Rhaetian

The Rhaetian was the last part of the Triassic Period. It came after the Norian and before the Hettangian, which is the first part of the Jurassic. Scientists know the end of the Rhaetian very well, but they still discuss when it began.

In 2010, scientists tried to decide the beginning of the Rhaetian by looking for a special tiny sea creature called Misikella posthernsteini. But there is still debate about exactly when this happened. One important place for studying this time is the Newark basin in the eastern United States.

Recently, new studies using radiometric dating of fossils from Peru suggest that the Rhaetian might have started around 205.7 million years ago. This is different from older ideas.

During the Rhaetian, the big landmass called Pangaea started to break apart, but the Atlantic Ocean had not yet formed.

Stratigraphic definitions

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

The start of the Rhaetian is marked by special fossils, like a type of tiny sea creature called Misikella posthernsteini. These fossils help scientists know when this time period begins. At the end of the Rhaetian, another special fossil called Psiloceras appears, marking the start of the next time period, the Hettangian.

Duration

The Rhaetian is the last part of the Triassic Period, coming after the Norian and before the Hettangian, which is the first part of the Jurassic Period. Scientists have studied different places around the world to figure out exactly when the Rhaetian began and ended.

Some researchers thought the Rhaetian was a short time, lasting less than 5 million years. They looked at rock layers in places like Turkey and Sicily to compare with layers in North America. However, others believed the Rhaetian was longer, lasting between 5 to 10 million years. They studied rock layers in Italy, Austria, and Japan, finding clues in tiny fossils and magnetic patterns in the rocks.

More recent studies have tried to find a middle ground. By dating ash layers in Peru and British Columbia, scientists estimated the Rhaetian began around 205.5 million years ago. The end of the Rhaetian is thought to be about 201.4 million years ago, based on studies of fossil ammonites in Peru. These dates help us understand how long the Rhaetian was and how it fits into Earth's history.

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 this ancient time.