SUBTERRANEAN AMPHIPOD STYGONECTES 135 



to a peneplain would have been an opportune period for wide dis- 

 persal. Ancestral forms, however, might have been able to disperse 

 over a terrain comparable to that which characterizes the present 

 Appalachians, and one need only to look at the wide range of S. 

 allegheniensis (tenuis group) for a possible present-day parallel. 

 The extensive range of this species over much of the north-central 

 Appalachians offers good evidence for the potential vagility of species 

 in Stygonectes and shows clearly that under certain circimistances 

 some species in this genus are able to overcome potential barriers to 

 dispersal. That ancestral species of the emarginatus group had much 

 wider ranges than extant species is strongly indicated by distributional 

 data. 



Distributional data also indicate that the rather intense speciation 

 that has occurred in this group has resulted from the isolation of 

 different populations of ancestral species in valleys separated by 

 geological barriers, i.e., primarily ridges of insoluble elastics such as 

 shales, sandstones, and conglomerates. Factors that operated to 

 force ancestral species into these limestone valleys are not clear, 

 but events during Pleistocene giaciation would appear to offer the 

 best explanation. Presumably dm'ing periods of giaciation, moun- 

 tainous regions, such as the central Appalachians, lying just south 

 of continental ice masses, were much colder and wetter than during 

 corresponding interglacial periods. This being the case, ground 

 waters would have been elevated and more extensively developed in 

 horizons near the surface. Under these cu'cumstances, phreatic 

 faunas would have been more widespread, and being distributed near 

 to the surface, they could more easily have spanned what are now 

 apparently insurmountable barriers to dispersal. During any one of 

 the three interglacial periods when climates became substantially 

 warmer and drier, ground waters would have receded and their as- 

 sociated faunas would have been forced either to migrate vertically 

 or face extermination. Assuming vertical migration by an am- 

 phipod fauna already well adapted to ground-water habitats, the 

 most likely destination would have been the extensive solution 

 channels and caves situated in valley floors. Attendant with lower- 

 ing of the ground-water table and colonization of deeper phreatic 

 habitats, horizontal movement by amphipod populations would have 

 become greatly limited and generaUy restricted to the limits of 

 ground-water circulation in stratigraphically continuous limestones 

 on opposite sides of ridges. Isolation of populations to restricted 

 limestone valleys over an extended period of time would have 

 resulted in genetic changes and subsequent speciation. 



The ranges of both S. emarginatus and S. gracilipes demonstrate 

 distribution along highly linear, northeast-southwest oriented valleys 



