MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 53 



V. THE GENUS HYSTRICHOSPHAERIDIUM AND ITS ALLIES 



By R. J. DAVEY & G. L. WILLIAMS 



INTRODUCTION 



The early history of the study of fossil dinoflagellate cysts, first described by 

 Ehrenberg (1838), is discussed in the previous chapter. 



White (1842) was the first to describe the forms possessing tubular processes in his 

 section on types of Xanthidia tubifera. In 1933, O. Wetzel placed all the then 

 regarded species of fossil microplankton in the new genus Hystrichosphaera. Deflan- 

 dre (1937) subdivided this genus separating those forms possessing an equatorial 

 girdle and polygonal fields, which he placed in the genus Hystrichosphaera emend., 

 and those without surface ornamentation which he placed in a new genus Hystri- 

 chosphaeridium. The diagnosis of the genus Hystrichosphaeridium was given as 

 follows : " This genus comprises all the hystrichospheres totally destitute of an 

 equatorial system of elongate plates and whose shell, in general, does not bear 

 fields or plates limited by sutures. The shell, of dimensions greater than 20[x, is 

 most often spherical or spheroidal ; some species, however, are more or less elongate." 



Eisenack (1958) emended and restricted this genus as follows : " Hystricho- 

 spheres with spherical to oval, non-tabulate central shell and with more or less 

 numerous, mostly well separated and in general similar appendages, the ends 

 being open and often expanded in funnel-like fashion." Those species not included 

 in Hystrichosphaeridium by Eisenack have been revised by Downie & Sarjeant 

 (1963)- 



Before classification of the spiny spheres placed in the genus Hystrichosphae- 

 ridium Deflandre could be attempted with any precision, their affinities had to be 

 determined. Certain organisms such as forms belonging to the genus Gonaulacysta, 

 are associated with species placed in Hystrichosphaeridium, and have a similar two 

 layered body-wall apparently composed of the same or a very similar organic 

 substance. These organisms are obviously related to the dinoflagellates and are 

 either resting or reproductive cysts. They possess a characteristic ornamentation 

 which can be related to the theca of modern dinoflagellates ; for example the 

 crests marking the tabulation as seen in Gonyaulacysta and Hystrichosphaera or less 

 obvious the distinctive equatorial region as possessed by Deflandrea or Palaeo- 

 hystrichophora. 



Thus many forms of fossil microplankton could definitely be said to be cysts of 

 dinoflagellates, but what of the genus Hystrichosphaeridium ? Numerous forms had 

 been attributed to this genus but dinoflagellate features had not been definitely 

 noted in any of them. In fact they appeared to be simple spherical shells possessing 

 a number of randomly arranged tubular appendages, usually with an opening or 

 pylome of some kind which no doubt was used by the organisms as an exit. It was 

 not until 1961 that Evitt pointed out the importance of these openings. He drew 

 attention to the fact that most of the openings had angular margins and realized that 

 here was evidence of breakage along a definite line. These openings, or " archaeo- 



