Triassic

The Triassic (/traɪˈæsɪk/; sometimes symbolized as 🝈) was a geologic period and a stratigraphic system. It lasted for about 50 million years, from 251.9 million years ago to 201.4 million years ago. It was the first and shortest period of the Mesozoic Era. It came after the Permian Period and before the Jurassic Period.

The Triassic began after a big event called the Permian–Triassic extinction event. This event caused many plants and animals to disappear.

During the Triassic, many new animals appeared. Reptiles, especially a group called archosaurs, were the main land animals. Some archosaurs were the ancestors of today's crocodilians. Others were the first animals able to fly, known as pterosaurs. The very first dinosaurs appeared toward the end of this period.

One big feature of the Triassic was the huge landmass called Pangaea. It 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 moved.

The Triassic Period ended with another big extinction event, the Triassic–Jurassic extinction event. After this, dinosaurs became the main animals in the Jurassic Period.

Etymology

The Triassic Period was named in 1834 by Friedrich August von Alberti. He chose this name because he saw three different 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," for these three rock layers.

Dating and subdivisions

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

Paleogeography

At the start of the Triassic, all the big continents were joined together into one huge landmass called Pangea. This giant land stretched from the north to the south pole. Laurussia was in the north and Gondwana was in the south. Oceans such as the Paleo- and Neo-Tethys were inside this supercontinent, while the big Panthalassa Ocean was 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 wore away.

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

Climate

The Triassic period had a hot and dry climate in its interior areas. This caused red sandstones and evaporites to form. The polar regions were moist and temperate, with 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 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, ending with a major warming event.

Flora

During the Triassic period, many kinds of plants grew on land and in the oceans. On land, plants called lycophytes, such as Pleuromeia, were important in the early Triassic. Later, 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 animals appeared on land and in water. In the oceans, new kinds of corals formed small reefs. Ammonites, creatures with shells, returned and changed in many ways. Fish also changed a lot. Ray-finned fishes became more common, and new kinds of fish appeared in rivers and lakes.

On land, some amphibians like temnospondyls survived, but many lived in water or near water. The first modern amphibians, like the ancestors of today’s frogs, also appeared. Reptiles changed a lot during this time. Some reptiles, such as tanystropheids with very long necks, thrived. True archosaurs split into two groups, one leading to birds and the other to crocodiles. Early pterosaurs and dinosaurs also appeared. Marine reptiles, such as sauropterygians and ichthyopterygians, lived in the oceans.

The Triassic was also when mammal-like reptiles known as therapsids began to disappear. Some cynodonts evolved into the first true mammals near 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. They show how life recovered after an event that affected 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. Fossils from this site help scientists learn about life from about 242 million years ago.

Triassic–Jurassic extinction event

The Triassic Period ended with a big loss of many plants and animals, especially in the oceans. 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.