SUBTERRANEAN AJMPHIPOD STYGONECTES 139 



believed to represent rather highly specialized characters. In general, 

 morphological and distributional data tend to indicate that this species 

 should be relegated to an evolutionary position near the emarginatus 

 group. 



S. spinatus occupies a greatty delimited range in Greenbrier Co., 

 W. Va., and occurs syn topically with S. emarginatus, but unlike the 

 latter, this species has never been taken in a cave south of the Green- 

 brier River or north of Greenbrier County. The invasion and 

 colonization of ground waters of the Greenbrier Valley by precursor 

 spinatus probably occurred at a different tmie than that of precursor 

 S. emarginatus. On the basis of its more restricted range, S. spinatus 

 might be regarded as a derivative of the older of the two postulated 

 invasions. This species is either the relict of a former species group 

 now largely extinct, or it is the single end product of an old branch 

 oflP of the ancestral line which gave rise to the emarginatus group. 



The flagellatus group. — The distribution of species in the 

 jiagellatus group is shown on the map in figure 33, and their suggested 

 phylogenies are diagrammed in figure 32. Patterns of speciation in 

 \h& jiagellatus group, like those in the emarginatus group, are insular, 

 but extrinsic barriers to gene fiow which appear to have affected 

 speciation of stygonectids in the Edwards Plateau region of central 

 Texas are of a different geological nature than those which were 

 operational in the central Appalachians. Distributional data mdicate 

 that ranges of species in ihe jiagellatus group reflect theoretical patterns 

 of delimited subterranean drainage systems. Perhaps chief among 

 dispersal barriers in the eastern part of the Edwards Plateau region 

 are extensive faiUt systems which developed dm'ing the Miocene and 

 subsequent epochs. These faidts are believed to have been largely 

 associated with the formation of the more extensive, northeasts 

 southwest trending Balcones fault zone, the approximate outline of 

 which is shown in figure 33, 



The Balcones fault zone marks the approximate boundary between 

 cavernous Cretaceous formations to the north and west and non- 

 cavernous formations to the east and south. Soluble, Lower Creta- 

 ceous limestones of the Edwards Plateau are generally characterized 

 by strata with a gentle regional dip to the southeast (Frank, 1964). 

 Primarily on the basis of Uvalde gravel deposits which occiu: in 

 terraces high above present streams and which mark upper Pliocene 

 or early Pleistocene stream levels at the eastern edge of the Edwards 

 Plateau, Barr (1960b) suggested that caves developed along the 

 eastern margin of this plateau are no older than early Pleistocene. 

 The origin of subterranean waters resm"ging through a number of large 

 springs along the Balcones fault zone between Austin and San Antonio 

 is not fidly known. Sellards et al. (1932) pointed out that while many 



242-803—67 10 



