Agmata, a proposed extinct phyllum of Early Cambrian age

Although the view is changing, conventional thinking has been that there are no extinct taxa of phylum rank. Rather than propose orders, classes, and phyla for unusual extinct organisms, many paleontologists have preferred to place such fossils in extant groups. Taxonomy at a high level should be approached with caution, but whether at a high or low level, this exercise is the same--the judgment of the significance of similarities and differences. The type lot of Salterella rugosa Billings is redescribed, for the first time from thin-sectioned specimens. The organism consists of a narrow, radially symmetrical calcareous cone, mostly filled with laminae composed in part of detrital quartz grains. This combination of structure is judged to be unique; the forms are not mollusks, nor, probably are they annelid worms. The structure is so novel at such a major systematic level that a new phylum, Agmata, is proposed to include Salterella, on the basis of these features. The phylum occurs in the late Early Cambrian in North America and drift fragments of the continent. Hydroconozoa and Vologdinella may be organisms that appear superficially similar; Volborthella may be slightly older than Salterella. Known specimens of Volborthella lack an outer calcareous shell, but this is likely the result of diagenetic phenomena. Specimens have layers of detrital grains constructed around a central tube, as in Salterella. Volborthella is assigned with question to the phylum Agmata; no other fossil forms are placed in this phylum. Thus, the taxon is distinctive, as all taxa must be, but has quite limited diversity.