The Triassic-Jurassic extinction event is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. 20% of all marine families and all large Crurotarsi (non-dinosaurian archosaurs), some remaining therapsids, and many of the large amphibians were wiped out. The opening of so many ecological niches allowed the dinosaurs to assume the dominant roles in the Jurassic.

Several explanations for this event have been suggested, but all have unanswered challenges. Gradual climate change or sea-level change during the late Triassic does not explain the suddenness of the extinctions in the marine realm. Asteroid impact is possible, but no impact structure can be dated to coincide with the Triassic-Jurassic boundary. Massive volcanic eruptions, specifically the flood basalts of the Central Atlantic Magmatic Province , would release carbon dioxide or sulfur dioxide which would cause either intense global warming (from the former) or cooling (from the latter). However, the isotopic composition of fossil soils of Late Triassic and Early Jurassic show no evidence of any change in the CO₂ composition of the atmosphere. More recently however, some evidence has been retrieved from near the Triassic-Jurassic boundary suggesting that there was a rise in atmospheric CO₂ and some researchers have suggested that the cause of this rise, and of the mass extinction itself, could have been a combination of volcanic CO₂ outgassing and catastrophic dissociation of gas hydrates. Gas hydrates have also been suggested as one possible cause of the largest mass extinction of all time; the so-called "Great Dying" at the end of the Permian Era.