<P> The Carboniferous spans from 360 million to 300 million years ago . During this period, average global temperatures were exceedingly high: the early Carboniferous averaged at about 20 degrees Celsius (but cooled to 10 degrees during the Middle Carboniferous). Tropical swamps dominated the Earth, and the large amounts of trees created much of the carbon that became coal deposits (hence the name Carboniferous). The high oxygen levels caused by these swamps allowed massive arthropods, normally limited in size by their respiratory systems, to proliferate . Perhaps the most important evolutionary development of the time was the evolution of amniotic eggs, which allowed amphibians to move farther inland and remain the dominant vertebrates throughout the period . Also, the first reptiles and synapsids evolved in the swamps . Throughout the Carboniferous, there was a cooling pattern, which eventually led to the glaciation of Gondwana as much of it was situated around the south pole, in an event known as the Permo - Carboniferous glaciation or the Carboniferous Rainforest Collapse . </P> <P> The Permian spans from 300 million to 250 million years ago and was the last period of the Paleozoic Era . At its beginning, all continents came together to form the super-continent Pangaea, surrounded by one ocean called Panthalassa . The Earth was very dry during this time, with harsh seasons, as the climate of the interior of Pangaea wasn't regulated by large bodies of water . Reptiles and synapsids flourished in the new dry climate . Creatures such as Dimetrodon and Edaphosaurus ruled the new continent . The first conifers evolved, then dominated the terrestrial landscape . Nearing the end of the period, Scutosaurus and gorgonopsids filled the empty desert . Eventually, they disappeared, along with 95% of all life on Earth in an event simply known as "the Great Dying", the world's third mass extinction event and the largest in its history . </P> <P> The Mesozoic ranges from 252 million to 66 million years ago . Also known as "the Age of the dinosaurs", the Mesozoic features the rise of reptiles on their 150 million year conquest of the Earth on the land, in the seas, and in the air . There are three periods in the Mesozoic: Triassic, Jurassic, and Cretaceous . </P> <P> The Triassic ranges from 250 million to 200 million years ago . The Triassic is a desolate transitional time in Earth's history between the Permian Extinction and the lush Jurassic Period . It has three major epochs: Early Triassic, Middle Triassic and Late Triassic . </P>

The appearance of hard parts in the fossil record is at the beginning of the phanerozoic