The Posidonia shale provides a snapshot of the time during the Mesozoic when a fundamental change occurred in osseous fish. More advanced, skilled predatory fish gained the upper hand over primitive, ganoid-scale fish. The latter still had an armour of thick, bony scales reinforced by a shiny enamel layer.
The scales of more advanced forms had already assumed a finer form. Their internal skeleton was far more ossified and their powerful jaws contained many pointed teeth. To judge by their anatomy, they were fast, agile predatory fish, like today’s trout or pike.