The Triassic-Jurassic extinction event, which occurred worldwide about 200 million years ago, is one of five major mass extinctions throughout the past 600 million years. It is estimated that 50 percent of all species went extinct. Some paleontologists call the Triassic-Jurassic extinction the second greatest mass extinction of prehistory. It occurred in the blink of a geological eye, over no more than 10,000 years.
This extinction event wiped out about 20 percent of marine families and 30 percent of marine genera. It wiped out many therapsids, which also have been called "mammal-like reptiles," although they were neither mammals nor reptiles. Also wiped out were all large crurotarsans — non-dinosaur archosaurs, the ancestors of modern-day crocodiles, alligators and gavials — and most large amphibians, which up until that point had been the dominant terrestrial fauna.
The Triassic-Jurassic extinction came just 50 million years after the greatest extinction event in the era of animal life, the Permian-Triassic extinction of about 250 million years ago. The first mass extinction of the Mesozoic era, the Triassic-Jurassic extinction event is often seen as setting the stage for the dominance of the dinosaurs. Before the mass extinction, dinosaurs represented about 1-2 percent of the Earth's fauna, but after it, as they took over niches from extinct species, they came to represent about 50-90 percent of the fauna.
The cause of the Triassic-Jurassic extinction event is unknown. Unlike some of the other mass extinctions of the past, little evidence has coalesced around any particular interpretation. Some hypotheses include meteor impact and volcanic traps, or massive sustained eruptions over the course of a million years.
Volcanic eruptions could have triggered secondary and tertiary effects such as global warming or cooling, methane hydrate release, the reduction of oxygen levels in the oceans and more. Until more evidence emerges, scientists can't be sure. That might not occur, however. Ocean crust recycles itself about every 50 million years, so any major impact craters from meteors or comets likely has been erased.
The period of time immediately after the Triassic-Jurassic extinction was crucial, because the empty niches could have been filled either by reptiles such as the dinosaurs, or by the therapsids, which includes the ancestors of mammals. It ended up with the dinosaurs being victorious, but if the therapsids had prospered and diversified instead, mammals might have evolved more than 150 million years earlier than they actually did. It might have just been a fluke that delayed mammalian evolution until the dinosaurs themselves succumbed to another mass extinction about 135 million years later.