How Do We Know the Paleozoic Started 542 Million Years Ago?

The geologic time scale is a living document. As geologists discover outcrops with key fossils and datable materials they move closer and closer to the absolute numerical age of key biostratigraphic occurrences that define the eras and epochs. Over the last 100 years of research the start of the Paleozoic Era, sometimes called the Cambrian Explosion has been estimated at 600 million years (Ma), 530 Ma, d543 Ma and 542 Ma. Each of these numerical ages was as precise as possible and advanced our understanding of when, in years, key changes in the fossil record occurred. Our presentation guides teachers through a set of geologic cross-sections, radiometric age determinations, and interpretations that span over 100 years of scientific study in countries from Morocco to Egypt/Israel to Namibia to Oman. The activities mirror the historic development of our understanding of the timing from soft-bodied to shelled organisms. This presentation provides a deeper understanding of deep time, a science topic considered controversial in some districts. We also model simple ways to engage students in the primary geologic literature