Vol 51, No 6 (2017)

Eastern Europe shows the most complete in the world continuous sequence of continental Permian and Triassic deposits, which allows the development of tetrapod faunas over more than 17 successive stages to be traced. The newly obtained data on transitional Vyazniki and Sundyr tetrapod faunas provide more complete characteristics of the Severodvinian (Late Guadalupian, pre-Lopingian) and Permian-Triassic ecological crises and the ways of replacement of the dominant vertebrate groups of Eastern Europe.

Permian–Triassic event is usually regarded as the greatest mass extinction in the Earth’s history, although detailed studies have shown that it was not very severe. Localities of fossil insects in European Russia, Tunguska and Kuznetsk basins, and Mongolia provide a unique (the best in the world) opportunity to study the preparation, course of the crisis, and restoration of the biota after it. It is generally believed that climatic changes causing the crisis resulted from eruption of the Siberian traps, so that localities of intertrappean deposits were undoubtedly formed during the crisis. Sedimentation conditions of volcanogenic deposits provide the most detailed time resolution, so that the crisis processes can be investigated in detail. The dynamics of insect diversity in the Paleozoic and basal Mesozoic shows that mass extinctions were absent, although many groups disappeared for some time from the taphonomic window. The crisis events in ecosystems appear earlier than events usually considered as the reason for crisis. The analysis of oryctocoenoses from localities of the intertrappean beds has shown that, during the formation of traps on the mountain plateau, rather diverse ecosystems were retained, including those of the forest formations. They are a source of information, allowing restoration of ecosystems at the end of the Early Triassic.

The erect feeding appendages of paracrinoids, brachioles of typical blastozoans and arms of crinoids are morphologically similar in their terminal growth, biserial cover plates, and pinnulation. This is attributed to the inducing effect of the radial ambulacral canal on their growth mode. The uniserial brachioles of Laurentian paracrinoids are homologous to the biserial brachioles of the Baltic Achradocystites and Heckerites, and those of other blastozoans. Based on this assumption, the two Baltic genera, which have a brachiole system plesiomorphic for paracrinoids, and a similar morphology of the theca, are assigned to this class. Brachiolars in brachioles are a new development, homologous to the flooring plates of the food groove and, where present, are the continuations of these plates beyond the theca. The uniserial brachioles of Laurentian paracrinoids evolved from the biserial brachioles as a result of a gradual shift of brachiolars in the neighboring rows and their subsequent fusion in pairs. Brachials in crinoidal arms are a new development that evolved as distal serial growth of radial plates under the induced influence of the incipient radial canals emerging from the closed vestibular cavity, which was an ontogenetic innovation in crinoids. The transformation of a nonorganized small-plated theca into a large-plated, and completely or partly symmetrized theca, or vice versa is possible and results from accelerated or retarded growth of some plate generation in relation to the growth rate of the theca.