16 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 461 



soma antrosa (anceps ) occurs in Kansan and Illinoian deposits in the Great Plains 

 but is not known from Wisconsina fossiliferous sediments, partly, at least, be- 

 cause the latter are almost everywhere composed of eolian loess. The species 

 is known to have occurred in the eastern part of the Plains Border Province until 

 very recent time. A meaningful comparison therefore must be made between faunas 

 from similar sedimentary environments. One cannot directly relate fossil assemb- 

 lages from upland eolian sediments with those from fluvial and lacustrine deposits. 



In the Great Plains the evidence is clear that major, and perhaps catastroph- 

 ic, oscillations of climate so critically changed the required environmental condi- 

 tions for mollusks that each large segment of time is marked by an assemblage 

 that is uniquely characteristic of that particular interval. It seems probable that 

 these pulsating faunal restrictions were occasioned by the fact that the climate 

 was near the threshold conditions for many species and when it fluctuated past 

 a critical point many of them were eliminated from the region (Frye and Leonard, 

 1967) and reinvasions produced different assemblages. These oscillations of 

 environment in the Great Plains, through the several rhythmic oscillations in 

 Pleistocene history from the Nebraskan to the present, resulted in the reduction 

 of the molluscan fauna to the depauperate assemblage that now lives there. In 

 the Bignell Loess, we have essentially the Holocene faunal assemblage. 



Not so in Illinois. Pre-Kansan faunas are not known, but from the Kansan 

 to the present, no consistent change in the character of molluscan faunal assemb- 

 lages is discernible. To be sure, there are modifications. A few species came 

 and went but basically the molluscan faunas re-established themselves after each 

 stage of extirpation by the Pleistocene glaciers. The changes in molluscan faunas 

 within the span of the Wisconsinan in Illinois (Leonard and Frye, 1960) are as 

 great as any that occur within the major oscillations of Pleistocene environments, 

 and it would seem evident that the climatic changes implied between the Altonian- 

 Farmdalian and Woodfordian Substages reported in figure 4 represent environmental 

 cataclysms as striking as those that characterize the contrast between the Kansan 

 and Illinoian episodes. 



Molluscan faunas on the Great Plains during Kansan time were not unlike 

 those in Illinois at that time. The Great Plains fauna gradually evolved toward 

 that of the Holocene, largely by the slow but inexorable process of extinction. 

 Essentially, Illinois has retained its Kansan molluscan fauna up to the present 

 day True, there are mollusks living in Illinois that are not known from the 

 Pleistocene, but these are relatively few and do not serve to change the general 

 aspect of the fauna. 



The remarkable feature of the molluscan faunas from the Pleistocene of 

 Illinois is their incredible ability to reinvade and repopulate terrain from which 

 they had been driven by advancing glaciers with so little admixture of "foreign" 

 populations. The magnitude of the emigration and immigration routes must have 

 involved more than three hundred miles. It has been pointed out that mollusks 

 characteristic of these faunas were at times living very close to the margin of 

 the glacier (Frye and Willman, 1958; Leonard and Frye, 1960). We conclude that 

 the molluscan populations moved in and out with the sweeps of glaciation and 

 changed remarkably little over a very long interval of time. 



