In the Devonian Period, forests and land plants evolved and vertebrates made their appearance. Four-legged vertebrates evolved during the Carboniferous Period, which caused an increase in the number of land-based organisms. Devonian Period, in geologic time, an interval of the Paleozoic Era that follows the Silurian Period and precedes the Carboniferous Period, spanning between about 419.2 million and 358.9 million years ago. The Devonian Period is sometimes called the โAge of Fishesโ because of the diverse, abundant, and, in some cases, bizarre types of these creatures that swam Devonian seas. Forests and the coiled shell-bearing marine organisms known as ammonites first appeared early in the Devonian. Late in the period the first four-legged amphibians appeared, indicating the colonization of land by vertebrates. The Carboniferous Period lasted from about 359.2 to 299 million years ago* during the late Paleozoic Era. The term "Carboniferous" comes from England, in reference to the rich deposits of coal that occur there. These deposits of coal occur throughout northern Europe, Asia, and midwestern and eastern North America. The term "Carboniferous" is used throughout the world to describe this period, although in the United States it has been separated into the Mississippian (early Carboniferous) and the Pennsylvanian (late Carboniferous) Subsystems. This division was established to distinguish the coal-bearing layers of the Pennsylvanian from the mostly limestone Mississippian, and is a result of differing stratigraphy on the different continents. The Mississippian and Pennsylvanian, in turn, are subdivided into a number of internationally recognized stages based on evolutionary successions of fossil groups . These stages are (from early to late) Tournaisian, Visean, and Serpukhovian for the Mississippian โ and Bashkirian, Moscovian, Kasimovian, and Gzhelian for the Pennsylvanian.
