Mississippian metazoan reef recovery was the longest in reef history [27]. Skeletons of reef-forming biota prior to the Late Devonian crises were constructed of low magnesium calcite. There is evidence that «calcite» to «aragonite» seas transition occurred in the mid-Tournaisian (most likely carbonate sediments of that time composed primarily of aragonite). Thereby reef recovery could be hindered by shift in chemical composition of seawater throughout the mid-Tournaisian. But further changes remains unclear.