Darriwilian (Middle Ordovician) graptolites from the northern margin of the Qaidam Basin (Qinghai, China)

A restudy of the early Darriwilian graptolites from the Dameigou section in the Qaidam Basin provides some new information for graptolite biostratigraphy. The section contains a number of biserial faunas and easily identifiable Holmograptus Kozlowski, 1957, which are the marked faunas of the early Darriwilian. According to the biostratigraphic occurrences of the species, the Levisograptus dentatus Zone and the Holmograptus lentus Zone are identified from the Qaidam Basin. At the same time, the identification of these two graptolite zones provides materials for replacing the old, long-ranging Amplexograptus confertus Zone.