A Middle Triassic Cassian‐type fauna (Pelsa‐Vazzoler Lagerstätte) and the adaptive radiation of the Modern evolutionary fauna

Abstract

We describe a high‐diversity silicified assemblage of marine molluscs (Pelsa‐Vazzoler Lagerstätte) from the upper Ladinian of the Agordo Dolomites (northeastern Italy). New data on the Triassic rebound, after the end‐Permian mass extinction, constrain it to an interval of relatively stable climatic conditions. This Lagerstätte, in the Sciliar Formation, yields a structure comparable to the famous lower Carnian San Cassiano Lagerstätte and suggests that the radiation of benthic molluscs may have occurred as early as the late Middle Triassic. We classified more than 4800 Cassian‐type molluscs, measuring abundance distributions of 109 species, including one new family (Rhaetidiidae), three new genera (Pelsia, Gaetania, Agordozyga) and 21 new species: Grammatodon egortinus, Modiolus friesenbichlerae, Myoconcha busattae, Schizogonium letiziae, Predazzella? monarii, Eucycloscala nitida, Tricolnaticopsis elongatus, Cortinella stricta, Triadoskenea alpicornu, Trachynerita tenuicostata, Coelostylina civettae, Gaetania coronata, Agordozyga caprina, Euthystylus dincae, Zygopleura elongata, Diatrypesis agordina, Cryptaulax pelsae, Pseudoscalites karapunari, Promathildia gracile, Camponaxis ladinica and Striactaeonina ingens. In this fauna, associated with tropical carbonate platforms, epifaunal filter‐feeding bivalves adopted new antipredatory features and gastropods conquered new ecospace, including parasitism and microcarnivory on sponges and scleractinian corals. Small size was an advantage in an ecosystem of small, isolated patch reefs. This is how, where and when caenogastropod and heterobranch snails (groups that today dominate global marine diversity) began their rise in the marine benthos. The origins of some evolutionary innovations that are key to our understanding of the time and place of the Mesozoic Marine Revolution, are therefore pushed back to the Middle Triassic.