SUBTERRANEAN AMPHIPOD STYGONECTES 159 



cured as recently as the late Tertiary. Ancestral alabamensis stock 

 had probably reached its height of disperasl by the early Quaternary, 

 and events throughout the Pleistocene, discussed below, could have 

 easily resulted in isolation of a number of these populations. At 

 least two events occurred durmg the Pleistocene, which, theoretically, 

 would have affected both local and regional isolation: (1) erosion 

 and general downcutting of laud forms in the Ozark region, and (2) 

 progression toward a drier climate in central Texas. 



The latter event has already been suggested by me (Holsinger, 

 1966) as having been largely responsible for the isolation of a segment 

 of alabamensis stock to central Texas. At that time I referred to 

 this progenitor stock as americanus, but a nomenclatural change in 

 this paper (see Systematics) has necessitated a change from americanus 

 to alabamensis; underlying principles remain the same, however. A 

 hypothesis for the isolation of populations of alabamensis stock to 

 the Edwards Plateau region of central Texas is as follows: During 

 the late Tertiary when a humid climate presumably prevailed thi-ough- 

 out central Texas, much as it still does today in eastern Oklahoma, 

 Arkansas, and Missouri, alabamensis stock ranged throughout most 

 of central Texas and southern Oklahoma as an interstitial biotype, 

 which was able to maintain contiguous distribution by means of 

 dispersal thi'ough shallow ground waters. Mechanisms of wide dis- 

 persal were probably similar to those currently utilized by widespread, 

 phreatobitic species such as S. alabamensis, S. allegheniensis, and S. 

 tenuis. As the climate in Texas became progressively drier during 

 the Pleistocene (Hibbard, 1960; Schuchert and Dunbar, 1950), 

 ground-water tables slowly dropped and near-sm-face habitats largely 

 disappeared. Populations inhabitating these biotopes would have 

 been able to survive by migrating into deeper subterranean waters 

 provided by the numerous caves and solution channels developed in 

 the cavernous Edwards Plateau. 



Throughout most of the more than 100-mile stretch that presently 

 exists between populations of stygonectids in northeastern Texas 

 (Dallas Co.) and those in central Texas (Coryell Co.), deeper ground- 

 water habitats were apparently not available for colonization by 

 subterranean amphipods. This area is not cavernous and is pre- 

 siunably devoid of any extensive subsurface drainage; seeps and 

 springs are uncommon. During a period of more humid climate, 

 however, interstitial amphipods could have existed there in habitats 

 developed near the siu-face. But as the climate became drier and 

 available habitats disappeared, interstitial inhabitants of this area 

 woidd have been exterminated. Thus, it is suggested that by late 

 Pleistocene or early Recent times, surviving populations of this stock 

 in central Texas were well isolated from populations of the same stock 



