14 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 461 



so far as can be determined, a north-temperate climate and apparently a partially- 

 wooded terrain having open savannas interspersed with woodland but not deep 

 forest cover. 



The Kansan assemblage reported here contains several gastropods not 

 previously known; these include Gastrocopta venusta 3 Punctum parvula, Succinea 

 exile, and Vertigo occulta (Leonard, 1971). None of these four species that 

 make their first appearance in the Kansan are restricted to it; all extend upward 

 into the immediately overlying Petersburg Silt (Liman Substage of the Illinoian 

 Stage), and one of them, Gastrocopta venusta, extends into the overlying Glas- 

 ford Formation. 



Only four species in the Kansan assemblage are restricted to it: Deroceras 

 cf. laeve, Lymnaea humilis , Vallonia albula , and Vertigo tridentata (fig. 4). Al- 

 though seemingly restricted to the Kansan in Illinois, all occur in Wisconsinan and 

 Holocene faunas elsewhere in North America. Three of the four species occur only 

 in the Belgium Member of the Banner Formation at a locality in extreme eastern 

 Illinois. The fourth species likewise occurs in only one locality. The geographic 

 limitation of these four seems likely to be an accident of collecting. Certainly, 

 the large assemblage of restricted molluscan species that characterize the Kansan 

 deposits in the Great Plains does not occur in Illinois. This contrast is judged 

 to be the result of the different climatic histories of the two regions. Although 

 the Kansan climate of the central Great Plains was similar to the climates in Illi- 

 nois that immediately preceded and followed the episodes of glacial advance and 

 retreat, the climate of the Great Plains was much more inhospitable during inter- 

 glacial episodes. The present climate of Illinois is not strikingly different from 

 that which existed in the area during parts of Kansan time when glaciers were not 

 present in the state. The result of these relatively similar climates on the snail 

 faunas is that Kansan molluscan faunas cannot be differentiated from Illinoian 

 faunas in the field and can be recognized only with great difficulty in the laboratory. 



Illinoian Molluscan Faunas 



The known Illinoian molluscan assemblage in Illinois, listed by geographic 

 location and stratigraphic position, is shown in figure 3. Of a total of 61 species, 

 18 are not known from deposits younger than the Illinoian although 1 1 of these 

 range downward into the Kansan. Only seven species are seemingly restricted to 

 the Illinoian, and of these, only one, Vertigo briarensis, seems to be generally 

 characteristic of Illinoian age deposits. This species occurs in ten of the 21 Illi- 

 noian localities studied and commonly in considerable abundance. It is related 

 to Vertigo occulta , which extends from the Kansan into the Petersburg Silt, the 

 basal Illinoian deposit. Among the remainder of the seven species, Acella halder- 

 mani and Succinea avara occur in two localities and the remaining four species, 

 including Armiger exigua, Deroceras aenigma, Helicodiscus singleyanus , and 

 Lymnaea exilis , are limited to a single locality. Thus their occurrences bear 

 little significance in the interpretation of faunal assemblages, and their presence 

 or absence must be regarded as accidents to be expected in random sampling. 

 Among the Illinoian assemblage 40 species range upward into deposits of Wiscon- 

 sinan age, but the published Wisconsinan fossil molluscan fauna of Illinois (Leonard 

 and Frye, 1960) includes 24 species that have not so far been found in Illinoian 

 or older deposits (fig. 4). 



