Working in the field you encounter a series of fossil-rich sedimentary layers. You identify: paradoxides [para-DOX-ih-dees] trilobites, mucrospirifer [mah-krohSPEER-ih-fur] brachiopods [BRAK-ee-oh-pods], horn corals, and crinoids [CRY-noyds]. You recognize them as index fossils representing what era of geologic time?