Bryozoan diversity in Southern Siberia at the Devonian–Carboniferous transition: New data confirm a resistivity to two mass extinctions

Two spectacular mass extinctions, namely the Kellwasser and the Hangenberg events, affected the Earthʹs biota during the Late Devonian–Early Carboniferous. Diverse assemblages of bryozoans are known from the Fasnian–Tournaisian deposits of Southern Siberia, which includes such regions as the Kuznetsk Basin, the Kolyvanʹ–Tomʹ Zone, the Rudny Altay, and the Gorny Altay. Our new data establish the stratigraphic ranges of 154 species, which belong to 56 genera and 21 families. These data have been analyzed quantitatively to measure the patterns of diversity dynamics and the changes in the taxonomic diversity structure. The species diversity increased at the Frasnian–Famennian transition. After a short-term, but dramatic decrease in the middle Famennian, the total number of species accelerated in the late Famennian and the Tournaisian. A similar pattern occurred for both genera and families, although the radiation at the Frasnian–Famennian transition was not as large, and total family diversity decreased slightly in the Tournaisian. Changes in total species diversity differed between orders of bryozoans. Changes in taxonomic diversity structure were moderately rapid. The Tournaisian species diversity was governed by the other genera comparing to the early time intervals. Thus, our regional data suggest that bryozoans generally survived both the Frasnian/Famennian and the Devonian/Carboniferous mass extinctions, confirming similar conclusions made earlier with global data. However, a mid-Famennian regional crisis (diversity drop) in the evolution of bryozoans is documented within Southern Siberia. Bryozoan radiations coincided with pulses of basin deepening that occurred during both mass extinction intervals, perhaps explaining why bryozoans were resistant to extinction. The mid-Famennian crisis might have been caused by a rapid regression coupled with basin shallowing.