ELECTRON-MICROSCOPY OF FOSSILS — HAY 413 



many of the "guide fossils" used to determine the age of a particular 

 stratum. In many of the species the pores in the test are large 

 enough to be readily visible in the light microscope. Others, how- 

 ever, have such fine pores that there was some question as to whether 

 the pores even existed or not. In the light microscope, the finely 

 porous wall presents a "fibrous" appearance. Electronmicroscopic 

 investigations reveal that the "fibers" of the wall are not calcite 

 ci*ystals at aU, but are the fine pore canals, which are only l^ /* in 

 diameter, and thus cannot be clearly observed in the light microscope. 

 The boundaries between the calcite crystals remain invisible, and 

 the fine pore canals produce the image of a "fibrous" wall. 



The pore canals of the hyaline Foraminifera are lined by an organic 

 membrane and at intervals disks of organic material cover the pores. 

 These disks can be seen vaguely in the light microscope. However, 

 in the electron microscope they can be observed to be perforated by 

 a number of openings and the term "sieve-plate" has been applied 

 to them (pi. 2). The sieve-plates may even be preserved in fossil 

 foraminifera. Very little is known about these structures at the pres- 

 ent time. The function of the sieve-plates is unknown, but since they 

 are located along the channels through which the protoplasm inside 

 the test commmiicates with the outside, they may play an important 

 physiological role. 



Another area of paleontologic investigation wliich has been opened 

 up through the development of replication teclmiques for use with 

 the electron microscope is the examination of very small fossils. The 

 term nannofossil (nannus= dwarf) is usually applied to these forms. 

 The calcareous nannofossils constitute a heterogenous group of ob- 

 jects ranging from 14 to 20/1. in size. Eecently it has become appar- 

 ent that many forms of calcareous nannofossils are of particular 

 importance to paleontologists in establishing the age equivalence of 

 rocks in distant places. When these fossils were first described dur- 

 ing the 19th century, it was thought that such small fossils must be 

 produced by exceedingly simple organisms, and that these organisms 

 could hardly be expected to show any evolutionaiy changes. Modern 

 studies have shown just the opposite to be true. Calcareous nanno- 

 fossils are among the most complexly constructed skeletal elements 

 yet studied, and evolution has wrought radical changes on them. 



The most common sort of calcareous nannofossil is the coccolith 

 (pi. 3). Coccoliths are button-shaped objects, mostly between 

 1 and 5/x. in diameter although they may range anywhere from 14 

 to 20/x. On close examination, coccoliths are found to consist of two 

 disk or shieldlike parts connected by a very short tube. Usually one 

 of the disks is smaller than the other, so that the coccolith resembles 

 a collar button. Coccoliths are made of calcimn carbonate and are 

 skeletal elements produced by a single-celled marine organism known 



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