Short answers, pulled from the story.
The Paleozoic Era began 538.8 million years ago and ended 251.9 million years ago, spanning nearly 287 million years. It is the first of three geological eras of the Phanerozoic Eon, preceded by the Neoproterozoic and followed by the Mesozoic Era.
From oldest to youngest, the six periods are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The first three form the Early Paleozoic sub-era, while the latter three form the Late Paleozoic sub-era.
Adam Sedgwick first used the name Paleozoic in 1838 to describe the Cambrian and Ordovician periods. John Phillips redefined it in 1840 to cover the full Cambrian to Permian span. The name comes from the Greek words for "old" and "life," meaning "ancient life."
The Cambrian explosion was the most rapid and widespread diversification of life in Earth's history, occurring during the Cambrian Period (539-485 million years ago). Nearly all major invertebrate animal phyla appeared in this single period, making it the greatest evolutionary boom in the fossil record.
The Paleozoic ended with the Permian-Triassic extinction event, known informally as "The Great Dying," in which approximately 95% of all species on Earth disappeared. It is the largest extinction event of the Phanerozoic Eon, and life on land took 30 million years into the Mesozoic Era to recover.
The base of the Paleozoic is defined at Fortune Head on the Burin Peninsula in Newfoundland, Canada, designated as the Global Stratotype Section and Point (GSSP) by the International Commission on Stratigraphy. The marker is the base of the Treptichnus pedum trace fossil assemblage, dated to 538.8 plus or minus 0.2 million years ago.