Bill Huffstatler, age 16, of Biloxi, Miss., for his question:
WHAT WAS THE PRECAMBRIAN ERA?
Scientists tell us that the first 4 billion years of the earth's earliest history was during the Azoic, Archeozoic and Proterozoic eras. This period of time is also often called the Precambrian Era. The length of time it covers makes up about 80 percent of the earth's total history.
During the Azoic Era, the earth had its very beginning. It may first have been part of a cloud of gas and dust and then a ball of liquid.
After millions of years, a rock crust finally formed. Geological evidence shows that the crust repeatedly melted and hardened. The continental shields took shape and the continents began to grow. The oceans and atmosphere also were formed during the Azoic Era. No evidence of life has been found in the rocks of this era.
Rocks of the Archeozoic Era contain fossils of the first and most primitive life: bacteria and tiny plants called algae. Archeozoic rocks include types of marble, slate and other metamorphic rocks. These rocks were made from sandstones, shales, limestones, iron ores, lava and volcanic ash by the action of heat, pressure and chemicals in the earth's crust.
Among the Archeozoic rocks were great blocks of granite. These blocks were formed when melted rocks flowed through cracks in the crust during periods of mountain building. The mountains themselves have been worn away, leaving only their bases in the shield areas.
Fossils of the first animals appeared in rocks of the Proterozoic Era. These animals included worms, sponges, jellyfish, corals and other animals without backbones called primitive invertebrates.
Algae and bacteria were also still plentiful during the Proterozoic Era.
In southern Canada, Proterozoic rocks including sandstones, shales, limestones, lava and volcanic ash form strata more than 80,000 feet thick.
The Paleozoic Era started 600 million years ago and ended 225 million years ago. It included seven periods. These periods were, from the oldest to the youngest: Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian and Permian. The Mississippian and Pennsylvanian periods are sometimes considered a single unit called the Carboniferous Period. _
Rocks of the Paleozoic Era contain a large number and variety of fossils. The appearance of such a great number of fossils of invertebrates suggests that much of the earlier fossil record was destroyed by erosion at the end of the Proterozoic Era.
Plants of the Cambrian Period include water dwelling algae from the Proterozoic Era. By Devonian times, the first land plants had developed. These plants were fern trees that grew on swampy land and reproduced by means of tiny bodies called spores, not seeds.
Mosses appeared during the Mississippian Period.
During the Pennsylvanian Period there was a great variety of ferns. Some grew 100 feet high in the coal making swamps.
The first true seed plants, the conifers, are found as fossils in strata formed during the Permian Period.