Michael Mcfadden, age 12, of Williamsport., Pa ., for his question:
What is meant by the paleozoic era?
The eras and periods and ages are man made divisions of geological time. However, they were made after thoughtful study of the earths long history. The divisions, their names and their durations, are based upon events and evidence recorded in the rocks by the earth herself.
Our wonderful old world keeps a detailed diary of her doings. Its pages are layers of her rocky crust, her inks are running rivers and shallow seas, her pens are scratchy gravels and the frozen fingers of icy glaciers. The durable pages are poured from seething volcanoes and hardened from the silty deposits that sift to the floors of quiet seas. Here and there fossil specimens of plants and animals are pressed and preserved within and between the rocky layers.
The code of the diary is deciphered by geologists, paleontologists and anthropologists. A pile of gravel is the signature of a long gone glacier, a layer of lava is the footprint of an ancient volcano. The age of a fossil is dated by radioactivity. Earth scientists can read the eventful diary back through almost four billion years. The earth herself provided the natural divisions to separate her long story into chapters and paragraphs. The names of these geological eras and periods were selected to give hints about the events and the subject matter they reveal. The records show that the earth has been remaking and reshaping herself through five long eras of geological activity. After each of these eras there was a restful pause before a new and different chapter began. The first 2.5 billion years is covered in the Archeozoic era, meaning dawn of life, and the Proterozic era, meaning early life. Their deep rocks, crushed and changed by later levels, bear no clear fossils, but simp1e creatures may have helped make their deposits of limestone and graphite.
After a geological pause, the Paleozoic era opened some 520 million years ago. Its title is coined from words meaning old and lift, and its rocks bear the earliest c1ear fossils. This third era covers some 330 million years and falls naturally into six periods. The Cambrian period saw the sea dwelling trilobites, and the following Ordovician period preserved the oldest fossils of backboned animals. The Silurian period recorded the brave story of the first land animals.
The Devonian period saved samples of fishes and amphibians, and the carbonaceous period produced the ferny forests that later became coal. The final Permian period saved fossils of the first reptiles, recorded a great ice age and the birth of the southern arm of the Appalachians. Then the earth paused again before starting the fourth chapter of her diary, the Mesozoic era that opened about 185 million years ago.
The names of the geological eras make use of the Greek word for animal. Or lift, the ancestor of our words zoo and zoologist. Each era records a chapter in the unfolding story of the earthlings that share our planet. Most of the geological periods art named for places where their typical rock formations were first discovered. The name of the carbonaceous period, however, tells us that it was the great carbonizing or coal making epoch of the earth's eventful history.