20 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 304 



Total numbers of molluscan species in any single Peoria fauna range from 

 as many as 26 (locality 57a) to as few as three (locality 25); a typical assemblage 

 comprises approximately 12 species. 



The Peoria fauna is distinctive and readily recognizable, even in the field. 

 The predominant species are small mollusks; the only relatively large shells that 

 occur are Aniuispira alternate!, Triodopsis alionquinensis , and occasionally T. 

 multilineata. The last two, as far as present information goes, are restricted to the 

 Woodfordian. Succinea ielida, Etenotrema hirsutum , S. leal, Hendersonia occulta, 

 and either Discus cronkhitei or D. macclintocki , although of only moderate size, 

 frequently are conspicuous in a local fauna. Among the minute species Columella 

 alticola^ Euconulus fulvus , and Vertiio modesta are most commonly recovered in 

 the Peoria loess. 



The Peoria fauna shows significant variation from north to south in the Illinois 

 Valley region. Among the species of reasonably frequent occurrence, Carychium 

 exile canadense , Columella alticola, Discus cronkhitei , and Pupilla muscorum are 

 either restricted to the northernmost part of the region, or reach their greatest popu- 

 lation density there, while Discus macclintocki, Hawaiia winuscula , Haplotrema 

 concavum, Vertiio hubrichti, and Aniuispira alternata occur most frequently in the 

 southern localities. 



Ecological Considerations 



A complete picture of the paleoecological conditions prevailing in Illinois 

 during the time of deposition of the Peoria loess cannot be fully reconstructed. The 

 thick, highly fossiliferous loess exposures are near the major valleys, and the loess 

 thins rapidly away from them. Because relief is relatively less pronounced away from 

 the valleys, suitable exposures are less frequent and those available generally are 

 so deeply leached that all snail shells have been destroyed. It is obvious from the 

 species composition of Peoria assemblages that the great deciduous forests of Al- 

 tonian time (at least as they appeared in the southern half of Illinois) did not occur 

 in Woodfordian in northern Illinois. This is not to say that the vegetative cover was 

 reduced to a prairie situation; the presence of Aniuispira alternata, Mesodon clausus, 

 and the several species of Stenotrema in the Peoria loess attests to the presence of 

 stands of trees. The general vegetational cover must have consisted of stands of 

 trees along stream courses, and perhaps elsewhere, interspersed with large areas 

 of open prairie. Correspondingly, the climate was likely less stable and perhaps 

 less humid than that existing in Altonian (and perhaps Farmdalian) time, although 

 the occurrence of Carychium exile canadense , Lymnaea parua , Pomatiopsis scalaris, 

 Triodopsis alionquinensis , and T. multilineata clearly show that the climate could 

 not have been deficient in rainfall. Carychium exile canadense , Euconulus fulvus, 

 Pupilla muscorum t and Vertiio alpestris ouihtoni support the thesis that the Wood- 

 fordian climate was somewhat cooler than today, even in northern Illinois,for these 

 species are today characteristic of molluscan faunas in the Canadian Life Zone. 



Faunal Assemblages from Morton Loess 



The post-Farmdale — pre-Shelbyville Morton loess (formerly classified as 

 the Iowan loess) in our studies is represented by two fossiliferous localities (10 

 and 23, fig. 2). From these a total of 12 species is known; Columella alticola. 

 Discus cronkhitei, Haplotrema concavum , Hendersonia occulta, Lymnaea dalli , 



