A number of extinctions occurred at, or near to the close of the Triassic period (Table 3), and the end-Triassic event,based on a number of families that died out per million years, was almost as extensive as the K-T extinction. In fact, if the criterion used is the percentage of genera per million years, the extinction peak is even greater than that at the end of the Cretaceous. This, however, is made particularly apparent by the short time-span of the most recent Triassic unit. Estimates of its age range from 1-4 my, and are based on the extinction of marine invertebrate animals (Benton 1997a). As with other mass extinctions, the main possible causes involved are climate change, volcanism, bolide impact and changes in sea level. The latter would have been associated with anoxia. The evidence for each of these hypotheses has been reviewed by Hallam and Wignall (1997), but no conclusion was reached as to which is the more plausible.
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