The Triassic-Jurassic extinction event, which occurred worldwide 200 million years ago, is one of five major mass extinctions throughout the last 600 million years. The extinction wiped out 20% of marine families, 30% of marine genera, many therapsids (formerly called "mammal-like reptiles", although they are neither), all large crurotarsans (non-dinosaur archosaurs, ancestors of modern-day crocodiles, alligators, and gavials), and most large amphibians, which up until that point had been the dominant terrestrial fauna. It is estimated that 50% 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.

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 250 million years ago. The first mass extinction of the Mesozoic era, the Triassic-Jurassic extinction is often seen as setting the stage for the dominance of the dinosaurs. Before the mass extinction, dinosaurs represented about 1-2% of faunas, but after, as they took over niches from extinct species, they came to represent about 50-90% of faunas.

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 like global warming/cooling, methane hydrate release, anoxia in the oceans, and more. Until more evidence emerges, we can't really be sure. Unfortunately, as ocean crust recycles itself about every 50 million years, any major impact craters from meteors or comets would likely be erased.

The period of time immediately after the Triassic-Jurassic extinction was crucial, because the empty niches could have been filled either by reptiles like the dinosaurs, or 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 (and even humans) could have evolved over 150 million years earlier than they actually did. It may have just been a fluke that delayed mammalian evolution until the dinosaurs themselves succumbed to another mass extinction 135 million years later.