WISCONSINAN MOLLUSCAN FAUNAS 13 



Ecological Considerations 



Although the limited number and erratic geographical distribution of avail- 

 able Farmdalian fossiliferous localities restrict generalizations about ecological 

 conditions during Farmdalian time, a few observations seem pertinent. "The Rocks" 

 section (locality 57b) reveals a woodland fauna of the type found in well established 

 hardwood forests. It would appear that the heavy forests of Altonian time persisted 

 through the Farmdalian Substage until the beginning of, or perhaps slightly into, 

 earliest Woodfordian time, at least in southern Illinois. There is no evidence of 

 heavy forest cover in northern Illinois at any time during the Wisconsinan Stage. 



The abundance of Aniuispira kochi and the robust individuals of Allogona 

 profunda, Aniuispira alternata, and Succinea ovalis confirm the concept of deep 

 forests at the southern locality at the time the shells were buried in the loess. It 

 is evident that the terrain was well watered, but there is little to indicate that the 

 climate was anything more than slightly cooler than at present. 



The absence of terrestrial species in the two northern localities (8, 24) does 

 not permit speculation concerning vegetational cover of the uplands of this part of 

 Illinois in Farmdalian time. There is reason, however, to doubt the occurrence 

 there of forest cover, because it seems likely that the large shells of forest species 

 would almost certainly have found their way into pond deposits. All that can be 

 conjectured is the presence of permanent, clear, ponded water at the sites from 

 which the collections came. There is nothing to indicate that these waters were 

 more than slightly cooler than lakes and ponds existing in the area today. Pending 

 the discovery of fossiliferous upland Farmdalian silt, the terrestrial molluscan 

 fauna (and a more complete paleoecological picture) of the Farmdalian Substage 

 in the northern parts of Illinois must remain unknown. 



Woodfordian Faunas 



Woodfordian molluscan assemblages comprising an aggregate of 55 species 

 have been recovered from a total of 50 localities; at some of them, Altonian or 

 Farmdalian faunas occur in their proper stratigraphic positions. Of the total assem- 

 blage, 22 species were restricted to the Woodfordian. These include: Aplexa 

 hypnorum , Carychium exile canadense , Columella alttcola, Euconulus fulvus , 

 Gastrocopta holzingeri, G. procera, G. pent odor. , Gyraulus parvus, Helisoma 

 trivolvls Mesodon clausus , Pisidium nitidum, P. uariabile, Planorbula armiiera, 

 Pupilla muscorum , Pupoides albilabris, Stenotrema leai, Triodopsis alionquinensis , 

 T. multilineata, Vertigo alpestris oughtoni, V. milium, and V. modesta . 



Woodfordian faunas are divided into several categories, based on whether 

 they were recovered from Peoria loess, Morton loess, Richland loess, terrace de- 

 posits, or moraines (fig. 2). 



Faunal Assemblages from the Peoria Loess 



The Peoria loess, which includes all post-Farmdalian loessial deposits 

 outside the borders of the Shelbyville moraine, comprises silts ranging in age 

 throughout the Woodfordian Substage. This gray to gray-tan or grayish yellow silt 

 is almost everywhere fossiliferous except where it has been leached. Fossil 

 mollusks from the Peoria loess at 37 localities are here reported (fig. 2); the 

 number could be increased almost indefinitely by additional collections. 



