october utc the fossil record does provide some constraint as explained in the ref as i recall in simpler terms the argument was that given the constraints that could be put into place on the clocks they show that more diversification happened during the cambrian explosion than usual incedentally doi b might be of interest it is an overview of the cambrian explosion whose structure bears uncanny resemblance to this article it may contain some snippets worth including or ideas for how to contract expand the main article talk