The Triassic-Jurassic extinction event took place 199.6 million years ago, shortly after the start of the Age of the Dinosaurs. It is one of the five largest mass extinctions in history, of similar magnitude to the K-T extinction, which would wipe out the dinosaurs 135 million years later. This mass extinction would pave the way for dinosaurs to become even more dominant than they were before it. The Triassic-Jurassic extinction is the event that separates the Triassic and Jurassic periods, both part of the longer Mesozoic era. The Triassic-Jurassic extinction is among the least-studied of the mass extinctions, as it lacks the magnitude of the Permian-Triassic extinction (which wiped out 99.7% of all species) or the proximity of the Cretaceous-Tertiary extinction.
The cause of the Triassic-Jurassic extinction is unknown, and all current proposals -- gradual climate change, asteroid impact, and supervolcano eruptions (the usual suspects) -- have a lack of evidence. For instance, climate change seems unlikely due to the sudden die-off of marine life, while the idea of an asteroid impact is speculative at best, as no known large craters date to around 199.6 million years ago. The Central Atlantic Magmatic Province did erupt around this time, but palynological studies suggest that the mass extinction occurred before the earliest volcanic eruptions there.
The vertebrate casualties of the Triassic-Jurassic were all the giant amphibians (only the Gondwanan -- Antarctic, South American, and Australian -- taxons, Brachyopidae and Chigutisauridae, survived), early erect-limbed archosaurs that competed with the dinosaurs (thecodonts and crurotarsans), and many large families of therapsids (the descendants of mammals, formerly known as "mammal-like reptiles," though they are not reptiles). These openings left niches poorly defended and ripe for dinosaur takeover. This allowed dinosaurs to consolidate their position as the leading taxa (superorder) of terrestrial animals, a reign they would continue for over a hundred million years.
The Triassic-Jurassic extinction event 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. Many simple invertebrates died, as well as a few more complex ones, include most straight-shelled nautiloid cephalopods.