The Paleozoic Era is the earliest geologic era of the Phanerozoic Eon and extends from 542 to 251 million years ago. Paleozoic or Palaeozoic is derived from the Greek terms for ancient, paleo, and animal zo (Borror, 1988, pp. 69 & 111). The Paleozoic Era is divided into six periods, including the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The start of the Paleozoic Era saw the appearance of almost all animal phyla. The 300-plus million years of the Paleozoic is characterized by the appearance and evolution of marine and land invertebrates, fishes, land plants, and reptiles. Much of the coal and limestone that we mine today was formed in the ancient seas and swamps of this geologic era. The end of the Paleozoic Era is defined by the largest known mass extinction.