Biostratigraphy of the Cambrian System-A Progress Report

The ultimate goal of biostratigraphy is the development of a sound basis for cor­ relation of Phanerozoic rocks. Good biostratigraphy is needed to deal effectively with practical geological problems and with the more purely abstract problems of earth history. On a local scale, biostratrigraphic data can often be integrated with physical stratigraphic data to provide extremely precise correlation of particular historical events. On a global scale, biostratigraphic correlation may be less precise because it depends on the presence of rapidly evolving, widely distributed, and morphologically distinctive organisms-often an ideal rather than a practical reality. Nevertheless, precision of biostratigraphic correlation in the best circum­ stances for Early Paleozoic time is almost an order of magnitude better than by radiometric means. The potential for providing a precise independent check on biostratigraphically derived correlations comes from the rapidly developing field of magnetostratigraphy. However, until a larger body of data from this source is available for Cambrian time, biostratigraphic correlation is the most powerful available tool for unraveling Cambrian history. This article evaluates the current status of research on Cambrian biostratigraphy from a global viewpoint. Except for the use of Archaeocyatha, Hyolitha, and acritarchs in zonal divisions of the Early Cambrian in the USSR, Cambrian biostratigraphy is almost entirely based on the spatial distribution of trilobite·s. This situation has some practical advantages. Tn younger systems, biostratigraphers have the constant problem of weighing the biostratigraphic value of a variety of coexisting phyla. Only the in­ articulate brachiopoda, which are moderately abundant, but whose study is still in its infancy, and the Archaeocyatha, which are restricted to the early Cambrian, have some realistic potential in biostratigraphic competition with trilobites through significant parts of the Cambrian system. Figure 1 shows the preser.t distribution of occurrences of fossiliferous Cambrian rocks. There is an obvious potential for globally integrated biostratigraphic