Figure 1

The history of the diversity of the marine animal families over the past 600 million years. The solid line connects 77 data points, each showing the total number of well-skeletonized families known from a particular geological epoch (each of whose duration is indicated by the width along the × -xis). The numbered arrows indicate the five recognized episodes of 'mass extinction'; the one labeled 3 is that which separates the Paleozoic from the Mesozoic, and the one labeled 5 is the one that ended the dinosaurs and ushered in our own era.