After the mass extinction at the end of the Ordovician Period, diversity of life on Earth needed to work its way back up. Animals were able to swim and feed closer to the surface than ever before in the history of life on Earth. Many different types of jawless fish and even the first finned fish with rays were prevalent. While life on the land was still lacking beyond single-celled bacteria, diversity was beginning to rebound.
Oxygen levels in the atmosphere were also nearly at our modern levels, so the stage was being set for more types of species and even land species to begin to appear. Toward the end of the Silurian Period, some types of vascular land plants as well as the first animals, the arthropods, were seen on the continents.
Diversification was rapid and widespread during the Devonian Period. Land plants became more common and included ferns, mosses, and even seeded plants. The roots of these early land plants helped to make weathered rock into the soil and that created even more of an opportunity for plants to take root and grow on land. Lots of insects began to be seen during the Devonian Period as well.
Towards the end, amphibians made their way onto land. Since the continents were moving even closer together, the new land animals could easily spread out and find a niche. Meanwhile, back in the oceans, jawless fish had adapted and evolved to have jaws and scales like the modern fish we are familiar with today. Unfortunately, the Devonian Period ended when large meteorites hit the Earth.
The Carboniferous Period was a time in which species diversity yet again had to rebuild from a previous mass extinction. Amphibians adapted even more and split off into the early ancestors of reptiles. The continents were still coming together and the southernmost lands were covered by glaciers once again. However, there were tropical climates as well where land plants grew large and lush and evolved into many unique species. These plants in the swampy marshes are the ones that would decay into the coal we now use in our modern times for fuels and other purposes.
As for the life in the oceans, the rate of evolution seems to have been markedly slower than times before. While the species that managed to survive the last mass extinction continued to grow and branch off into new, similar species, many of the kinds of animals that were lost to extinction never returned. Finally, in the Permian Period, all of the continents on Earth came together completely to form the super-continent known as Pangaea.
During the early parts of this period, life continued to evolve and new species came into existence. Reptiles were fully formed and they even split off into a branch that would eventually give rise to mammals in the Mesozoic Era. The fish from the saltwater oceans also adapted to be able to live in the freshwater pockets throughout the continent of Pangaea giving rise to freshwater aquatic animals. There were several well-defined types of ostracoderms were in existence.
Two events of great biological importance occurred. The land plants evolved and air-breathing animals appeared.
The first land plants were leafless and similar to ferns. The first land-breathing animals were arachnids. This period began about million years ago and lasted for about 50 million years.
There was a great expansion of fishes, land plants, and first land animals — the primitive amphibians in this period. The original ostracoderms evolved into a great variety of fish. All kinds of fishes — bony, cartilaginous, were all established in marine and freshwater. By the late Devonian, two groups of bony fishes, the lungfishes and lobe-finned fishes adapted to land environments. Towards the end of the Devonian period, ostracoderms completely disappeared and trilobites and branchiopods declined in variety and number.
One of the important events in this period is the emergence of the first land vertebrates, amphibians called Stegocephalians.
Next, throughout the Carboniferous Duration — million years ago , prevalent forests of massive plants left enormous deposits of carbon that ultimately resorted to coal. The first amphibians evolved to move out of the water and colonize land, however, they had to return to the water to reproduce. Right after amphibians emerged, the initial reptiles evolved.
They were the first animals that could reproduce on dry land. You can learn more by going to your library or searching the Internet for words like "Paleozoic" or the names of each of the periods. Here in the Paleozoic, Earth's interior has cooled down to something like modern levels, so that volcanic activity is usually about as humanity experiences it: a few minor eruptions like Mount St. Helens each year, and major ones like Krakatoa every century or so.
However, gigantic "hot-spot" type eruptions still occur every hundred million years or so. Plate tectonics continues to push land masses across Earth's surface. At this particular time--the middle of the Silurian Period--most of the land is still locked in two supercontinents called Gondwanaland, which happens to be wandering over the South Pole in our view, and Laurasia, which is on the other side of the globe.
Huge glaciers cover the interior of Gondwanaland, and Earth is experiencing one of its ice ages. Over the next hundred million years, Gondwanaland will move north over the equator and begin to break up, and the climate will warm up substantially See the Mesozoic. The composition of the atmosphere has continued to slowly change, mostly due to the increase of oxygen produced by photosynthetic algae floating on the ocean. At long last the air is capable of supporting large animals, and almost in response, life explodes into the Paleozoic!
At the beginning of the Paleozoic, life existed only in or near the ocean. Trilobites, shellfish, corals, and sponges appeared, followed by the first fish.
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