Pithonelloideae, the dinoflagellates that dominated the boreal shelf-assemblages in the Maastrichtian, rapidly became extinct in the Danian. Typical bloom abundances directly below the clay confirm that there is no gradual decline in number well before the event. A few specimens can be found above the clay layer. Those may indicate a short-term recovery but more likely are reworked. Despite the mass mortality of Pithonelloideae the clay layer shows a relatively high diversity of calcareous dinoflagellates.
Many species appear to have been well adapted to the exceptional environmental conditions.
The forms that became dominant in the Danian were already present in the Late Maastrichtian but cannot be found in the Fish Clay.