Stage 5 of the Cambrian is the first stage of the 3rd series of the Cambrian (or Middle Cambrian). Its lower boundary has not been formally defined by the ICS. The most promising definition is the first appearance of either the trilobite species Oryctocephalus indicus or Ovatoryctocara granulata which is currently estimated to be around 509 million years ago. The end of Cambrian Stage 5 and the beginning of the Drumian stage is marked by the first appearance of the trilobite Ptychagnostus atavus around 504.5 million years ago.
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Naming
The International Commission on Stratigraphy has not officially named the 5th stage of the Cambrian. A likely candidate for the name is "Molodian", named after the river Molodo in Bulunsky District, Sakha Republic, Russia.
GSSP
Three sections are currently discussed as GSSP candidates: the Wuliu-Zengjiayan section near Balang in Guizhou province (China), a section on Split Mountain in Nevada (USA) and the "Molodo river section" along the Molodo river. The Wuliu-Zengjiayan section is an outcrop of the Kaili Formation in the Wuliu quarry. The first candidate for the beginning of Stage 5 is the trilobite Oryctocephalus indicus, the second candidate is the trilobite Ovatoryctocara granulata.