Reid Smith, age 13, of Rockingham, North Carolina, for his. question:

Which was the Paleozoic Era?

The five long geological eras are broad chapters in the story of life on our planet. They are subdivided into periods that take their names from geological events in the earth's crust. The Paleozoic Era is the third chapter and the longest recorded sequence of the life story.

This geological era followed the Archeozoic and Proterozoic Eras that date back billions of years into the earth's dim, remote past. Its name, the Paleozoic Era, is coined from words meaning "old" and "life" and its rocks contain some of the earliest fossil records that can be identified clearly as the remains of living things. We can, however, infer from certain minerals that simpler life forms existed through countless ages during the two earlier chapters. The Paleozoic Era dawned more than 500 million years ago and it inherited from the past a multitude of assorted simple life forms, perhaps similar to the one celled organisms that still exist in our teeming word.

Most modern scientists suspect that the dry land of those far off days was more or less linked together in a single continent over the South Pole. Certainly the earth's geography did not resemble a modern map. The world's major mountains were in what are now Canada, Wisconsin and Minnesota. During later periods of the Paleozoic chapter, there were massive uplifts of the earth's crust in Greenland and Northern Europe and India. The southern and finally the northern arm of Appalachians arose. Several times vast areas of land were invaded by the sea, and near the end of the long chapter, parts of India and Africa, Australia and South America were covered by the glaciers of a monstrous ice age.

The Paleozoic Era spanned more than 300 million years and is subdivided into more than half a dozen geological periods. It opened with the Cambrian Period during which the most complex creatures were the assorted trilobites that lived in the ancient, almost fresh water seas. The first backboned animals arrived in a later period and, sometime during the Silurian Period, some 300 million years ago, certain plants and animals first left the seas to cope with a new way of life on the dry land.

Fossil records of later Paleozoic periods reveal that life on the land    prospered and improved by leaps and bounds. By the Carboniferous Period, tall scraggly plants thrived in swampy forests that later formed our major coal deposits. Before the end of the era, this primitive plant life was replaced by forests of cycads and conifers. The first brave scorpions to dare live on land were followed by assorted insects and amphibians. When the Paleozoic Era closed some 200 million years ago, the first small reptiles had arrived. The story of life was ready for the next major chapter, the Mesozoic Era, which was the Age of Reptiles.

Naturally a detailed account of the immense Paleozoic Era cannot be detailed in a few paragraphs. What's more, the records of much of this remote era are not easy to de¬fine. A million years or so is but a wink in geological history. The general pattern of the advancing life story is plain. But earth scientists cannot estimate precisely the length of each advancing stage. The most recent estimates of the immense time span in the Paleozoic Era and its periods are sure to be revised again and again as researchers unearth more about the remote past.