Scientists find fossils of a wide variety of dinosaur species throughout Mesozoic rocks, which date from approximately 250 million to 65 million years ago. Above the Mesozoic rocks lie Cenozoic rocks, which date from approximately 65 million years ago to the present day. In the overlying Cenozoic rocks, scientists find no dinosaur species. Instead, they find a great number of new mammal species. Which of the following best describes what is preserved in the Cenozoic rocks?