. Bulletin - United States National Museum. Figure 35.—Distribution of tenuis group of Stygonectes in south-central United States.[O S. a. alabamensis; O S. a. occidentalis; ® S. barri; A S. hoiumani; X S. clantoni;A 5. elatus; ? S. montanus; + S. ozarkensis. Two symbols in a circle indicate twospecies from the same locahty.] 152 NATIONAL MUSEUM BULLETIN 259. Figure 36.—Suggested phylogeny of tenuis group of Stygonedes. Ao) was presumably a widely distributed brackish-water form, whichinhabited coastal areas during the Mississippian embayment andpossibly even earlier. It is impossible to
. Bulletin - United States National Museum. Figure 35.—Distribution of tenuis group of Stygonectes in south-central United States.[O S. a. alabamensis; O S. a. occidentalis; ® S. barri; A S. hoiumani; X S. clantoni;A 5. elatus; ? S. montanus; + S. ozarkensis. Two symbols in a circle indicate twospecies from the same locahty.] 152 NATIONAL MUSEUM BULLETIN 259. Figure 36.—Suggested phylogeny of tenuis group of Stygonedes. Ao) was presumably a widely distributed brackish-water form, whichinhabited coastal areas during the Mississippian embayment andpossibly even earlier. It is impossible to assign a specific time to theearly bifurcation of hne Ao into the divergent stocks represented byAi and A2, but it seems reasonable to suppose that such a spht occurredsometime near the beginning of the fresh-water invasion of tenuis groupprogenitors. The subsequent splitting of line Ai probably resultedfrom isolation of ancestral populations to fresh-water habitats adjacentto the Atlantic Coast (line Bi) and corresponding isolation of otherancestral populations to fresh-water habitats adjacent to the Gulf SUBTERRANEAN AlVIPHIPOD STYGONECTES 153 Coast (line B2). The large area between the ranges of eastern andsouthcentral species of Stygonectes shown in figure 30 may have neverbeen mvaded and colonized by ancestral tenuis group populations, andif not, it
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