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The massive extinction of invertebrate shallow marine organisms during late Permian time and the resurgence of new families during Triassic time are well documented and appear to be related to sharp reduction in the habitat areas of the major groups as shallow sea environments became increasingly reduced. Sharp changes occurred in the terrestrial vertebrates, but the patterns of change were distinctly different and through Permian and Triassic periods occurred four times. The first major decrease in numbers of families took place at the end of early Permian time (the Kungurian-Guadalupian transition). The loss comprised the majority of families and their immediate derivatives that had persisted from the coal measures into the lower Permian. A second sharp decrease of reptilian families took place at the end of Permian time and included primarily therapsid reptiles involved in the therapsid radiation of the middle and upper Permian. A few “advanced” families existed. A third episode of loss of families occurred at the end of Early Triassic time and a fourth at the end of Triassic time. The last involved primarily archosaurian reptiles, which had replaced the therapsids.

The changes that took place and the causes of severe decreases in families may be viewed from four perspectives: stochastic events; physical changes extrinsic to the organisms; biological changes, both intrinsic and extrinsic to the organisms; and catastrophic events. The strong tendency toward phyletic attainment of large size clearly played an important role in familial decreases as physical and biological changes and geographic distributions changed. No definitive evidence that catastrophe had any role in any of the familial extinctions exists at present, but that this may have played a role in the loss of families during Permian and Triassic times cannot be fully ruled out.

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