DWARF FAUNAS 



PROFESSOR HERVEY W. SHIMER 

 Massachusetts Institute of Technology 



Faunas in which all the individuals are uniformly so 

 small as to be notable for this reason are common both 

 in recent and past times. Such dwarf faunas may be 

 merely an association of normally small species ; or they 

 may be individuals much smaller than the normal size 

 for that species, prevented for some reason from attain- 

 ing full size. 



The following is an attempt to summarize some of the 

 principal recent and dwarf faunas with the probable 

 causes producing them. It is confined to invertebrate, 

 water-living faunas. The article is primarily a sum- 

 mary of such literature as chanced to be seen ; that very 

 much of importance was overlooked is undoubtedly true, 

 but it is thought that the major causes of dwarfing are 

 here noted. 



The first part of the article is a discussion of the chief 

 agencies of dwarfing using recent examples as illustra- 

 tions; the second part considers a few fossil examples of 

 dwarf faunas with their probable causes. 



The following are the chief agencies of dwarfing as 

 noted in recent and fossil faunas : . 



1. A change in the normal chemical content of the 



water. 



(a) Due to a freshening of the sea water. 



(b) Due to a concentration of the salt, iron, etc. 



(c) Due to an increase in H 2 S and other gases. 



2. Presence of mud and other mechanical impurities in 



the water. 



3. A floatiiig habitat. 



4. Variations in temperature. 



5. Extremes in depth of water. 



