156 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



crumpled. Study of the damaged specimens suggests that archaeopyle formation 

 may occur by loss of the anterior ventral surface (see Text-fig. 40) : such a method 

 has not been observed in any other known dinoflagellate cyst. Further studies are 

 needed before this suggestion can be confirmed ; but certainly, no specimens yet 

 observed give suggestion of archaeopyle formation by more familiar methods. 



The holotype and paratypes of this species are in the collections of the Micro- 

 palaeontology Laboratory, Department of Geology, University of Sheffield. 



CONCLUSIONS 



The characteristics and known stratigraphic distribution of the twenty-one valid 

 genera here considered are summarized in the accompanying Table. At present, no 

 coherent picture emerges regarding the stratigraphical distribution of the different 

 archaeopyle types : fuller studies of critical intermediate assemblages are clearly 

 necessary. All four principal modes of archaeopyle formation were operative by the 

 Upper Jurassic and it seems likely that these represent four divergent lines of 

 evolution. Rhaetogonyaulax may represent a trend towards non-tabulate cysts of 

 spindle-shape ; Acanthogonyaulax towards non-tabulate, densely spinose forms ; 

 and the ancestors of Hystrichosphaera may well be found in species of Gonyaulacysta 

 with progressively lower crests bearing progressively higher spines. 



Dinoflagellate cysts basically having a Gonyaulax-type tabulation are shown to be 

 dominant elements in Upper Jurassic and Lower Cretaceous assemblages, declining 

 in importance (Hystrichosphaera excepted) in the Upper Cretaceous ; rare in the 

 lowest Tertiary ; and apparently not represented after the Eocene. Their strati- 

 graphic importance is thus greatest in the Upper Jurassic and Lower Cretaceous, 

 where rapid evolution and limited vertical range combine to render many species 

 satisfactory zonal indices. 



