Psephosthenaspis microspinosa Zone

Adrain, Jonathan M. & Karim, Talia S., 2012, 3467, Zootaxa 3467, pp. 1-97 : 6-7

Psephosthenaspis microspinosa Zone , Psephosthenaspis pseudobathyurus Zone ,

Psephosthenaspis glabrior Zone

These zones are as outlined by Fortey and Droser (1996), except that they were originally proposed as subzones of a " Psephosthenaspis Zone ". Although the use of the term "zones" based on the ranges of genera has been standard practice in Laurentian Cambrian and Lower Ordovician biostratigraphy, it reflects a pre-modern approach to systematics. Genera are human constructs. They are based on explicit hypotheses of synapomorphy, though the vast majority of trilobite genera have yet to be treated in modern systematic terms. Even when genera have been supported by modern phylogenetic analysis, their inclusivity (i.e., where to draw the basal node) remains a matter of subjective human decision. For this reason, "zones" based on the ranges of genera have essentially arbitrary bases. Further, definition of biostratigraphic units based on higher taxon ranges risks allowing the biostratigraphic scheme to drive the systematics: if everyone knows that a taxon is from a given "zone", then it should not occur outside that "zone" and when encountered there it is given a different name. This tail wagging the dog effect has plagued Laurentian Cambrian trilobite systematics, in which stratigraphic (and geographic) range has been explicitly advocated as a criterion for supraspecific taxonomy (e.g., Palmer, 1960, pp. 57–59).

The "subzones" recognized by Fortey and Droser (1996), on the other hand, are species-level units based on the first appearance of particular species and containing substantially distinct sets of species which only rarely cross zonal boundaries. They are directly equivalent in practice to the zones recognized by Adrain et al. (2009) and hence we elevate them here to full zonal level. All are Dapingian, and fall within the Tripodus laevis conodont Zone ( Ross et al., 1997). The species content of each zone was described in detail by Fortey and Droser (1996).