Triassic

The Triassic (/traɪˈæsɪk/; sometimes symbolized as 🝈) was a geologic period and a stratigraphic system that lasted for 50.5 million years, from 251.902 million years ago to 201.4 million years ago. It was the first and shortest period of the Mesozoic Era, coming after the Permian Period and before the Jurassic Period. The Triassic began after a huge event called the Permian–Triassic extinction event, which greatly reduced life on Earth.

During the Triassic, many new kinds of animals appeared and began to thrive. Reptiles, especially a group called archosaurs, were the main land animals. Some of these archosaurs were the ancestors of today's crocodilians, and others were the first animals able to fly, known as pterosaurs. Also, the very first dinosaurs appeared toward the end of this period, though they did not become the top animals until later.

One of the biggest features of the Triassic was the huge landmass called Pangaea, which covered most of the Earth. As the Triassic ended, Pangaea began to split into two smaller landmasses: Laurasia to the north and Gondwana to the south. The climate was mostly hot and dry, but it became more humid as the land started to drift apart. The Triassic Period ended with another major extinction event, the Triassic–Jurassic extinction event, which allowed dinosaurs to become the dominant animals in the following Jurassic Period.

Etymology

The Triassic Period was named in 1834 by Friedrich August von Alberti. He chose this name because he noticed three distinct layers of rock in southern Germany. These layers are called the lower Buntsandstein (colorful sandstone), the middle Muschelkalk (shell-bearing limestone), and the upper Keuper (coloured clay). The word "Triassic" comes from the Greek word triás, meaning "triad," referring to these three rock layers.

Dating and subdivisions

The Triassic is split into three main parts called Early, Middle, and Late Triassic Epochs. Scientists use these names when talking about the rocks formed during each part of this time. The Triassic also has special steps called faunal stages, which help experts study the fossils and changes that happened during this period.

Paleogeography

At the beginning of the Triassic, all the major continents were joined together into a supercontinent called Pangea. This huge landmass stretched from the north to the south pole, with Laurussia in the north and Gondwana in the south. Oceans such as the Paleo- and Neo-Tethys were inside this supercontinent, while the massive Panthalassa Ocean lay beyond.

Pangea was surrounded by areas where the Earth's crust was moving, called subduction zones. Over time, Pangea changed its movement, rotating in different directions. This movement was influenced by the opening and closing of ocean passages. As Pangea moved, mountain ranges formed and then eroded away.

In the northern part of Pangea, called Laurussia, volcanic activity created large areas covered in lava. Rivers and deltas spread out across shallow ocean areas. In the southern part, called Gondwana, Africa remained mostly above sea level, but some areas had lakes and rivers. The land continued to shift, creating new patterns of mountains and valleys.

Climate

The Triassic period had a hot and dry climate in its interior areas, leading to the formation of red sandstones and evaporites. Unlike today, the polar regions were moist and temperate, supporting forests and many animals, including reptiles. Because the supercontinent Pangaea was so large, it had strong seasonal changes, with very hot summers and cool winters. This difference between Pangaea and the surrounding oceans caused powerful monsoon rains.

While the Triassic was mostly dry, there were times with more rain, especially around the Tethys Sea and parts of North America, China, and Argentina. One of the most notable wet periods was the Carnian Pluvial Event, which brought widespread humidity and changed the climate for a time. The climate continued to shift throughout the Triassic, with periods of cooling and warming, ending with a major warming event that led to a mass extinction at the close of the period.

Flora

During the Triassic period, many different kinds of plants grew on land and in the oceans. On land, plants called lycophytes, such as Pleuromeia, became important in the early Triassic. Later, other plants like Bennettitales and conifers became common. One special plant from the southern lands was Dicroidium, a type of seed fern that grew in forests.

In the oceans, tiny plants called phytoplankton changed during the Triassic. Before this time, red and green algae were the main phytoplankton. But during the Triassic, new types of algae called secondary endosymbiotic algae became the most important plankton in the sea.

No coal deposits formed at the very start of the Triassic, a time known as the "coal gap." This may have been because of changes in climate, sea levels, or the loss of plants that usually helped create peat, which turns into coal.

Fauna

In the Triassic period, many new types of animals appeared in both land and water environments. Marine life saw the emergence of new kinds of corals forming small reefs, and ammonites, a type of shelled creature, returned and began to diversify. Fish also went through significant changes, with ray-finned fishes becoming more diverse, and new types of predatory fish appearing in rivers and lakes.

On land, amphibians such as temnospondyls survived, though many were now aquatic or semi-aquatic. The first modern amphibians, like the ancestors of today’s frogs, also appeared. Reptiles underwent huge changes, with archosauromorphs such as tanystropheids—known for their extremely long necks—and rhynchosaurs thriving. True archosaurs split into two main branches, one leading to birds and the other to crocodiles. Early pterosaurs and dinosaurs also appeared during this time. Marine reptiles included diverse groups like sauropterygians, ichthyopterygians, and the first plesiosaurs, all adapting to life in the oceans.

The Triassic was also a time when mammal-like reptiles, known as therapsids, began to decline, making way for the rise of true archosaurs. Some cynodonts, however, evolved into the very first mammals towards the end of this period.

Lagerstätten

Two important fossil sites from the early Triassic — the Guiyang biota and the Paris biota — are known for their well-preserved fossils and wide variety of life forms. They show how life recovered after a big event that wiped out many species.

Another famous fossil site is Monte San Giorgio, located in Italy and Switzerland. During the middle Triassic, this area was a calm lagoon where many fish and marine reptiles, like Neusticosaurus and Tanystropheus, were preserved perfectly because there were no animals to disturb their remains. Fossils from this site help scientists understand life from about 242 million years ago.

Triassic–Jurassic extinction event

The Triassic Period ended with a mass extinction that was especially hard on ocean life. Many sea creatures, including groups called conodonts, and most marine reptiles, disappeared. Invertebrates such as brachiopods and molluscs were also greatly affected.

On land, several important groups of reptiles vanished, and some early dinosaurs went extinct. However, other dinosaurs survived and later became dominant during the Jurassic Period. The exact cause of this extinction is still uncertain, but huge volcanic eruptions as the supercontinent Pangaea began to break apart are a leading theory.