Agnostoid biostratigraphy across the Middle-Upper Cambrian boundary in Hunan, China

Many trilobite species of the order Agnostida have short stratigraphic rang es and global distribution in open-marine deposits. They are most abundant and most diverse from about the middle Middle Cambrian to the middle Upper Cambrian where they are proving to be excellent biostratigraphic indices. W e describe unusually rich agnostoid faunas of that age from two thick, well -exposed sections, one near Paibi and one near Wangcun, in northwestern Hun an Province, China. All collections are from the Huaqiao Formation, the def inition of which is revised and its stratigraphic content is expanded. Most of the fossils are well preserved in dark, laminated. argillaceous carbona tes that were deposited in lower slope environments. These are assigned to 76 species and 33 genera. New species described are Agnostus? babcocki, Amm agnostus? cryptus, Ammagnostus histus, Ammagnostus hunanensis, Ammagnostus wangcunensis, Baltagnostus? ambonus, Linguagnostus stenus, Lisogoragnostus hybus, Lisogoragnostus mictus, and Utagnostus songae. Species reassigned in a new generic combination are Ammagnostus laiwuensis (Lorenz, 1906), Glabe ragnostus? cicer (Tullberg, 1880), Kormagnostus minutus (Schrank, 1975), an d Pseudophalacroma scanense (Westergard, 1946). The subfamily Ammagnostinae Opik, 1967, is elevated to family rank. The bases of eight agnostoid biozones are defined by the lowest occurrence of a common species having a relatively short stratigraphic range and wide geographic distribution. In ascending order, these zones are named for Ptyc hagnostus atavus, Ptychagnostus punctuosus, Goniagnostus nathorsti, Lejopyg e laevigata, Proagnostus bulbus, Linguagnostus reconditus, Glyptagnostus st olidotus, and Glyptagnostus reticulatus. Each zone is further characterized by an assemblage of agnostoid species that may vary from place to place. B y extrapolating from recently published uranium-lead geochronology, we esti mate that the average duration of these agnostoid zones was approximately o ne million years. Formal division of the Cambrian System into global series and stages is a c urrent project of the International Subcommission on Cambrian Stratigraphy. If the base of an upper Cambrian series of whatever name is selected to co rrelate closely with the traditional Middle-Upper Cambrian boundary in Euro pe, we propose the base of the Linguagnostus reconditus Zone in the Wangcun section as a potential Global Standard Stratotype-section and Point (GSSP) . Alternatively, if the base of an upper Cambrian series is selected to coi ncide with thr base of the Glyptagnostus reticulatus Zone, we propose the b ase of that zone in the Paibi section as a potential GSSP.