Viséan

The Visean, Viséan or Visian is an important age in Earth's history, part of the way scientists describe and divide up long stretches of time. It belongs to the ICS geologic timescale, which helps us understand how old rocks, fossils, and other features of the Earth are.

As a stage in the stratigraphic column, the Visean was the second part of the Mississippian period. This period is the lower subsystem of the larger Carboniferous era, a time when much of the world had vast forests and many new kinds of life were appearing.

The Visean lasted from about 346.7 to 330.3 million years ago, marked as Ma. It came after the Tournaisian stage and was followed by the Serpukhovian stage. This period helps scientists and historians understand the order of events and changes that happened on Earth long ago.

Name and definitions

The Viséan Stage was named by Belgian geologist André Dumont in 1832 after the city of Visé in Belgium’s Liège Province. Before it became an international stage, the Viséan was part of the European regional geologic time scale. It came after the Tournaisian Stage and was followed by the Namurian Stage. In North America, it matches parts of the Osagean, Meramecian, and Chesterian stages, and in China, it matches the lower and middle Tatangian series.

The start of the Viséan Stage is marked by the first appearance of a specific fusulinid species called Eoparastaffella simplex. Originally, the type location was near Dinant, Belgium, but this was not good for matching rock layers in different places. A Global Boundary Stratotype Section and Point (GSSP) has been suggested in the Luzhai Formation near Penchong in Guangxi, China. The end of the Viséan is marked by the first appearance of a conodont called Lochriea ziegleri or the start of the biozone of the goniatite Cravenoceras leion.

Biota

The Carboniferous-Earliest Permian Biodiversification Event began during the Viséan, matching the start of the main phase of the Late Palaeozoic Ice Age.

During the late Viséan, reefs made by animal life reappeared after being damaged during the Hangenberg Event. One of the animals that lived at this time was Westlothiana, which looked like a reptile but was actually an amphibian. At first, it was thought to be one of the earliest animals with an amniotic egg, but newer studies question this idea.

Biostratigraphy

The Visean period includes four special areas defined by tiny fossils called conodonts. These areas are named after specific types of these fossils: Lochriea nodosa, Lochriea mononodosa, Gnathodus bilineatus, and Gnathodus texanus.

In British geology, the Visean is divided into five smaller parts. Starting from the most recent to the oldest, these parts are called Brigantian, Asbian, Holkerian, Arundian, and Chadian. The earliest part of Chadian overlaps with the Tournaisian period.