Calculations of the diversity development of Carboniferous ammonoids on the basis of species have been
made possible by utilising the data stored in the database GONIAT. GONIAT is especially designed as a tool
for investigations on the systematics of Palaeozoic ammonoids, their palaeogeographic distribution and their
 biostratigraphic range.
 
  The quantitative and qualitative analysis of the present study concentrates on the epochs before and after two
fundamental critical intervals in ammonold evolution: the time around the Devonian/Carboniferous boundary
(DCB) and around the Mid-Carboniferous boundary (MCB) between the Lower and the Upper Carboniferous
subsystems. In both cases several major groups became extinct some time before or at the respective boundary,
and other groups entered after the boundary.  Only a few species may have crossed the DCB and the MCB,
respectively.

   The ammonoid changeover in both critical intervals have characteristics in common: The morphological
differences concern mainly the shell characteristics of their early whorls; significant characters are also the
ornamentation and the generel outline of the conch including the size. The similarities in ammonoid evolution are

 1) the stepwise decrease in diversity before the boundary from a high diversity phase to a low diversity phase,

 2) after the low point the origin of new groups with wide, but small whorls and a low grade of involution,

 3) the onset of a high diversity phase with complicated conch forms and ornamentation.  

Possible causes are discussed.