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The Triassic–Jurassic extinction event marks the boundary between the Triassic and Jurassic periods, 199.6 million years ago, and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. A whole class (conodonts), twenty percent of all marine families and all large crurotarsans (non-dinosaurian archosaurs), some remaining therapsids, and many of the large amphibians were wiped out. At least half of the species now known to have been living on Earth at that time went extinct. This event vacated ecological niches, allowing the dinosaurs to assume the dominant roles in the Jurassic period. This event happened in less than 10,000 years and occurred just before Pangaea started to break apart. This marked the divide between the Triassic dinosaurs and the Jurassic dinosaurs.

Statistical analysis of marine losses at this time suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions.

Several explanations for this event have been suggested, but all have unanswered challenges:

  • Gradual climate change or sea-level fluctuations during the late Triassic. However, this does not explain the suddenness of the extinctions in the marine realm.
  • Asteroid impact, but no impact crater has been dated to coincide with the Triassic–Jurassic boundary (the impact responsible for annular Manicouagan Reservoir occurred about 12 million years before the extinction event).
  • 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).
  • 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 hydrate. 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 Period.

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