Science

https://stratigraphy.org/chart/

The International Commission on Stratigraphy says the Cambrian Period started 538.8 ± 0.6 million years ago. If you look at older webpages, they tend to give older dates still, such as 541 million years ago (back in the 1990s, it was 570 million years ago; I have a 1981 encyclopedia that says 590 million years ago, and Carl Sagan say 600 in Cosmos (perhaps rounding)).

So I'd like to know when the start had moved from 539 (to the nearest million, and inside the range from the international body defining it) to 535, for this "complex animals started in the Ediacaran" claim to be meaningful.