438 Annals New York Academy of Sciences 



It seems valuable to reiterate, with fossil protists in mind, that "we may 

 assume that during the period for which we have good fossil evidence, the sea 

 has remained very much the same in overall chemical composition."^*^ Cer- 

 tainly, this is applicable to the Tertiary and Mesozoic. By extrapolation, for 

 protists such as the radiolarians, it may be referred back to the older Paleozoic. 

 Ecological studies of living marine biotas suggest "dim outlines of food chains 

 that must have had links similar to those of the present day"' in the geological 

 past. 



In thinking about assemblages of fossil protists, their growth and form in 

 ancient seas, coastal and inland waters, one can refer to the same or equivalent 

 physical-chemical factors known to influence living phytoplankton. 



The Diatom Frustule and Dinoflagellale Armor 



Certain physical realities of the environment have to be satisfied to ensure 

 survival for various protists including pelagic diatoms and dinoflagellates. 

 We may speak of these as "fence" or limiting conditions. These restrictions 

 influence not only distribution but growth and form as well. The first "fence" 

 is the specific weight of living protoplasm, which is 1.02 to 1.06, and hence 

 heavier than pure water."* There will be a tendency to sink if the added incre- 

 ment of a skeleton (test or armor) is superimposed on this naked weight. 

 Whether the protist is a passive floater like the pelagic diatoms, or capable of 

 feeble flagellar locomotion like the dinoflagellates, the fence condition will 

 apply. The second "fence" is established by the requirements of photosynthe- 

 sis. Pelagic protists need to be physically positioned, or located in a specific 

 zone of the sea, the photic zone, or both. 



Given these fence conditions, a selective advantage will favor individual 

 pelagic diatoms and dinoflagellates with slight variations in skeletal morphology 

 that tend to retard the rate of sinking. Natural selection would then become 

 effective within the available band of skeletal variation characterizing a given 

 population. 



Projecting spines, chains of cells, disc-shaped tests or needle and hair types, 

 curvature of cells, bevelled ends of tests, are all structural adaptations to resist 

 the gravitational force. Spines, for example, aid flotation, as do spiral or 

 flattened chains of cells. This last feature produces more surface area and 

 hence greater frictional resistance.'^ It should be emphasized that test shape 

 and modification of the ends of tests do not prevent sinking. Rather, these 

 features either facilitate a return to the horizontal from a vertical position, or 

 expand the path of passive descent from a straight line to a zigzag path or a 

 widely circular one. In this way, removal from the photic zone is slowed down 

 or delayed.** 'i^''**^ 



Weight and spination of diatom frustules have been observed to vary ac- 

 cording to species, season, and habitat. Generally, pelagic species tend to be 

 thin shelled, whereas bottom and littoral forms are not. Viscosity, which 

 varies inversely as the temperature, is a factor in flotation of pelagic protists. 

 Heavier frustules tend to sink under reduced viscosity. It follows then, that 

 cold water or winter forms will have heavier shells.'' '^'^ In all such instances, 

 silicon metabolism and the supply of silicon are also involved.'"" 



