The Mississippian Period, 360 to 320 million years ago, and the Pennsylvanian Period, 320 to 284 million years ago, are divisions of the Paleozoic Era. They are sometimes combined and called simply the Carboniferous Period, because of the large coal deposits found in rocks of this age throughout the world. In North America, the rocks of the Mississippian (or lower Carboniferous) are mostly limestones, whereas the rocks of the Pennsylvanian (or upper Carboniferous) are coal-bearing strata. The Appalachians were formed when North America and Europe, which were attached at the time, crashed into another ancient landmass, Gondwanaland (now Africa and South America). During this time period, land animals developed the amniote egg, which allowed them to reproduce freely on land without the danger of drying out the embryo.
Missippian and Pennsylvanian features of the United States: