Triassic
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Triassic

Started off very slow due to the Permian extinction event. The first mammals evolved at the start, the pterosaurs evolved at the end. Ends with another mass extinction that allowed dinosaurs to take hold.

Climate

At the beginning of the Triassic, most of the continents were concentrated in a giant supercontinent called Pangaea.

Image credit: Rainer Lesniewski via Getty Images
Image credit: Rainer Lesniewski via Getty Images

The Triassic climate was generally very dry, with a vast desert interior with very hot summers and cold winters. Coastal areas experienced a highly seasonal monsoon climate.

Late in the Triassic, rifting occurred between the northern and southern parts of Pangaea. Eventually this would seperate the Pangea into two smaller supercontinents; Laurasia and Gondwana, which would be completed in the Jurassic.

Evolutionary traits and species

Marine life

The oceans were massively depopulated following the Permian-Triassic extinction, where an estimated 95% of species went extinct due to high carbon dioxide levels.

Later in the Triassic rocky corals evolved, and (check this) the first coral reefs were built.

A group of reptiles (the ichthyosaurs) returned to the oceans. They evolved to look like modern day dolphins, breathed air and gave birth to live young. Eventually they became the top predator of the Triassic oceans.

Ichthyosaurs

During the early Triassic Period, ichthyosaurs evolved from a group of, as yet, unidentified land reptiles that returned to the sea, in a development parallel to that of the ancestors of modern-day dolphins and whales, which they gradually came to resemble in a case of convergent evolution. They were particularly abundant in the later Triassic and early Jurassic Period, until they were replaced as the top aquatic predators by another marine reptilian group, the Plesiosauria, in the later Jurassic and Cretaceous Period. In the Late Cretaceous ichthyosaurs became extinct for as yet unclear reasons. -Wikipedia, Ichthyosaur

Ichthyosaurs by John Sibbick
Ichthyosaurs by John Sibbick

Insects

Grasshoppers evolved during the Triassic.

Plants

Mosses and ferns survived in coastal regions.

Pterosaurs

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First dinosaurs

The first dinosaurs diverged from a branch of archosaurs during the Middle to Late Triassic epochs.

First mammals

The first mammals originated in the late Triassic, and were nocturnal insectivores.

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Lystrosaurus

Lystrosaurus was by far the most common terrestrial vertebrate of the Early Triassic, accounting for as many as 95% of the total individuals in some fossil beds.

Very few large synapsids survived the Permian-Triassic extinction event, and one form, Lystrosaurus attained a widespread distribution soon after the extinction.

Lystrosaurus had only two teeth, a pair of tusk-like canines, and is thought to have had a horny beak that was used for biting off pieces of vegetation. Lystrosaurus was a heavily built, herbivorous animal, approximately the size of a pig. The structure of its shoulders and hip joints suggest that Lystrosaurus moved with a semi-sprawling gait. The forelimbs were even more robust than the hindlimbs, and the animal is thought to have been a powerful digger that nested in burrows.

Despite its wide distribution, it was the archosaurs who became the dominant land vertebrates in the early Triassic.

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Archosaurs

Archosaurs dominated the Triassic.

From the Early Triassic:

And the late Triassic:

Triassic–Jurassic extinction event

The Triassic–Jurassic (Tr-J) extinction event, sometimes called the end-Triassic extinction, marks the boundary between the Triassic and Jurassic periods, 201.3 million years ago,[1] and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans.

[T]he most well-supported and widely-held theory for the cause of the Tr-J extinction places the blame on the start of volcanic eruptions in the Central Atlantic Magmatic Province (CAMP). The CAMP is the geographically largest known large igneous province, and was responsible for outputting a high amount of carbon dioxide to induce profound global warming and ocean acidification.

The really intriguing thing is all early dinosaurs are very rare. Outnumbered by the land crocs. It looks like the late triassic extinction of land crocs released dinosaur potential. -Robert Bakker, Reddit AMA