The Triassic period, which lasted from 251 to 199 million years ago, is the first of the Mesozoic era’s geologic periods. During the Triassic, most of the Earth’s landmass was encased in Pangaea, a supercontinent. The continent’s interiors were vast, dry deserts, with swamps and forests circling the edges. There was no evidence of glaciation during the Triassic, which was a warm and dry period. Indeed, the poles were moist and warm, providing ideal conditions for reptiles.
The Triassic began immediately after the world’s worst mass extinction, the Permian-Triassic extinction event. 96 percent of marine species and 70% of terrestrial vertebrate species went extinct in less than 10,000 years. Many paleontologists refer to this event as “the Great Dying” because it was so dramatic. It’s thought to have been caused by a number of factors, including a series of extreme and continuous volcanic eruptions known as “traps” today.
The Permian-Triassic extinction event wreaked havoc on marine life, wiping out vast swaths of biodiversity. Trilobites, eurypterids (sea scorpions), blastoids (an ancient echidnoderm), jawless fish, and armored fish (placoderms) all died out. Ammonites, a cephalopod resembling the modern nautilus on the surface, were nearly extinct, but a single lineage survived and diversified. Several coral species have become extinct. All of the large amphibians died on land, making room for reptiles and synapsids, the ancestors of mammals. Almost all large herbivores died, despite the fact that they needed the most healthy foliage to survive.
Despite the fact that the largest amphibians died, many medium-sized amphibians survived, indicating that the land extinction was not as severe as the marine extinction. The temnospondyls, a type of amphibian, thrived during this time period. The Permian-Triassic period is also known as the only known mass extinction of insects, with many Mesozoic insect fossil groups differing significantly from Paleozoic groups.
The most important evolutionary event of the Triassic was the gradual takeover of terrestrial ecosystems by reptiles, particularly the menacing archosaurs — today’s birds and crocodiles — who would eventually evolve into true dinosaurs near the end of the period. Reptiles competed against synapsids, which had dominated the Permian period before essentially defeating them and competing only against themselves. Synapsids would be relegated to stepping in the shadows of large reptiles for hundreds of millions of years, only to reappear 65 million years ago after the K-T extinction event. The “Triassic Takeover” was a progressive event that occurred around the middle of the Triassic Period.
The change in animal life between before and during the Triassic is so dramatic that it is placed at the start of one of the three major geologic periods with multicellular life, the Paleozoic.