The beginning and end of each chunk of time in the geologic time scale is determined by when some species appeared or disappeared from the fossil record ... The end of the Paleozoic and beginning of ...

3 min read The Cambrian period, part of the Paleozoic era, produced the most ... which left a huge number of fossils. Trilobites had flattened, segmented, plated bodies that helped to protect ...

3 min read During the Ordovician period, part of the Paleozoic era ... known mainly from the tiny fossil teeth they left behind. The few complete fossils that have been found suggest they were ...