Pleistocene Mammals Central Florida 81 



Miller found in competition experiments that, in its preferred deeper 

 soils, Geomys excluded species of other genera, whereas Thomomys was 

 the superior competitor in shallow soils. Hence, presence of Geomys 

 and Thomomys in the Rock Springs deposit suggests local occurrence 

 of two markedly different substrates. The deep, sandy soils predominat- 

 ing the vicinity today favor Geomys and generally represent sediments 

 of marine terraces of interglacial periods. Shallower, gravelly soils often 

 occupied by Thomomys in western North America are uncommon in 

 Florida today. However, such soils could be developed via erosion of 

 deep sands overlying Umestone. Shallow sands mixed with gravel formed 

 of eroding limestone could form a substrate inhabitable by Thomomys 

 but not by Geomys. 



ACKNOWLEDGMENTS.— I thank Richard Franz, Jeff Franz, 

 Gary Morgan and Ann Pratt for field assistance. Discussions with 

 Richard Franz, Richard Hulbert, Gary Morgan and David Webb aided 

 me in identifying specimens and in making paleoecological interpreta- 

 tions. The efforts of Mary Ellen Ahearn, Jon Becker, Richard Hulbert, 

 Bruce MacFadden, Gary Morgan, Ann Pratt and David Webb in 

 reviewing drafts of this manuscript are greatly appreciated. This paper is 

 University of Florida Contribution to Vertebrate Paleontology No. 223. 



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