Fossils can and do provide evidence for phylogenetic hypotheses, showing, for example, that groups that branch early in phylogenies appear early in the geologic record. But the fossil record does more than this. First, fossils enable us to calibrate phylogenies in terms of time. It is one thing to infer that mammals diverged from the common ancestor of birds, crocodiles, turtles, and lizards and snakes before crocodiles and birds diverged from a common ancestor (see Fig. 23.7), but another matter to state that birds and crocodiles diverged from each other about 220 million years ago, whereas the group represented today by mammals branched from other vertebrates about 100 million years earlier. As we saw in Chapter 21, estimates of divergence time can be made using molecular sequence data, but all such estimates must be calibrated using fossils.

The evolutionary relationship between birds and crocodiles highlights a second kind of information provided by fossils. Not only do fossils record past life, they also provide our only record of extinct species. The phylogeny in Fig. 23.7 contains a great deal of information, but it is silent about dinosaurs. Fossils demonstrate that dinosaurs once roamed Earth, and details of skeletal structure place birds among the dinosaurs in the vertebrate tree. Indeed, some remarkable fossils from China show that the dinosaurs most closely related to birds had feathers (Fig. 23.11).

A third, and also unique, contribution of fossils is that they place evolutionary events in the context of Earth’s dynamic environmental history. Again, dinosaurs illustrate the point. As discussed in Chapter 1, geologic evidence from several continents suggests that a large meteorite triggered drastic changes in the global environment 66 million years ago, leading to the extinction of dinosaurs (other than birds). In fact, at five times in the past, large environmental disturbances sharply decreased Earth’s biological diversity. These events, called mass extinctions, have played a major role in shaping the course of evolution.