The Permian was a period of extremely sharp climatic zonation and consequent faunal differentiation. Definite trends in marine faunal composition and evolution are recognized in both the Permian polar regions (Boreal and Notal) and also in the tropical region.

In the Permian polar regions there was a significant faunal impoverishment and many groups of higher taxonomic rank were absent. The fauna of the Boreal and Notal areas consisted mainly of endemic genera or, rarely, families. Thus groups with a bipolar distribution, but absent from the tropical area, represented a major part of this fauna. A characteristic feature of the polar regions in the Late Permian is the reduction in the rate of the appearance of new forms and the gradual extinction of already existing Paleozoic groups. Thus only a few taxa survived the close of the Permian and passed into the Triassic in these regions.

In the tropical area, many endemic orders and superfamilies were present. Here the extinction of most of the Paleozoic groups began later and proceeded more rapidly than in the polar regions. Strata containing surviving Paleozoic groups will often yield typical Mesozoic faunal elements causing difficulties for those attempting to accurately define the Permian-Triassic boundary. A significant number of taxa of high rank thus pass from the Permian into the Triassic. Late in the Permian, the tropical area appears to have been a refuge for marine faunas. In Early Triassic times a general climatic amelioration took place, the seas transgressed and an abundant fauna migrated north and south from Tethys to occupy the polar regions. The beginning of this widespread transgression coincides with the base of the Otoceras Zone and it therefore seems appropriate to draw the Permian-Triassic boundary at this level.

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