The last two geological divisions of the Triassic Period, the Carnian and Norian stages, are marked by high rates of extinction and include at least one and possibly two episodes of mass extinction, separated by 12 to 17 million years. The Late Norian (the terminal period of the Triassic) is marked by one of the five largest mass extinctions of the Phanerozoic Era. At least 23 percent of all marine and terrestrial families and 54 percent of marine genera became extinct during the latest Triassic. This extinction, which occurred about 200 million years ago, had a dramatic impact on both terrestrial and marine organisms, although the effects may have been felt on land several hundred thousand years before the effects were felt in the sea. Although the cause of this event is unclear, it is likely to be related to marine regression and widespread anoxia (that is, lack of oxygen) in the oceans. In addition, the boundary between the Middle and Late Carnian stages marks a dramatic faunal turnover in terrestrial vertebrates as well as some marine groups, and it may represent an important extinction event. This event may have been climatically induced.
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