. Their results were expanded on and presented as evolutionary trees in Fortey and Owens (1997) and Fortey (2001). “Ptychopariida” is posited to emerge out of the paraphyletic mass of “Redlichiina” with the set of relationships suggested by Fortey (2001). Fortey and Chatterton (1988) and Edgecombe (1992) discussed in greater detail some of the specific families of “Ptychopariida” that might be most closely related to the Asaphida. Although phylogenetic analyses of trilobites within this paraphyletic grade have proven challenging, some of the structure of the individual groups has been examined in cladistic studies.