Interior Low Plateau Distributional Patterns 173 



To the west and southwest the Plateau is separated from the Coast- 

 al Plain Province by the Tennessee and Cumberland rivers in Kentucky, 

 except along a narrow isthmus that abuts Missouri after crossing 

 extreme southern Illinois, and by the Tennessee River in Tennessee and 

 northwestern Alabama. The whole plateau is an ecotonal region, past 

 and present, that lies in the western mesophytic forests, although slender 

 extensions of the southeastern forests penetrate along rivers. The nar- 

 row connection between the Ozark Plateau and the Low Plateau 

 through Indiana and Illinois has exerted an important influence on the 

 floras and faunas, both east and west (Conant 1960). 



In postglacial times, particularly during the so-called Climatic 

 Optimum, the mesic forest and its streams, many of which are now 

 buried (Wayne 1952), were apparently much more extensive than at 

 present. They extended well up into the glaciated parts of Ohio, Indiana 

 and Illinois. During the Xerothermic period that followed, the mesic 

 forest vanished from most of the area above the Ohio River (Conant 

 1960), forcing a shrinkage in ranges of many organisms and leaving 

 many of them in the narrow isthmus, an area thought to have been 

 stable along the upper margin of the Mississippian Embayment since at 

 least Tertiary times. Conant (1960) believed that many organisms used 

 this narrow isthmus to extend their ranges east and west, including var- 

 ious species of Eurycea, Plethodon, and Natrix, and the fishes Hybopsis 

 dissimilis (Kirtland), Noturus eleutherus (Jordan), and Notropis galac- 

 turus (Cope). Etheostoma microperca Jordan and Gilbert, certain spe- 

 cies of the percid subgenus Nothonotus, and Notropis telescopus (Cope) 

 may have been involved in such exchanges as well. However, Pflieger 

 (1971) noted that the hill country around the head of the Mississippi 

 Embayment may have played a role in various eustatic changes that 

 allowed fish exchanges through the Embayment rather than around it 

 via streams now absent. 



Older geologic changes doubtless played a role in determining var- 

 ious aspects of the Interior Low Plateau's fauna, particularly pre- 

 Cretaceous and Cretaceous Appalachian peneplanation (Griswold 1895, 

 and others). One portion of such peneplains has been mapped westward 

 through the Mississippi Embayment into Arkansas and northern Ala- 

 bama, an important fact considering the putative sources of many Low 

 Plateau organisms. Not only is the ancestral Cumberland River sup- 

 posed to have crossed the northwestern Alabama axis, but there is a 

 possibility that a Lower Mississippi stream, working backward from the 

 embayment, captured much of the interior drainage from the old Teays 

 system (Griswold 1895), transferring many aquatic species with it. Later 

 events would have been of greater importance. 



