Gzhelian

Gzhelian is a stage of the Late Carboniferous, also known as the upper Pennsylvanian of the geological time scale. The stage spans roughly from 303.7 to 298.9 million years ago, marking the final interval before the transition into the early Permian. It was defined in 1949 and formally adopted in 1966, with its boundary dates revised in the International Chronostratigraphic Chart, reflecting newer radiometric evidence. The Gzhelian is named after the village of Gzhel in Russia, where characteristic rock assemblages were first catalogued.

Lithologically, the Gzhelian is represented by widespread coal-bearing strata interbedded with sandstones and shales, especially in the Appalachian Basin, the Baltic Basin, and the Cantabrian Basin. In the Western Interior of North America, the stage includes the continued development of extensive coal fields, such as the Morrow Formation and the associated vertical fissure coal layers. The associate depositional environments ranged from beach and shallow marine to swampy pellicular conditions that promoted peat accumulation.

The paleontological record of the Gzhelian shows a diverse fauna of amphibians, early reptiles, and a variety of invertebrates. Trilobites, mollusks, and early seed plants exhibit a clear evolutionary trend influenced by climatic cooling and shifts in sea level. The stage is notable for the extinction of certain brachiopod and crinoid groups, marking a turnover that sets the stage for the Permian biota. Moreover, the Gzhelian plays a key role in the study of the Pennsylvanian–Permian transition, offering insights into climatic, tectonic, and biotic dynamics of the late Paleozoic.