Paleozoic

Cambrian

Late cambiran pix

Panotia, the supercontinent that formed at the end of the Precambrian Era, approximately 600 million      
years ago, had already begun to break apart by the beginning of the Paleozoic Era. A new ocean, the Iapetus Ocean, widened between the ancient continents of Laurentia (North America), Baltica (Northern Europe), and Siberia. Gondwana, the supercontinent that was assembled during the Pan-African orogeny, was the largest continent at this time, stretching from the Equator to the South Pole.

Animals with hard-shells appeared in great numbers for the first time during the Cambrian when the continents were flooded by shallow seas. The supercontinent of Gondwana had just formed and was located near the South Pole. Because Trilobites dominate the Cambrian fossil record, they are helpful as index fossils and to characterize biofacies and paleobiogeography. Despite the relatively strong faunal provincialism, Cambrian trilobites constitute the biostratigraphical framework that allows comparison and dating of rock successions throughout much of the world where Cambrian strata exist.

Ordovician
 

Middle Ordovician Pix

During the Ordovician, oceans separated the continents of Laurentia, Baltica, Siberia and Gondwana. Gondwana contained most land area, and comprised Southern Europe, Africa, South America, Antarctica, and Australia. Throughout the period, Gondwana drifted south. North America lay on the equator and was mainly submerged until the Middle Ordovician. Western and Central Europe were separated and located in the southern tropics. Europe moved southward towards North America. 

During the Middle Ordovician, uplifts occurred in most of the areas that had been shallow shelf. Such uplists are known to be predictors of forthcoming glaciation. Increased sea floor spreading and ridge activity accompanied by volcanic activity occurred. Ocean currents changed as a result of lateral continental plate motions causing the opening of the Atlantic Ocean. Sea levels underwent regression and transgression globally, causing flooding of the Gondwana craton , inturn causing cessation of carbonate sedimentation. 

Late in the Ordovician, a major glaciation began. This pervasive global event caused a profound drop in sea level, draining nearly all craton platforms. The end of the Ordovician was one of the coldest times in Earth history. Ice covered much of the southern region of Gondwana.

Silurian (440 to 410 mya)

Middle Silurian Pix

During the Silurian, Laurentia collided with Baltica closing the northern section of the Iapetus Ocean. 
This collision was preceded in many places by the abduction of marginal island arcs, resulting in the formation of the Caledonide mountains in Scandinavia, northern Great Britain and Greenland, and the Northern Appalachian mountains of Eastern North America. North China and South China had rifted away from the Indo-Australian margin of Gondwana, drifting North across the Paleo-Tethys Ocean. Throughout the Early and Middle Paleozoic, the expansive Panthalassic Ocean covered much of the northern hemisphere.  Surrounding this ocean was a subduction zone, much like the modern "ring-of-fire" that surrounds the Pacific Ocean. 

Devonian (410 to 360 mya)

Early Devonian Pix

	The Devonian realized profound changes in the world's geography. Landmasses were concentrated in two supercontinents, Gondwana and Euramerica. These vast continents close to other in the lower hemisphere, while a vast ocean consumed the remainder of the Earth. These supercontinents were surrounded on all sides by subduction zones. With the development of the subduction zone between Gondwana and Euramerica, a major collision was set in motion that would bring the two together to form the single world-continent Pangea in the Permian. The collision of what is now North America and Europe, causes in huge granite intrusions and the uplifting of the Appalachian Mountains. Erosion of new mountains provided great volumes of sediment hat were deposited in the also and nearby lowlands and shallow seas. 
	Extensive reef building, producing some of the world's largest reef complexes, proceeded as stromatoporoids and corals appeared in increasing numbers. These were built in the equatorial seas between the continents. Large areas of shallow sea in North America, central Asia, and Australia became basins in which great quantities of rock salt, gypsum, and other minerals precipitated. 
	Near the end of the Devonian, a mass extinction event occurred. Glaciation and the lowering of the global sea level may have triggered this crisis, since the evidence suggests warm water marine species were most affected. Meteorite impacts have also been blamed for the mass extinction, or changes in atmospheric carbon dioxide. It is even conceivable that it was the evolution and spread of forests and the first plants with complex root systems that may have altered the global climate. Whatever the cause, it was about this time that the first vertebrates moved onto the land.