This large bivalve mollusc ruled the mud flats of the largest Triassic carbonate platform, for good reason called Dolomia Principale (Main Dolomite), stretched throughout the Dolomites and over a good part of the western Tethys. The dolomitization process almost always dissolved its thick shell, leaving only the sediment that filled the shell itself.

This bivalve mollusc makes up 90 percent of the fossils in the Pachycardia Tuff layers, which explains where they got their name from! These layers of coarse sediments (sandstone and conglomerates with rounded gravel of volcanic origin) are only found on Alpe di Siusi and represent deposits containing organisms that lived on the beaches of the volcanic islands, where the waves were very strong. Pachycardia has a very thick shell that made it resistant to the force of water.

Daonella are typical bivalve molluscs from the Middle Triassic period (247-237 million years). Their thin, flat shells were reinforced with radiating ribs that, beside their circular to sub-triangular shell outline, characterize the species. D. lommeli is one of the largest and most widespread species. As it is often found in rocks deposited in the open sea, it is presumed that it lived afloat the water thanks to its thin, wide shell.

Along with few other molluscs, the bivalve genus Claraia is among the organisms that best adapted to the critical environmental conditions of the coastal sea waters after the extinction event taking place 252 million years ago. Remains of other organisms are only occasionally found in association with Claraia, showing just how extreme the environment was. It was apparently the only one surviving under these conditions.