Pennsylvanian

The Pennsylvanian is the Upper Carboniferous epoch in the strata of North America. It comes after (above) the Mississippian, and before the Permian. The Pennsylvanian started about 323 million years ago (mya), and ended about 299 mya.
The "coal measures", which give the Carboniferous its name, are the remains of peat formed by dense tropical wetland forests. They were formed in the Pennsylvanian, from about 315–300 million years years ago.
These forests were on the equator, and the wetlands, which are always low-lying, stretched from North America in the west, through what is now Europe to China in the east, because these continents were all together at the time (Laurussia). The river plain which was the heart of the wetland stretched 5000km from eastern Canada to Ukraine, and was 700km wide.p6 This would make the Pennsylvanian basin larger than any wetlands on Earth today.
The characteristic vertebrates of the coal measures were amphibia, and the plants were mostly giant clubmosses such as "Lepidodendron".
Change of climate.
The rainforest system collapsed about halfway through the Pennsylvanian, and was replaced by a cooler, drier climate. The wetland forests ended when the land level was raised by the pressure of the Gondwana continent against Laurussia, causing the zone of contact to be raised. The end of the Coal Measures marks the end of the Carboniferous period. China was too far away to be affected, and there the wetland forests continued for another 50 million years, into the early Permian.p30
As the climate became drier, egg-laying amniotes (Synapsids and Sauropsids) became more common than amphibia. These are the lines that would lead to reptiles and mammals. These egg-laying land vertebrates probably split from the early amniotes in the Mississippian.