Cambrian Series 2

The Cambrian Series 2, also called the Mid-Early Cambrian, is an important time period in Earth's history. It is the second major part of the Cambrian era, sitting right above the Terreneuvian series and below the Miaolingian series. Even though scientists know where it fits, this series has not been officially defined by the International Commission on Stratigraphy. This means we do not have a clear starting point or exact way to divide it into smaller parts.

One way scientists think about the start of Cambrian Series 2 is by looking for the first appearance of a group of ancient sea creatures called trilobites. These small, multi-legged animals were some of the first complex life forms to live in the oceans. The appearance of trilobites is estimated to have happened around 521 million years ago, marking a key moment in the early Cambrian world. This time period helps us understand how life on Earth began to grow and change in ancient seas.

Naming

The International Commission on Stratigraphy has not yet given an official name to the 2nd series of the Cambrian period. When it is named, it will replace older terms like "Lower Cambrian" and "Early Cambrian". In some areas, such as Siberia, this time period is called the "Yakutian".

Subdivisions

The 2nd series of the Cambrian period is divided into two main stages: Cambrian Stage 3 and Cambrian Stage 4. However, these stages have not yet been fully defined by scientists. In Siberia, three different stages are used: Atdabanian, Botomian, and Toyonian. Most ways to split this series up depend on studying fossils, especially trilobite zones.

In Australia, there is a stage called the Ordian. It was once thought to be the first stage of the Miaolingian, but it might actually belong to the upper part of Series 2. As of 2024, the start of the Ordian stage has not been officially defined yet.

Biostratigraphy

The beginning of the second series of the Cambrian period is identified by the first appearance of trilobites. This event helps scientists study and compare rock layers across different continents, though doing so can be challenging. The oldest known trilobite, Lemdadella, signals the start of the Fallotaspis zone.

The end of this series is marked by a significant loss of marine life, known as the first major extinction of the Paleozoic era. Changes in ocean chemistry and marine conditions are believed to be the main reasons for this extinction. A notable event at the boundary between Series 2 and the Miaolingian is the disappearance of many trilobite families, including Olenellidae in Laurentia and Redlichiidae in South China. After this extinction, new trilobites such as O. indicus appeared, and where their fossils are not found, scientists use chemostratigraphic data to identify the boundary.