Because many animals form mineralized skeletons, sedimentary rocks deposited throughout their evolutionary history over the past 541 million years contain a rich fossil record of shells and bones. Animals also leave a sedimentary calling card in their tracks, trails, and burrows, again widespread in sedimentary rocks formed after 541 million years ago. Not all animal phyla are well represented in the fossil record—fossil annelids, for example, are rare and flatworms are essentially unknown as fossils because they lack mineralized body parts (Chapter 23). In contrast, the fossilized shells and bones of bivalve mollusks, brachiopods, echinoderms, and mammals preserve an excellent record of evolutionary history within these groups.

Phylogenies make predictions about the evolutionary history of animals, and fossils provide more than a confirmation of comparative biology. The remains of ancient animals record when innovations in morphology and function first appeared, preserving the remains of now extinct species that resemble the ancestors of modern groups. They often show intermediate combinations of traits not seen in modern animals and chronicle the relationship between Earth and life through time. And they underscore the evolutionary importance of extinctions, including a small number of events that removed a majority of existing species, paving the way for renewed diversification among survivors. If we step back and look at the big picture, what do fossils tell us about the evolutionary history of the animal kingdom?