The Triassic period, which extends from 251 to 199 million years ago, is the first geologic period of the Mesozoic era. During the Triassic, most of the Earth's land mass was locked in a supercontinent known as Pangaea. The interiors of the continent were vast, dry deserts, while swamps and forests circled the fringes. The Triassic was a warm and dry period, with no evidence of glaciation. In fact, the poles were moist and warm, a perfect climate for reptiles.
The Triassic started immediately after the worst mass extinction the Earth has ever seen, the Permian-Triassic extinction event. In 10,000 years or less, 96% of all marine species and 70% of all terrestrial vertebrate species went extinct. This event was so extreme that it is called "the Great Dying" by many paleontologists. It is thought to have been caused by a combination of factors, especially extreme and continuous volcanic eruptions known today as "traps."
The Permian-Triassic extinction event hit marine fauna hardest, wiping out huge amounts of biodiversity. Trilobites, eurypterids (sea scorpions), blastoids (an ancient echidnoderm), trilobites, jawless fish, and armored fish (placoderms), went completely extinct. Ammonites, a cephalopod superficially similar to the modern nautilus, almost went extinct, but a single lineage survived and diversified. Several types of coral went extinct. On land, all the large amphibians died, making room for the diversifying reptiles and synapsids, early relatives of mammals. In general, just about all large herbivores bit the dust, requiring the largest quantities of healthy foliage to survive.
Though the largest amphibians died, many medium-sized amphibians survived, as the extinction on land was clearly not as extreme as the marine extinction. The class of amphibians called temnospondyls was successful throughout the period. However, the Permian-Triassic is also the only known mass extinction of insects, with many Mesozoic insect fossil groups being distinctly different than Paleozoic groups.
The most significant evolutionary event of the Triassic was the progressive takeover of terrestrial ecosystems by reptiles, especially the menacing archosaurs — represented today by birds and crocodiles — which would evolve into true dinosaurs around the end of the period. Reptiles initially competed against synapsids, which had dominated the earlier Permian, then essentially won over them and competed just against themselves. For hundreds of millions of years, synapsids would be relegated to stepping in the shadows of large reptiles, only to emerge once again 65 million years ago, after the K-T extinction event. This progressive event, which happened around the mid-Triassic, is known as the "Triassic Takeover."
The change in animal life from before and during the Triassic is so extreme that one of the three major divisions in geologic periods with multicellular life — the Paleozoic — is put at the beginning of the period.