The Triassic-Jurassic extinction occurred 199.6 million years ago, just after the Age of the Dinosaurs began. It is one of history’s five largest mass extinctions, comparable to the K-T extinction, which wiped out dinosaurs 135 million years later. Dinosaurs would become even more dominant as a result of this mass extinction than they were previously. The Triassic-Jurassic extinction marks the end of the Triassic and the beginning of the Jurassic periods, both of which are part of the larger Mesozoic era. Because it lacked the magnitude of the Permian-Triassic extinction (which wiped out 99.7% of all species) and the proximity of the Cretaceous-Tertiary extinction, the Triassic-Jurassic extinction is one of the least studied of the mass extinctions.
The cause of the Triassic-Jurassic extinction is unknown, and all current theories — gradual climate change, asteroid impact, and supervolcano eruptions (the usual suspects) — are based on speculation rather than evidence. Climate change, for example, appears unlikely due to the sudden extinction of marine life, while the idea of an asteroid impact is speculative at best, as no large craters dating back to around 199.6 million years ago have been discovered. Although the Central Atlantic Magmatic Province erupted around this time, palynological studies indicate that the mass extinction occurred prior to the earliest volcanic eruptions there.
All of the giant amphibians (only the Gondwanan — Antarctic, South American, and Australian — taxons, Brachyopidae and Chigutisauridae, survived), early erect-limbed archosaurs that competed with dinosaurs (thecodonts and crurotarsans), and many large families of therapsids were among the vertebrate victims of the Triassic-Jurassic (the descendants of mammals, formerly known as “mammal-like reptiles,” though they are not reptiles). These voids left niches undefended, ripe for dinosaur encroachment. Dinosaurs were able to solidify their position as the most important taxon (superorder) of terrestrial animals, a position they would hold for over a hundred million years.
The extinction event that occurred between the Triassic and Jurassic periods is thought to have wiped out 50% of all living species, including 20% of marine families and 30% of marine genera, including many ammonite families. The majority of straight-shelled nautiloid cephalopods, as well as a few more complex invertebrates, died.