AGNOSTOID BIOSTRATIGRAPHY ACROSS THE MIDDLE–UPPER CAMBRIAN BOUNDARY IN HUNAN, CHINA

Abstract Many trilobite species of the order Agnostida have short stratigraphic ranges 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. We describe unusually rich agnostoid faunas of that age from two thick, well-exposed sections, one near Paibi and one near Wangcun, in northwestern Hunan Province, China. All collections are from the Huaqiao Formation, the definition of which is revised and its stratigraphic content is expanded. Most of the fossils are well preserved in dark, laminated, argillaceous carbonates that were deposited in lower slope environments. These are assigned to 76 species and 33 genera. New species described are Agnostus? babcocki, Ammagnostus? 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), Glaberagnostus? cicer (Tullberg, 1880), Kormagnostus minutus (Schrank, 1975), and Pseudophalacroma scanense (Westergård, 1946). The subfamily Ammagnostinae Öpik, 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 Ptychagnostus atavus, Ptychagnostus punctuosus, Goniagnostus nathorsti, Lejopyge laevigata, Proagnostus bulbus, Linguagnostus reconditus, Glyptagnostus stolidotus, and Glyptagnostus reticulatus. Each zone is further characterized by an assemblage of agnostoid species that may vary from place to place. By extrapolating from recently published uranium-lead geochronology, we estimate that the average duration of these agnostoid zones was approximately one million years. Formal division of the Cambrian System into global series and stages is a current project of the International Subcommission on Cambrian Stratigraphy. If the base of an upper Cambrian series of whatever name is selected to correlate closely with the traditional Middle–Upper Cambrian boundary in Europe, 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 coincide with the base of the Glyptagnostus reticulatus Zone, we propose the base of that zone in the Paibi section as a potential GSSP.