When did the Cambrian Period begin after the Precambrian?

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A detailed geologic time scale chart displaying Earth's history from the Hadean Eon over 4.5 billion years ago to the present, organized by eons, eras, periods, epochs, and ages with color-coded columns and corresponding dates in millions of years. — Precambrian The stratigraphic chart of geologic time.

The Cambrian Period is generally recognized as beginning approximately 538.8 million years ago, marking the start of the Paleozoic Era and the Phanerozoic Eon.12

While some historical and geological contexts cite the transition from the Precambrian at 541 million years ago, the lower boundary of the Cambrian System is formally defined by a global stratotype section at Fortune Head, Newfoundland. This boundary is characterized by the first appearance of the fossil Trichophycus pedum, which serves as the benchmark for the beginning of the Cambrian and the Phanerozoic Eon.345