Tasch: Growth and P'orm of Fossil Protists 439 



The shapes of some nonmarine diatoms can be influenced by other ecological 

 factors. Individuals of the genus Desmogoniurn were long and had barely 

 capitate ends in fast flowing water but were short and had broadly capitate 

 ends in standing water. ^" 



Similar considerations also apply to the morphology of armored dinoflagel- 

 lates.^'^^-^"'''^ Kofoid studied skeletal development {i.e., an armor of loosely 

 cemented cellulose plates) in the genus Gouyaulax.-^ He found that all modifi- 

 cations in this genus were variants of the spherical configuration (cf., Lejeune- 

 Carpentier-' for fossil Gonyaula.x). In turn, this ensures least surface area, 

 and hence confers an advantage on protists leading a pelagic existence. 



Braarud^'^ stresses that form variation is observed in every species. Study 

 of Schiller's work on dinoflagellates brings this out clearly. Dinophysis 

 hastata^^ for example, shows a whole spectrum of variation from ovate to sub- 

 ovate and subelliptical conligurations, and corresponding size and shape varia- 

 tions in epithecal and hornlike structures. Braarud''' noted that in some 

 instances, form variation appeared to be "phenotypic" and "tentatively re- 

 lated" to a whole series of ecological factors such as salinity, temperature, 

 nutrient salt concentration, and day length. An excellent example of such 

 infraspecific form variation is found in the fossil record of the dinoflagellate 

 Nannoceralopsis recovered from beds of Jurassic age.-^ 



Twenty specimens of N . deflandrei show variations in form from ovate to 

 subovate hanging drop configurations. These may bear weakly defined an- 

 tapical horns or lack them. Other forms are broadly and acutely subtriangular 

 with the base faintly or markedly concave between short horns. This strik- 

 ingly contrasts with the long horn type, X . pelliicida.^^ 



It is apparent from our previous discussion that these variants are adaptive 

 modifications for flotation. Something similar to pelagic diatom adaptation 

 in thickness of test is found in the armored dinoflagellates. Thus, in colder 

 waters of the South Ecjuatorial Currents in which viscosity was greater, the 

 horns of Cerialia were found to be longer than those of equivalents taken in the 

 warm water of the Guinea stream."* 



The short horn, X. deflandrei, may be related to warmer waters, whereas 

 .V. peUucida, the long horn species, would then indicate colder waters. A third 

 type tentatively assigned to Xannoceraiopsis has been found in the Permian. 

 This form is flask-shaped and bears stubs in place of horns (Tasch, in press). 



Other structural modifications that have adaptive value in armored dino- 

 flagellates, include a variety of specializations to ensure suspension or flotation 

 when the flagella are at rest or swimming is feeble.^ •-^■"•^■-'^ In the genus 

 Triposolenia, the ends of the antapical horns are deflected. The significance of 

 this asymmetry has engendered speculation. Kofoid"* thought that the asym- 

 metry must bear a "profound relationship" to the survival of the forms in 

 which it appears. It occurs in more than one species and in the genus Amphiso- 

 lenia also.-'' Still other genera have analogous structures. After a descent of 

 about 10 times the body length, the asymmetrical horns serve to orient the 

 long axis horizontally, i.e., the position of greatest resistance to downward 

 pun.4 19 



A few of the morphological variants in armored dinoflagellates include: 

 round and egg-shaped skeletons (Glenodinium) sometimes bearing spines on the 



