Rhaetian

The Rhaetian was the last age of the Triassic Period. It came after the Norian and was followed by the Hettangian, the first age of the Jurassic. Scientists use special methods to decide exactly when the Rhaetian began and ended, but they agree it ended about 201.4 million years ago.

In 2010, scientists tried to mark the start of the Rhaetian by looking for a tiny sea creature called Misikella posthernsteini. However, there is still debate about when this boundary occurred. Studies from places like the Newark basin in the eastern United States and Turkey have offered different ideas. Recently, in 2024, the start of the Rhaetian was updated to about 205.7 million years ago.

During the Rhaetian, big changes were happening to the Earth's continents. The supercontinent Pangaea was starting to break apart, although the Atlantic Ocean had not yet formed. This was a time of important changes in Earth's history.

Stratigraphic definitions

The Rhaetian is named after the Rhaetian Alps, a mountain chain in Switzerland, Italy, and Austria. It was first described by scientists in 1856.

The start of the Rhaetian is marked by certain fossils, like a type of tiny tooth-shaped creature called Misikella posthernsteini. These fossils help scientists know when this time period begins. The end of the Rhaetian is marked by the appearance of a specific type of ammonite called Psiloceras.

Duration

The Rhaetian is the last part of the Triassic Period, coming after the Norian and before the Hettangian, the first part of the Jurassic. Scientists have studied different ways to figure out exactly when the Rhaetian began, called the Norian-Rhaetian boundary.

Some scientists believe the Rhaetian was quite short, maybe only lasting around 2 to 4.5 million years. They looked at rock layers in places like Turkey and Sicily to compare magnetic patterns in the rocks. However, others think the Rhaetian was longer, lasting between 5 to 10 million years. They studied rock layers in Italy and Austria and found different signs that suggest a longer time span.

More recent studies have tried to find a middle ground. By dating ash layers in Peru and British Columbia, scientists estimated the Norian-Rhaetian boundary happened about 205.5 million years ago. This date fits both the short and long Rhaetian ideas, suggesting there might not have been a missing time period in the rock record as some earlier studies thought.

The end of the Rhaetian, called the Rhaetian-Hettangian boundary, is dated to about 201.4 million years ago. This comes from studying ash layers and fossils in Peru, showing when certain fossils first appeared.

Notable formations

Some important rock layers from the Rhaetian time include the Lower Elliot Formation in South Africa, the Exter Formation in Germany, the 'Grès infraliasiques' Formation near Saint-Nicolas-de-Port in France, and the Penarth Group in England and Wales, part of the UK. These formations help scientists understand what the Earth was like during the Rhaetian age.