ILLINOIAN AND KANSAN MOLLUSCAN FAUNAS 15 



A great number of the 65 species composing the combined Kansan and Illi- 

 noian molluscan assemblage occur abundantly in sediments of both ages. Among 

 these, the following should be mentioned: Carychium canadense , Cionella lubrica , 

 Columella alticola , Discus cronkhitei , D . macclintocki , Euconulus fulvus , Gastro- 

 copta pentodon , Gyraulus altissimus , Hendersonia occulta , Lymnaea dalli , L. 

 parva , Punctum parvula , Pupilla muscorum , Retinella electrina , Stenotrema mono- 

 don , Strobilops labyrinthica , Succinea gelida , Vallonia gracilicosta , Valvata 

 sincera , V. tricarinata , Vertigo elatior , Vertigo modesta , V. occulta , and perhaps 

 a few others that occur in both faunas if not in as many localities as those men- 

 tioned. These species, indicating the lacustrine, woodland, and woodland-border 

 types of environments, probably reflect most accurately the local environments 

 at the time of deposition of the sediments. 



From these populations it can be deduced that the broad aspects of the en- 

 vironment in the parts of Illinois represented by these collections differed little 

 between Kansan and Illinoian times and that adaptive radiation was active in only 

 a few genera. 



Comparison with Great Plains Faunas 



In studies made of fossil molluscan assemblages from deposits of Kansan 

 and Illinoian age in the Great Plains (Leonard, 1950; 1952), a total of 65 species 

 collected from 20 localities made up the composite fauna associated with the 

 Pearlette volcanic ash. Although the published work (Leonard, 1950) referred the 

 deposits to the Yarmouthian Stage, the deposits were subsequently placed in the 

 KanSan Stage and considered to be late Kansan in age (Frye and Leonard, 1952); 

 they clearly occur below the Yarmouth Soil. Among these faunas, some 13 species, 

 or 20 percent of the total fauna known from the central Great Plains, belong to 

 kinds restricted to Kansan sediments, and an additional three species, known from 

 Nebraskan deposits, are not known to range above the Kansan. Thus, in effect, 16 

 species among a total of 65, or approximately 25 percent, are restricted to the 

 Kansan, while in Illinois for practical utility in the field there are no restricted 

 species. In 1952 Leonard made a comparative study of Kansan, Illinoian, and 

 Wisconsinan molluscan faunas in the Great Plains. This study showed that among 

 74 species known from the Kansan through late Wisconsinan (Bignell Loess), 30 

 are restricted as fossils to the Sappa Silt of late Kansan age, although this figure 

 includes a few species that are living today. A relatively small number of the 

 species found in deposits of Kansan age are known to extend upward through the 

 Illinoian Stage, but not into deposits of Wisconsinan age. This faunal succession 

 in the central Great Plains presents a strong contrast to the faunal succession in 

 Illinois, where the great majority of the Kansan species extend into deposits of 

 Illinoian age or younger. 



Such comparisons are, of course, complicated by differences in environment 

 and in sedimentary facies. For example, comparison of faunal assemblages from 

 fluvial or lacustrine deposits with those from eolian sediments may be quite mis- 

 leading, because the environments are, at least in certain aspects, mutually 

 exclusive. On the other hand, the elimination of a species from deposits of a 

 certain age is for that stratigraphic unit truly an extinction, even though the 

 species is living elsewhere. For example, in the Great Plains, Valvata tricarinata 

 is a common species in Kansan deposits but is not known to occur in later sedi- 

 ments, although the species is living today elsewhere. Nevertheless, Valvata 

 is an "index fossil" for the Kansan in the Great Plains. On the other hand, Heli- 



