Insects at the borderline between the Permian and the Early Triassic (Urzhum - Olenek Age) and the problem of Permian-Triassic biodiversity crisis

Distribution of 115 insect families is considered in 15 local assemblages of European Russia, Siberia, Australia and South Africa. The assemblage ages embrace the Urzhum stage of the Middle Permian, the Late Permian, and the transitional Permian-Triassic interval. The assemblages are ordered statistically using two criteria. Ordination after the appearance of a fauna, that is, relation of the number of younger vs. older families, is found to be generally consistent with the stratigraphic data. The method of minimizing the gaps (ghost ranges) in distribution of the families is useful in interpreting the results. Urzhum time is characterized by the balance of emergence and extinction of families (counted as their first and latest appearances, respectively). In Severodvinsk and particularly in Vyatka time, the number of first appearances was decreasing resulted in prevailing extinction. In the transitional Permian-Triassic interval, the emergence of new families accelerated. Initially, the appearance of assemblages was typically Paleozoic (with older families prevailed). It changed gradually, so as by the end of Vyatka time it turned to be quite post-Paleozoic. Diversity was the highest in Severodvinsk time, and it halved at Vyatka time and at the transition interval. However, if we consider transitional families (those not found on a particular interval, but known before and after), the extinction rate reduces to one-third. And when normalized after the material volume, the diversity drop decreases up to a quarter. There was no mass extinction found at the end of the Permian, and the less so at the Permian-Triassic boundary and during the Lower Triassic. Structure of the Permian-Triassic diversity crisis is similar to that of the Cretaceous crisis in many respects. Since the Middle Triassic and up to now, the biodiversity kept increasing quickly and continuously. This implies that the Permian-Triassic crisis resulted in profound modification of the biosphere structure, so as its capacity increased manifold.