The Paleozoic was a time of dramatic geological, climatic, and evolutionary change. The Cambrian witnessed the most rapid and widespread diversification of life in Earth's history, known as the Cambrian explosion, in which most modern phyla first appeared. Arthropods, molluscs, fish, amphibians, synapsids and diapsids all evolved during the Paleozoic. Life began in the ocean but eventually transitioned onto land, and by the late Paleozoic, it was dominated by various forms of organisms. Great forests of primitive plants covered the continents, many of which formed the coal beds of Europe and eastern North America. Towards the end of the era, large, sophisticated diapsids and synapsids were dominant and the first modern plants (conifers) appeared.
The Paleozoic era began and ended with supercontinents and in between were the rise of mountains along the continental margins, and flooding and draining of shallow seas between the mountain ranges, in the interior of the continents. At its start, the supercontinent Pannotia broke up. Paleoclimatic studies and evidence of glaciers indicate that Central Africa was most likely in the polar regions during the early Paleozoic. During the early Paleozoic, the huge continent Gondwana (510 million years ago) formed or was forming. By mid-Paleozoic, the collision of North America and Europe produced the Acadian-Caledonian uplifts, and a subduction plate uplifted eastern Australia. By the late Paleozoic, continental collisions formed the supercontinent of Pangaea and resulted in some of the great mountain chains, including the Appalachians, Ural Mountains, and mountains of Tasmania.
The Cambrian spanned from 541 to 485 million years ago and is the first period of the Paleozoic era of the Phanerozoic. The Cambrian marked a boom in evolution in an event known as the Cambrian explosion in which the largest number of creatures evolved in any single period of the history of the Earth. Creatures like algae evolved, but the most ubiquitous of that period were the armored arthropods, like trilobites. Almost all marine phyla evolved in this period. During this time, the supercontinent Pannotia begins to break up, most of which later became the supercontinent Gondwana.
The Silurian spanned from 444 to 419 million years ago. The Silurian saw the rejuvenation of life as the Earth recovered from the previous glaciation. This period saw the mass evolution of fish, as jawless fish became more numerous, jawed fish evolved, and the first freshwater fish evolved, though arthropods, such as sea scorpions, were still apex predators. Fully terrestrial life evolved, including early arachnids, fungi, and centipedes. The evolution of vascular plants (Cooksonia) allowed plants to gain a foothold on land. These early plants were the forerunners of all plant life on land. During this time, there were four continents: Gondwana (Africa, South America, Australia, Antarctica, Siberia), Laurentia (North America), Baltica (Northern Europe), and Avalonia (Western Europe). The recent rise in sea levels allowed many new species to thrive in water.
The Permian spanned from 299 to 252 million years ago and was the last period of the Paleozoic Era. At the beginning of this period, all continents joined together to form the supercontinent Pangaea, which was encircled by one ocean called Panthalassa. The land mass was very dry during this time, with harsh seasons, as the climate of the interior of Pangaea was not regulated by large bodies of water. Diapsids and synapsids flourished in the new dry climate. Creatures such as Dimetrodon and Edaphosaurus ruled the new continent. The first conifers evolved, and dominated the terrestrial landscape. Near the end of the Permian, however, Pangaea grew drier. The interior was desert, and new species such as Scutosaurus and Gorgonopsids filled it. Eventually they disappeared, along with 95% of all life on Earth, in a cataclysm known as "The Great Dying", the third and most severe mass extinction.
The Ordovician and Silurian were warm greenhouse periods, with the highest sea levels of the Paleozoic (200 m above today's); the warm climate was interrupted only by a 30 million year cool period, the Early Palaeozoic Icehouse, culminating in the Hirnantian glaciation, 445 million years ago at the end of the Ordovician.
There are many unanswered questions about the late Paleozoic. The Mississippian (early Carboniferous Period) began with a spike in atmospheric oxygen, while carbon dioxide plummeted to new lows. This destabilized the climate and led to one, and perhaps two, ice ages during the Carboniferous. These were far more severe than the brief Late Ordovician ice age; but, this time, the effects on world biota were inconsequential. By the Cisuralian Epoch, both oxygen and carbon dioxide had recovered to more normal levels. On the other hand, the assembly of Pangaea created huge arid inland areas subject to temperature extremes. The Lopingian Epoch is associated with falling sea levels, increased carbon dioxide and general climatic deterioration, culminating in the devastation of the Permian extinction.
A noteworthy feature of Paleozoic life is the sudden appearance of nearly all of the invertebrate animal phyla in great abundance at the beginning of the Cambrian. The first vertebrates appeared in the form of primitive fish, which greatly diversified in the Silurian and Devonian Periods. The first animals to venture onto dry land were the arthropods. Some fish had lungs, and powerful bony fins that in the late Devonian, 367.5 million years ago, allowed them to crawl onto land. The bones in their fins eventually evolved into legs and they became the first tetrapods, 390 million years ago, and began to develop lungs. Amphibians were the dominant tetrapods until the mid-Carboniferous, when climate change greatly reduced their diversity. Later, reptiles prospered and continued to increase in number and variety by the late Permian period.