26 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487 



Baker (1918, p. 659) lists mollusks recovered from four ponded deposits 

 on the Champaign Moraine in Urbana and nearby Mahomet. Recognizable species 

 comparable to the Strawn-Batavia assemblages include: Valvata tricarinata, 

 Sphaerium sulcatum (= S. simile 1 ?), Pisidium compression, Helisoma antrosa, H. 

 campanulatum , H. trivolvis , Gyraulus altissimus , Physa gyrina, Lymnaea 

 caperata, and L. obrussa. In a later study, Baker (1930) reported these species, 

 which are common to the two bog assemblages: Sphaeriwn sulcatum ( = S. similel) , 

 Valvata tricarinata , Helisoma antrosa, and Ferrissia parallela. 



More recent studies of fossil molluscan assemblages include those of 

 Leonard and Frye (1960), who reported on Wisconsinan molluscan faunal assem- 

 blages in the Illinois Valley region. Species common to the Strawn-Batavia 

 faunal assemblages include: Carychium exile, Helisoma trivolvis, H. antrosa, 

 Planorbula armigera, Gyraulus altissimus , Lymnaea dalli, L. parva, L. palustris , 

 Valvata tricarinata , and Amnicola leightoni. Lymnaea dalli and L. parva were 

 frequently recovered from loess deposits; the animals apparently lived near small 

 ephemeral ponds on the loess surface, although other evidence of these ponds 

 was not always apparent. 



In a report on Illinoian and Kansan molluscan faunas, Leonard, Frye, and 

 Johnson (1971) listed 65 species of aquatic and terrestrial mollusks from 29 lo- 

 calities; among those species common to the Strawn-Batavia faunal assemblages 

 are: Aplexa hypnorum, Armiger exigua, Gyraulus altissimus , L. dalli, L. obrussa, 

 L. palustris, Pisidium compressum, Valvata tricarinata , and Vertigo modesta. 



A study of the geology and paleontology of Pleistocene Lake Saline in 

 southeastern Illinois (Frye, Leonard, Willman, and Glass, 1972) revealed several 

 species common to the Strawn-Batavia assemblages: Promenetus exacuous , 

 Amnicola gelida, A. leightoni, Gyraulus altissimus, and Valvata tricarinata. 



One of the conclusions to be drawn from these observations on previous 

 occurrences is that the faunal assemblages found at the Strawn and Batavia 

 localities, in spite of being relatively young, are composed of molluscan species 

 established in Illinois since much earlier Pleistocene times. Most of these 

 species also occur in Pleistocene faunal assemblages in other areas, as, for 

 example, in the studies in Ohio previously referred to. 



Inasmuch as it has been shown that freshwater mollusks are capable of 

 living in waters that vary greatly as to temperature, pH, dissolved minerals, 

 and trophic state, the ecological inferences which can be drawn from the mollus- 

 can fossils recovered from the two bogs in question are somewhat limited. 



1 . An unexpected result of an analysis of the molluscan assemblages is 

 that the molluscan faunas in the two bogs are strikingly similar; such differences 

 as do occur are apparently not important. For example, Helisoma campanulatum 

 is found at the Strawn NE locality in only 2 of 20 collections, while at Batavia 

 W the species occurs at every level investigated (fig. 3). The significance 

 of this difference in occurrence is not clear. 



2 . At neither of the two bogs, at any time in their history extending 

 over 10,000 radiocarbon years or more, were freshwater molluscan faunas as 

 abundant as the number of fossil shells per cubic unit would indicate, because 

 the rate of deposition was extremely slow (2 inches per century) . Since no 

 etched shells were observed at any level, it can be concluded that most of the 

 shells that settled to the bottom were preserved. The presence of intact pairs 

 of sphaeriid shells bolsters this conclusion. It therefore follows that molluscan 



