SUBTERRANEAN AMPHIPOD STYGONECTES 143 



have been interstitial and more vagile than present species; thus, 

 dispersal routes would have been less restricted and corresponding 

 ranges would have been wider. A broad interpretation of regional 

 climate dm'ing the early and middle Cenozoic is that warmer and more 

 humid conditions prevailed throughout most of the western United 

 States (Schuchert and Dunbar, 1950). Because of this, corresponding 

 water tables in central Texas would have been closer to the surface, 

 and dispersal by a presupposed interstitial amphipod fuana would 

 have been greatly facilitated. 



As already indicated, present patterns of species distribution reflect 

 isolation of ancestral populations to restricted subterranean drainage 

 systems. As the chmate became progressively drier during the 

 Cenozoic, populations would have been compelled to migrate vertically 

 as phreatic waters receded A\ith progressive decrease in precipitation. 

 Vertical migration would have resulted in limitation of the horizontal 

 movements of ground-water faunas. In the Edwards Plateau, dis- 

 persal through subterranean waters by faunas inhabiting deeper 

 phreatic systems is theoretically restricted by faults and stratigraphic 

 differentiation, the latter being largely the result of the former. 

 Geological barriers that have influenced present patterns of speciation 

 were probably initiated in the Miocene when faulting began in the 

 eastern Edwards Plateau. Faulting apparently continued over an 

 extended period of time and was accompanied by gradual do^vncutting 

 of the Edwards Plateau by erosion and weathering. Both surface and 

 subterranean drainage systems were undoubtedly altered by these 

 geological events throughout the late Tertiary and were further altered 

 in the Quaternary. Present cave systems in this area may be no 

 older than the early Pleistocene. 



Still another factor may have contributed to the extreme isolation 

 and apparent rarity of species in the flag ellatiLS group. Species of the 

 tenuis group of Stygonectes are well represented in central Texas by 

 two widespread forms and two comparatively isolated ones. Species 

 of this group are believed to have invaded subterranean waters of 

 Texas at a later time than those of the flagellatus group (see discussion 

 under tenuis group, p. 150). Even in the rarer two species of the 

 tenuis group, population sizes are relatively large (in numbers of 

 collected individuals) in comparison to population sizes of species 

 in the flagellatus group. Second, populations of species of the tenuis 

 group are often found in small pools and streams that appear to be 

 fed directly by vadose waters and seepage. The rarity of populations 

 of species in the flagellatus group, then, may be due in part to the 

 inability of species in this group to compete successfully wdth the 

 more abundant and widespread populations of species in the tenuis 

 group. 



