A major division of time in geologic history, extending from about 540 to 250 million years ago (Ma). It is the earliest era in which significant numbers of shelly fossils are found, and Paleozoic strata were among the first to be studied in detail for their biostratigraphic significance. Western Europe, especially the British Isles, was the cradle of historical geology. Early work with rock strata and their fossils was strictly practical; the relative ages of rock units were essential for correlating scattered outcrops to search for natural resources—particularly coal—in the early part of the nineteenth century.