io MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



II. THE MORPHOLOGY, TERMINOLOGY AND CLASSIFICATION OF 

 FOSSIL DINOFLAGELLATE CYSTS 



By C. DOWNIE & W. A. S. SARJEANT 



In the description of fossil dinoflagellates in the past, the terms used have been 

 largely borrowed from modern plankton descriptions. This is appropriate enough : 

 however, as studies of fossil dinoflagellates have developed, structures have been 

 discovered for which no terms exist and the use of descriptive terms, without 

 specification of precise meanings, has produced, on the one hand, ambiguity, on the 

 other hand, the failure to distinguish between different, albeit broadly similar, 

 structures — cf., for example, past usage of the terms " spine " and " tube " in 

 description of appendages. 



The work of Evitt (1961, 1965, in press) has gone some way towards the establish- 

 ment of precise terms for some morphological characters. In the present work, a 

 number of new terms are proposed which, it is hoped, will form a workable basis for 

 future descriptions. 



In addition, the existence of broad groupings of morphologically similar dino- 

 flagellate cysts has become apparent : these groupings appear to have considerable 

 stratigraphic meaning. The classification at present in use is criticized for not 

 taking cyst structure into account. 



Morphology and terminology, (i) Cysts and Motile Stage Thecae. 



Evitt & Davidson (1964) have described the process of cyst formation in some 

 Recent dinoflagellates and have shown that these resting cysts are of types closely 

 resembling some of the most common fossil species. They conclude that most, if not 

 all, of the fossil remains of dinoflagellates are cysts. These cysts are smaller than 

 the motile stage cell and are formed by the deposition of an ellipsoidal or spherical 

 wall some distance inside the motile stage envelope (or theca if hardened) . This wall 

 in fossil and Recent cysts is often seen to be constructed of two layers, which we 

 propose to call the endophragm and the periphragm. The outer layer, or peri- 

 phragm, usually carries extensions, either in the form of spines or as lists, which 

 extend out to the position of the formal thecal wall and appear to have acted as 

 supports during the period of cyst formation (Text-fig. 1). 



Many kinds of fossil dinoflagellate cysts are equipped with a special opening which 

 functions when the cyst contents are to be released. These openings, called archaeo- 

 pyles by Evitt, generally have a definite polygonal shape and are fixed in location in 

 any particular species. Their presence in a fossil demonstrates that it is a cyst. 

 Fossil remains of potentially motile dinoflagellates lack an archaeopyle and have a 

 cingulum, or transverse furrow, in the form of a continuous spiral groove which has no 

 impediments such as spines and septae crossing its track, indicating that this was the 

 former position of the transverse flagellum (Text-fig. 2). 



Tabulation, which is a striking feature of the living armoured dinoflagellates, can 

 also be represented in cysts, often highly modified, but its presence or absence is not 

 determinative in distinguishing cysts from motile stage thecae. 



