Eras Geologic Of The Earth

Precambrian

The Precambrian, dating 4.5-544 million years ago (mya), marks the period of Earth's history from its initial formation to the beginning of life. During this time range, our Earth went through vast changes. The Earth had solidified from the solar disk left over from the Sun's own formation. The Moon formed, setting the stage for later tidal fluctuations. As the Earth cooled, it developed its initial atmosphere that contained no oxygen, yet setting the stage for the development of the first life forms, prokaryotes. From their interaction with the then prevalently carbon dioxide and sulfur rich atmosphere, they slowly altered, developing into the first eukaryotes, and creating yet another profound change in Earth, the production of oxygen. As the atmosphere slowly became enriched in oxygen, complex multicellular organisms evolved, and the curtain had fallen upon one era of Earth's history, to arise in a new era, the Paleozoic. With one of the greatest events in life's theater on Earth, the Precambrian Explosion marked a moment when an abundance of fossils showed a vast increase in the numbers and diversity of life. Later shown to be a more gradual increase in Earth's diversity, this was still the most significant increase in life forms on our new, young planet.

Cambrian Period

Dating from 544-505mya, the Cambrian Period began with what has been called a "Cambrian Explosion" for the large diversity in animals seen over such a geologically short period in time. Latter seen to have been a slower evolving process than it was once thought to be, the fossils of the Burgess Shale still show a the development of vastly more complex organisms than was seen before. Animals evolved the ability to swim and crawl, developed more acute senses of smell and taste. The sense of sight appeared With a the small beginning of just sensing changes in light, sight became so useful in detecting a swimming predatory moving over head, that vision soon pushed the evolution of many different organisms. Trilobites, ahead of their time, developed vision as advanced as the best insect sight of today.

Ordovician Period

Dating from 505-440mya, the Ordovician Period saw one land mass, Gondwana, with nothing but a vast ocean lying to the north. Plants began a colonization of the land.The Ordovician Period saw the first arthropods appear on Earth. The first mass extinction marked the end of the Ordovician, when an ice age began about 440mya.

Silurian Period

Dating from 440-410mya, the Silurian Period saw the first comeback in life on Earth. Recovering from the first mass extinction, plants once again colonized land. Plants were soon followed by terrestrial animals, such as spiders and centipeds. The quick colonization of land was greatly helped by a stabilization of Earth's climate. Still, with the earlier glaciers melting, new habitat in the form of freshwater appeared, fish soon moving into this new area. Coral appeared, slowly growing higher as the melting glaciers added to the ocean level. With the newer habitats of deep and open oceans jawed fish soon dominated in oceans, growing larger with more room to move.

Devonian Period

Dating from 410-360mya, the Devonian Period saw a greater diversity on land. Ferns, horsetails, and the first seed plants, gymnosperms, appeared. Amphibians evolved during the Devonian also. Echinoderms and coral were very common by this time. The Devonian ended with another major extinction.

Carboniferous Period

Dating from 360-286mya, the Carboniferous Period began after the second major extinction of life on Earth. With the Carboniferous, insects evolved wings, and certain amphibians evolved amniotic eggs, allowing for the evolution of reptiles. Temperatures became milder, marking a decline in the number of ferns and large insects.
The Carboniferous Period can be further divided into two epochs, the Mississippian, or Lower Carboniferous, and the Pennsylvanian, or Upper Carboniferous. The Mississippian fossils are found in strata of limestone, while the Pennsylvanian fossils are found in coal-bearing layers of rocks. The Mississippian Epoch dates 360-325mya. The Pennsylvania Epoch dates 325-286mya.