Paleozoic a. : Of or pertaining to, or designating, the older division of geological time during which life is known to have existed, including the Silurian, Devonian, and Carboniferous ages, and also to the life or rocks of those ages. See Chart of Geology.

Example:
Whats the difference between fossils in Early Paleozoic and Middle Paleozoic?

Taxonomy
Paleozoic, Paleozoic era, climate, Devonian, Paleozoic life, the Paleozoic era, free encyclopedia, first appearance, Gondwana, carbon dioxide, Silurian, boundary, ice age, warmer, youngest, continents, plants, Late Ordovician.

Defination from the web:
Paleozoic or Palaeozoic Era (from the Greek palaios (παλαιός), "old" and zoe (ζωή), "life", meaning "ancient life") is the earliest of three ... The Paleozoic spanned from roughly 542 to 251 million years ago (ICS, 2004), and is subdivided into six geologic periods; from oldest to youngest ...