During the Devonian, Carboniferous, and Permian periods, several, severe
 changes occurred in the configuration of the tectonic plates, resulting in  
 major changes in the areas of oceans and epicontinental seaways. The relatively
 rapid rate at which ammonoids evolved allows detailed information on
 changes in the environmental conditions and geological events that influ-
 enced the habitats of these animals (House, 1985a,b, 1989; Kullmann, 1983,
 1985).
  Periods of mass extinction have been discussed intensively, first by Schin-
 dewolf (1954) and Newell (1967) and then in detailed ammonoid studies by
 House (1983, 1989).  Fluctuations of sea level in connection with anoxic
 oceanic overturns are regarded as major factors causing drastic changes in
 ammonoid diversity (House, 1989, Becker, 1993a,b).  Global geotectonic influ-
 ences,i.e.,intervals of tectonic unrest in various parts of the world, are also
 thought to be reflected by changes in diversity (Kullmann, 1985; Korn and
 Kullmann, 1988).  However, the fact that major diversity fluctuations seem to
 have occurred more frequently and more intensely during the Devonian than
 during the Carboniferous and the Permian, which were characterized by
 widespread geotectonic activities (Becker and Kullmann, Chapter 171 this
 volume), is in direct contrast to the idea th"t plate tectonic movements were
 the major source of perturbations in the ammonoid record (Kullmann, 1994).
 In addition, a number of as yet unidentified environmental, tectonic, and
 genetic factors (Ziegler and Lane, 1987) may also have contributed to changes
 in ammonoid diversity.
   The overall diversity of Paleozoic ammonoids is reflected by the number
 of recognized species during the major time units of the Devonlan, Carbonif-
 erous, and Permian (Table I).  In this paper, the number of Paleozoic taxa
 (species and genera) is based on data from the GONIAT data base, Tübingen,
 version 2.40 (Kullmann et al., 1993-1 Korn and Kullmann, 1993, Korn et al.,
 1994).  Because this database is not yet completed, these numbers represent
 approximations of the real numbers of taxa; taxa based on insufficient material
 or inadequate descriptions are omitted.  Representatives of the Bactritida are
 extremely rare and insufficiently known and are, therefore, not included in
 this study.