There are also green, and gray sandstones. This is the age of fish. There are armored fish, jawless fish, bony fish, air-breathing fish, and sharks. In Europe this period is called the Lower Carboniferous while in America it is called the Mississippian. Limestones thousands of meters deep formed in shallow warm seas as a result of Laurentia and Gondwanaland drifting closer together.
Limestone is formed by the the shells and skeletons of millions of marine organisms.
Pennsylvanian - Age of Plants and Giant Insects Huge coastal swamp forests thrived, were buried, and formed layers of coal. In Pennsylvania this period is over 4,000 feet thick in some places from which it gets its name. In Europe it is called the Upper Carboniferous. There were huge insects like the Meganeura, dragonfly that had a wingspan of one and one-half feet. Gondwanaland and Luarentia collide to form the Appalachian Mountains and shallowing the Tethys Sea.