Tasch: Growth and Form of Fossil Protists 447 



Mesozoic (U. Jurassic-Lower Cretaceous) pelagic deposits in the Mediterranean 

 area, Rumania, Cuba, and Mexico.^^ It is always associated with the pelagic 

 facies containing radiolarians and tintinnids, and occasionally, with smooth 

 ammonites. ^ 



The following affinities have been suggested: (1) the object represents an 

 embryonic stage of the tiask-shaped foraminifer Lagena; (2) it is a unicellular 

 chlorophyllous alga; (3) it is of inorganic origin having formed from calcite 

 crystals in a highly saturated medium; (4) it belongs to the oogonia of certain 

 Charophyta; and (5) it represents a little known coccolithophorid.''^'^^ 



There are then a whole set of constants and some variables to explain. Con- 

 stant factors include: distinctive wall composition and structure, 50 ju or less 

 in length; persistence of faunal associations; and occurrence in pelagic facies 

 intermittently deposited over a span of tens of millions of years in different 

 parts of the globe. Variable factors include: nine species of X annoconus based 

 upon variations in axial canal and basal cavity, overall shape and size; three 

 distinct Xaiiiiocoiius faunas in as many zones of the Lower Cretaceous.'^^ 



In light of our previous discussion on form in many protists, the likelihood 

 is that configurations in Xannoconus are variants of a sphere.^^ Thus, selective 

 modification of the sphere gives an elongate type or a cylindrical type. The 

 basal cavity of circular types have no axial canal when seen in thin section. 

 Circular types were spherical in life. Elongate, conical, and subovate types 

 do have an axial canal. The size and configuration of axial canal and basal 

 cavity could be a function of compression of an original sphere. Although this 

 can account for the variation in morphology and inner spatial relationships of 

 the object, it is unclear whether mechanical compaction or genetics was the 

 active agent. 



Although we assume the first of these possibilities, the list of constant fea- 

 tures still remains to be explained. Colum'^" notes that Xannoconus at times 

 appears in great numbers in pure limestone lithotopes. Population density is 

 thus another variable. 



What is the likelihood that inorganic precipitation of calcium carbonate and 

 mechanical distortion alone can account for Xannoconus? The nearest ap- 

 proach to a regular type of inorganic carbonate deposit is the example of oolites. 

 These may be radiate in internal structure or bear concentric bands around a 

 nucleus. In size range, oolites are also restricted wherever they are found. 

 Mineralogy of the bands tends to be relatively uniform although alterations 

 are known. There is a definite spherical-to-elliptical configuration. When 

 compressed, flat, pelletiferous shapes result. Why cannot Xajinoconus be an 

 object of this type? 



The best argument against an inorganic origin is the persistent crystallization 

 of minute calcite wedges, all of which are perpendicular to, and form a band 

 about a hollow basal cavity or axial canal. Inorganic origin cannot account 

 for the discrete thickness of the wall in this case as it can for the successive 

 bands of oolites. If the calcite wedges were invested in an organic membrane 

 that surrounded a cavity or canal, both wall thickness and mineral orientation 

 could be readily explained. 



Once the conclusion is reached that Xannoconus is of organic origin, the other 



