At the start of the Triassic period, 250 million years ago, the earth was just recovering from the Permian/Triassic Extinction, which witnessed the demise of over two-thirds of all land-dwelling species and a whopping 95 percent of ocean-dwelling species. In terms of animal life, the Triassic was most notable for the diversification of archosaurs into pterosaurs, crocodiles and the earliest dinosaurs, as well as the evolution of therapsids into the first true mammals.
Climate and Geography During the Triassic period, all of the earth's continents were joined together into a vast, north-south landmass called Pangaea (which was itself surrounded by the enormous ocean Panthalassa). There were no polar ice caps, and the climate at the equator was hot and dry, punctuated by violent monsoons; some estimates put the average air temperature in most of the continent at well above 100 degrees Fahrenheit. Conditions were wetter in the north (the part of Pangaea corresponding to modern-day Eurasia) and the south (Australia and Antarctica).
Terrestrial Life During the Triassic Period
The Permian period was dominated by amphibians, but the Triassic marked the rise of the reptiles--notably the archosaurs ("ruling lizards") and therapsids ("mammal-like reptiles"). For reasons that are still unclear, the archosaurs held the evolutionary edge, muscling out their "mammal-like" cousins and evolving by the middle Triassic into the first true dinosaurs like Eoraptor and Herrerasaurus. Some archosaurs, however, went in a different direction, branching out into the very first pterosaurs (Eudimorphodon being a good example) and a wide variety of ancestral crocodiles, some of them two-legged vegetarians. Therapsids, in the meantime, gradually shrunk in size, and the first mammals of the late Triassic period were represented by small, mouse-sized creatures like Eozostrodon and Sinoconodon.
Marine Life During the Triassic Period
Because the Permian Extinction depopulated the world's oceans, the Triassic period was ripe for the rise of early marine reptiles, not only unclassifiable, one-off genera like Placodus and Nothosaurus but the very first plesiosaurs and a flourishing breed of "fish lizards," the ichthyosaurs. (Some ichthyosaurs attained truly gigantic sizes; for example, Shonisaurus measured 50 feet long and weighed in the vicinity of 30 tons!) The vast Panthalassan Ocean soon found itself restocked with new species of prehistoric fish, as well as simple animals like corals and cephalopods.
Plant Life During the Triassic Period
The Triassic period wasn't nearly as lush and green as the later Jurassic and Cretaceous periods, but it did see an explosion of various land-dwelling plants, including cycads, ferns, Gingko-like trees and seed plants. (A new study has shown that flowering plants appeared toward the middle of the Triassic period, 100 million years earlier than previously believed.) Part of the reason there were no plus-sized Triassic herbivores (along the lines of the much later Brachiosaurus) is that there simply wasn’t enough vegetation to nourish their growth!
The Triassic/Jurassic Extinction Event
Not the most well-known extinction event--and certainly a fizzle compared to the earlier Permian/Triassic extinction and the later Cretaceous/Tertiary (K/T) extinction--the Triassic/Jurassic extinction witnessed the demise of various genera of marine reptiles, as well as large amphibians and certain branches of archosaurs. We don't know for sure, but this extinction may have been caused by volcanic eruptions, a global cooling trend, a meteor impact, or some combination thereof.
Next: The Jurassic Period