The Cambrian Period is in the Paleozoic Era. The Paleozoic Era was known as the "Age of Marine Life." This "Age of Marine Life" all began with what is called the "Cambrian Explosion." The "Cambrian Explosion" happened roughly 542 million years ago and obviously took place during the Cambrian Period. It was when most of the major groups of marine animals appeared in a relatively short period of time. According to fossil records, Bryozoa, algae, sponge (porfera), Arthropoda (trilobita), Mollusks (plecypoda, gastropoda, cephalopoda), and corals first appeared all in the Cambrian Period. The Ordovician Period followed the Cambrian Period, and this is when some of the first brachiopods appeared, such as clams, snails, and cephalopods. Also, ostracoderms, jawless armored fish, were the earliest vertebrates that appeared in this time period. "Typical Ordovician fish had large bony shields on the head, small, rod-shaped or platelike scales covering the tail, and a slitlike mouth at the anterior end of the animal" (www.ucmp.berkeley.edu). During the Silurian, there was a widespread radiation of crinoids, a continued proliferation and expansion of the brachiopods, and the oldest known fossils of coral reefs. There was also the appearances the first known freshwater fish. And finally, the first record of life on land! Fossil records show a group of fungi on land, and also arachnids and centipedes. According to www.ucmp.berkeley.edu, "Most Silurian plant fossils have been assigned to the genus Cooksonia, a collection of branching-stemmed plants which produced sporangia at their tips. None of these plants had leaves, and some appear to have lacked vascular tissue. Also from the Silurian of Australia comes a controversial fossil of Baragwanathia, a lycophyte. If such a complex plant with leaves and a fully-developed vascular system was present by this time, then surely plants must have been around already by the Ordovician. In any event, the Silurian was a time for important events in the history of evolution, including many 'firsts,' that would prove highly consequential for the future of life on earth." The Devonian Period is most remembered by the fish that appeared in it. The early fish in the Devonian were ostracoderms, but in the mid- Devonian, we see the first jawed fish. Most of these fish were large and predators of the sea. But probably the most interesting fish that appeared are the, sarcopterygiians, or lobe-finned fish, that appeared, wish would produce the first tetrapods that appeared just before the Devonian Period ended. Now we move on to the Carboniferous Period. The Carboniferous Period had a more uniform, tropical, humid climate than the climate we live in today. Maybe this is why we see animals such as marine reptiles, lizards, and snakes versus the archosaurs (crocodiles, dinosaurs, and birds). Also the first freshwater clams appeared. We also see a growing diversity among sharks and other bony fish. And now the last period of the Paleozoic Era, the Permian Period was largely characterized by the largest mass extinction in the history of Earth. By the end of the Permian, 90% of all known species became extinct. The species that were fortunate to survie did so in very low numbers. Atmospheric oxygen was cut from 33% to just 14%, a 19% difference! After the Permian Period, the Earth was never the same again. Now onto what is called the "Age of the Reptiles," the Mesozoic Era begins with the Triassic, which marked the beginning of many reptiles and dinosaurs. Groups of animals that emerged and flourished during the Triassic Period are: Temnospondyls, Rhynchosaurs, Phytosaurs, Aetosaurs, Rauisuchians, Theropods, and Cynodonts. Temnospondyls were up to 13feet high! And phytosaurs resemble todays alligators and crocodiles! Theropods included Tyrannosaurs and Velociraptors! The Jurassic Period gave us the first birds. Also, we see giant marine crocodiles, and sharks and rays that were nearly identical to the ones we see today! "Also prominent in the seas were cephalopods -- relatives of the squids, nautilus, and octopi of today. Jurassic cephalopods included the ammonites, with their coiled external shells (upper left), and the belemnites, close relatives of modern squid but with heavy, calcified, bullet-shaped, partially internal shells" (ucmp.berkeley.edu). The Cretaceous gave us another mass extinction. Many species became extinct at the end of the Cretaceous. However, insects began to diversify. We find the oldest ants and insects during the Cretaceous. Also we see grasshoppers, bees, wasps,and termites during this period. Also there were species that went practically untouched during this mass extinction, such as lizards, snails, clams, snakes, crocodiles, mammals, and amphibians. This starts the era that we are in today, the Cenozoic Era, also known as the "Age of the Mammals." The first period of the Cenozoic Era is the Paleogene Period. Not much change happened during this period. Mammals thrived on land and animals adapted to their environments. It is not until the Neogene Period, the period we are in today, when massive change occurs. In the Neogene Period we see the Pleistocene Ice Age. During this Ice Age, glaciers covered 30% of earths land masses. After is has all been said and done, the Cenozoic Era produced the great hunter cats, whales, humans and a whole other plethora of mammals. The Earth has seen a huge diversity of life throughout all of time and changed rapidly as well as slowly depending on the time period. And Earths life continues to change and will continue to change until the end of time.

Bibliography:
http://www.ucmp.berkeley.edu/