ILLINOIAN AND KANSAN MOLLUSCAN FAUNAS 11 



Stratigraphic Paleontology 



Although deposits of clearly demonstrated Nebraskan age are known in 

 Illinois, no fossiliferous Nebraskan deposits have been found. Fossiliferous de- 

 posits of Kansan age are rare in Illinois, but the eight localities from which collec- 

 tions were made span more than 200 miles east-west across the central part of 

 the state (fig. 1). Molluscan faunas of Illinoian age are widely distributed through- 

 out the central half of the state. 



Kansan Molluscan Faunas 



Kansan molluscan species and their stratigraphic and areal distribution 

 are shown in figures 3 and 4. Several distinctive features of the Kansan faunal 

 assemblages are worthy of comment. 



One distinctive feature is the preponderance of terrestrial species, despite 

 the fact that the exposures sampled consist of fluvial and lacustrine deposits. 

 Among the 47 kinds of mollusks here reported from Kansan sediments, 40 are ter- 

 restrial and only seven are aquatic in habit. Furthermore, among the seven aquatic 

 species, two of them, Gyraulus parvus and Lymnaea humilis , occur at only one 

 locality each. Lymnaea dalli, Valvata sincera, and V. tricarinata each occur at 

 five of the localities, Gvraulus altissimus , is found at four localities, and an 

 unidentified species of Pisidium occurs at two locations. These aquatic mollusks, 

 all associated' with lacustrine habitats, must have lived with fishes and amphibians, 

 but for unknown reasons no fossils of these vertebrates were observed. It is 

 equally difficult to explain the absence from the aquatic phase of the assemblages 

 of Helisoma antrosa and H_. trivolvis , which are common pond snails in Wiscon- 

 sinan deposits, and the several species of Amnicola that are at least locally 

 abundant in Wisconsinan sediments in Illinois (Leonard and Frye, 1960). 



Among the terrestrial snails, the majority indicate a forest habitat or a 

 forest-border environment. Such species as Cionella lubrica , Columella alticola , 

 Euconulus fulvus , Haplotrema concavum, Mesodon clausus , Stenotrema monodon , 

 S. hirsutum, S> stenotrema, Striatura milium, and Strobilops labyrinthica are 

 typical of mixed hardwood forests, although they may occasionally occur in forest- 

 border situations. On the other hand, the assemblage lacks many species that 

 should be expected to occur with them if deductions concerning the probable habi- 

 tat are correct: for example, Anguispira kochi, Polygyra profunda, Triodopsis al- 

 bolabris, and the several possible species of Mesomphix . Ventridens , and other 

 genera from woodland habitats. The remaining terrestrial species are less restricted 

 to woodland situations but vary among themselves as to moisture and other re- 

 quirements. 



There is little to be learned from the fauna as far as temperature tolerance 

 is concerned, except that there are no southern faunal elements. The only pos- 

 sible northern element may be Vertigo occulta , which is not recorded among the 

 present living pupillid fauna but seems to be closely related to Vertigo nylanderi , 

 an inhabitant of northern Maine and southern Canada. In less precise fashion, 

 the assemblage of terrestrial mollusks reflects a somewhat more northern latitude, 

 such as that of Michigan. For example, Carychium canadense , a widely distributed 

 and locally abundant species in both Kansan and Illinoian deposits, today inhabits 

 the northern tier of states and southern Canada east of the Rockies and occurs on 

 Vancouver Island. In summary, the Kansan faunal assemblage in Illinois indicates, 



