448 Annals New York Academy of Sciences 



array of factors readily supports the interpretation that it represents a pelagic 

 protist of uncertain affinities. 



Among other organized micro-objects of uncertain position are Favreina, 

 Globochaeta, Eothrix, Lomhardia,''^ Pilhonella,^^ and objects described by 

 Elliott.^i" 



Discoaster, an object 3 to 15 /x in diameter, is represented by calcareous, 

 stellate, or rosette-shaped plates. In many species the central area bears a 

 stem. These objects are abundant in pelagic sediments of Tertiary age. The 

 sediments containing discoasters also bear coccohths, Globigerina, and other 

 pelagic foraminifers.^*'^^ These objects are now thought to be the skeletal 

 remains of nannoplanktonic organisms of uncertain affinities. 



One ecological observation has been made about discoasterids. Across the 

 Eocene-Oligocene boundary, Riedel found not merely a change of radiolarian 

 fauna, but "surprisingly," a change in discoasterid assemblage. This is thought 

 to reflect "some change in surface waters. "^^ The active factor here might be 

 surface temperature. 



A whole series of related forms of uncertain position among the calcareous 

 nannoplankton include: Claihrolithus, Discoasteroides, Fasciculilhus, Heliolithus, 

 Tsthmolithus, Polycladolithus, Sphenolithus, and Rhomboasier.^^ Even though 

 Rhomboasler "is suggestive of some unusual habit of inorganic calcite growth," 

 three considerations refer it to the nannoplankton: specimens occur in abun- 

 dance; they are found only with coccolithophorids; their occurrence in time 

 is restricted. ^^ 



Numerous reports of minute sporelike and other types of bodies and "mesh- 

 work filaments" from Pre-Cambrian algal stromatolites are now at hand from 

 Russia, Scandinavia, France, West Africa, and the United States (Gunflint 

 formation of Northern Michigan). The biological organization of Barghoorn's 

 material "is supported by geochemical evidence" {i.e., the quantity of C'^ per 

 mil).^^ There is no equivalent verification of pyrite spherules thought to have 

 replaced microorganisms.^* 



Stimmary 



Pelagic protists tend to contigurations of least surface area. The sphere and 

 its modifications is a recurrent shape. The many shapes and structures 

 (spinosity, for example) of the scleratoma of radiolarians, the lorica in tin- 

 tinnids, the frustule of pelagic diatoms, the armored skeleton of dinoflagellates, 

 the test of planktonic foraminifers, the siliceous and calcareous skeleton of 

 chrysophytes, appear to be adaptations to resist sinking below optimal food 

 and photic levels of the sea. 



Examples of nongenetic factors affecting differential morphology of protists 

 include: variable oceanic temperature, salinity, depth, and turbidity; presence 

 of lead, excess iron and copper as well as carbon dioxide; condition of encyst- 

 ment; nature of substrate, barophilic property, nutrient salt concentration, 

 and amount of light penetration. Some of these factors apply only to specific 

 protists. 



Fossil microproblematica of Mesozoic age seem to be nannoplankton of un- 

 certain affinities among the protists (for example, Nannoconus). Micro- 

 objects of Pre-Cambrian age may represent spores and algae. 



