The Precambrian-Cambrian boundary global stratotype is the base of the Phycodes pedum Zone at Fortune Head, southeastern Newfoundland. The initial, two-stage radiation of the Phanerozoic fauna (the appearance of Phanerozoic-aspect trace producers and subsequent origin of diverse skeletalized metazoans) was earliest Cambrian. Fossil assemblages with deep, complex burrows or originally mineralized skeletal fossils (with exception of Cloudina) are Early Cambrian or younger. Correlation into Avalon indicates that strata once referred to the Proterozoic (e.g., Manykaian [or Nemakit-Daldynian] and Rovno stages) are lowest Cambrian and that the Siberian Tommotian Stage is middle Lower Cambrian. The thick sequence and complex epeirogenic history of the Placentian Series, as compared with the trilobite-bearing Branchian Series, suggest that most Early Cambrian time is represented by the subtrilobitic Lower Cambrian.

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