The date for the base of the Cambrian period has been revised a number of times in the past decade. The 543 MA, which is the base of the Cambrian now is well constrained. But the great increase in the rate of diversification associated with the Cambrian has been shown to have begun in the Neoproterozoic.
Note that this time chart does not show space proportional to time. The Hadean and Archean combined account for about 2100 million years. The Proterozoic lasted about 1960 million years. The Phanerozoic's paltry 543 million years is blown out of proportion.
