ASBOG Fundamentals of Geology (FG) Practice Exam

Trilobites first appear in the fossil record in Cambrian rocks and are iconic of Cambrian oceans. They were early, hard-shelled marine arthropods that thrived in shallow sea environments, so Cambrian marine deposits are where their fossils are most commonly found. While they do appear later in some groups into the Ordovician, their defining and earliest-rich context is Cambrian. They are not typical in Cretaceous rocks, since trilobites had largely disappeared by that time (extinct by the end of the Permian). So Cambrian marine deposits are the setting that best fits trilobites.