Figure 11.18Fossils that record the Cambrian explosion. Precambrian organisms such as Namacalathus (left) were the first organisms to use calcite in making shells. These organisms became extinct at the Precambrian-Cambrian boundary. Their extinction paved the way for a strange new group of organisms, including Hallucigenia (center) and the more familiar trilobites (right), that formed weak shells made of organic material similar to fingernails. In each example, the fossils are shown on top and the reconstructed organism is shown on the bottom.