WISCONSINAN MOLLUSCAN FAUNAS 7 



bearing deposits, has enabled us to elaborate criteria for the recognition of faunas 

 characteristic of the several Wisconsinan substages. Assignment of assemblages 

 to specific substages makes it possible to recognize within reasonable limits the 

 variations and discrepancies produced by local ecological conditions, geographic 

 differences, and hazards of random sampling. It allows a greater degree of assur- 

 ance in correlating the fauna contained in sediments at localities where the relative 

 age has not been determined by any other method. Bulk samples were processed 

 from most of the approximately 60 localities shown in figure 1 . Experience has 

 shown, however, that repeated bulk sampling is required in order to approach full 

 recovery of an assemblage. In one instance, pooling of two bulk samples taken at 

 the same locality at different times resulted in a 30 percent increase in the total 

 number of species recovered, although each sample contained more than 200 identi- 

 fiable specimens. Therefore, claim can not be made for more than reasonable com- 

 pleteness of the snail collections discussed here. 



Before focusing attention on the fossil mollusks discussed in this paper, it 

 seems appropriate to review the complex factors that influence the composition of 

 local faunal populations. Aside from the distortions induced by random and inade- 

 quate sampling and by the vagaries of preservation, assemblages of fossil organisms 

 reflect the combined and interacting effects of a wide variety of forces upon the 

 formerly living populations. Among these, perhaps the most obvious, if not always 

 the most important, are changing climates, local ecological conditions, including 

 details of sedimentation, and the relative genetic stability of the organisms involved. 



Climatic changes occur constantly (if sometimes slowly) under the influence 

 of numerous complex factors, and such changes keep the living organism in a per- 

 petual state of nonequilibrium with its environment. The frequency and distribution 

 of precipitation, the total annual precipitation, and the evaporation rate are of para- 

 mount importance to any animal or plant. Average temperatures and diurnal and 

 seasonal temperature extremes impose stresses upon the organism and make neces- 

 sary continual adjustments. Regional long-range climatic changes related to orogeny, 

 changes in mean sea level, fluctuations in the prevailing patterns of air-mass move- 

 ments, or other changes are additional hazards to survival. Climatic differences re- 

 lated to latitude or altitude and cyclic climatic changes coincident with repeated 

 continental glaciation constitute environmental stresses to which the organism must 

 adapt or succumb, or from which it must escape by migration. 



Local ecological conditions are notedly variable. Soils, important elements 

 of local environment, vary with parent materials, maturity, vegetative cover, and 

 relative position in the microtopography . Availability of appropriate food and cover, 

 inter- and intra-specific competition, and selective predation produce profound 

 effects upon animal populations. Moreover, small and sedentary animals, such as 

 gastropods, utilize microhabitats that may be considerably at variance with the 

 general environmental pattern of a region, making analysis of their precise ecological 

 requirements extremely difficult. 



Faunal change is not necessarily related to the physical separation of two 

 stratigraphic levels, but rather to the time that elapsed during their deposition and 

 to the concomitant ecological changes involved. These are the factors that provide 

 opportunities for the mutations, genetic drift, extinctions, or mass migrations of 

 organisms that serve to alter the composition of populations. Populations at dif- 

 ferent stratigraphic levels do not necessarily differ greatly, especially when the 

 amount of time involved is relatively small, provided that the ecological conditions 

 are reasonably similar. 



