Figure 11.17The diversity of animal fossils reveals both mass extinctions and radiations. This graph shows the number of “shelly” animal families found in the fossil record during the last 600 million years; each family comprises many species. During a radiation, such as the Cambrian explosion, the number of new families increases. During a mass extinction, such as the one at the end of the Cretaceous period, the number of families decreases. (Ma, million years ago.)