Figure 8.2Fossils are traces of living organisms preserved in the geologic record. (a) Ammonite fossils, ancient examples of a large group of invertebrate organisms that are now largely extinct. Their sole representative in the modern world is the chambered nautilus. (b) Petrified Forest, Arizona. These ancient logs are millions of years old. Their substance has been completely replaced by silica, which preserved the original details of their form.