Patterns of bivalve biodiversity from Early to Middle Triassic in the Southern Alps (Italy): regional vs. global events

The Triassic records the severe effects of the end-Permian mass extinction and the beginning of the Mesozoic / modern adaptive radiation of marine invertebrates. As far as bivalves are concerned, a group which was relatively unaffected by the extinction, diversification started in the Middle Triassic, more particularly during the Anisian. Thus, the interval between the basal Triassic (Induan) and Anisian is crucial in analysing the biodiversity changes and diversification patterns of these molluscs. This analysis is discussed at genus level and based on the regional records of the Southern Alps. Biodiversity changes are examined in connection with palaeogeographic evolution, the conquest of vacated ecologic niches, and climatic changes. The lowest point of generic richness, as an immediate aftermath of the P/T mass extinction, is recorded in the Induan units (Mazzin and Siusi Mbs) of the Werfen Formation, which include only five genera, numerically dominated by Claraia and Unionites. Twelve genera are represented in the Olenekian Campil, Val Badia and Cencenighe Members. Such an increasing richness is mostly connected with the recovery of pectinids and endobyssate bakevellids; typical marine biota such as ammonoids (Tirolites and Dinarites fauna) and crinoids also appear in this time. The richness and structure of the Early Anisian (Bithynian) bivalve assemblages contained in the Gracilis Formation (and equivalent units of Southern Alps) do not significantly change (15 genera) in comparison with those from Olenekian. The most relevant increase in bivalve genera (31 genera) occurs in the late Anisian, mostly in the Pelsonian Calcare di Recoaro and equivalent units, when a strong tectonic phase considerably differentiated the marine habitats and ecological niches in the Southern Alps. However, bivalve Pelsonian radiation is recorded in other Tethyan and peri-Tethyan areas, and coincides with the reappearance of marine stenotopic hard bottom dwellers and reef communities, including scleractianian corals. Therefore, it represents a global bioevent which indicates the fully recovery of the physical and chemical limiting factors of the sea water. In the Southern Alps, the diversification of bivalves after the end-Permian extinction occurred in two steps: at the end of Smithian (early Olenekian), when their recovery began and in the Pelsonian (late Anisian) when their radiation parallels with the recovery of hard bottom communities and metazoan reefs. Both the events occurred at the end of humid phases.