SUBTERRANEAN AJMPHIPOD STYGONECTES 125 



in loosely consolidated coastal plain sediments to caves and solution 

 channels situated in Paleozoic limestones. The general paucity of 

 locality records for this genus may to some extent be attributed to 

 the inaccessibility of habitats to direct investigation rather than to 

 an actual scarcity in the number of individuals. 



Origin of Stygonectes. — ^Any hypothesis proposed to explain the 

 origin of stygonectid fauna in the fresh ^vaters of North America, no 

 matter how broad in scope, is fraught with certain difficulties. Two 

 major obstacles to a completel}'' workable hypothesis are: (1) the 

 total lack of a fossil record, and (2) the absence of marine forms that 

 can be regarded as related to an ancestral stock. Both of these 

 factors demand that any acceptable theory must be based almost 

 entirely on patterns of distribution of extant, fresh-water species. 

 When, one plots the present distribution of these species, how- 

 ever, and compares it with the ancient shoreline of the Mississippian 

 embayment (fig. 30), a rather striking pattern of correlation be- 

 comes evident, and its possible zoogeographic significance cannot 

 be overlooked. I have already pointed out in an earlier paper on 

 Stygonectes of the south-central United States (Holsinger, 1966) that 

 the optimum time for marine ancestors to have migrated to fresh 

 water would have been at the maximum extent of the Mississippian 

 embayment during the Eocene. It now seems feasible, in light of the 

 more complete data available from all known species in the genus, to 

 broaden this theory and extend it to the entire group. 



Schuchert and Dunbar (1950) have pointed out that during much of 

 the Cenozoic Period the Gulf Coastal Plain was largely submerged, 

 and to a lesser extent the Atlantic Coastal Plain was also covered by a 

 shallow marginal sea. The greatest inland encroachment of this 

 shallow sea apparently reached its climax sometime during the Eocene, 

 and the outline shown in figiu^e 30 illustrates what Schuchert and 

 Dunbar (1950) and more recently Kummel (1961) have considered as 

 representing the fm-thest points of submergence at that time. 



Established limits of the distribution of Stygonectes indicate that 

 no presently known species of this genus ranges more than 300 miles 

 away from the old Mississippian embayment shoreline. Fm'thermore, 

 the ranges of most species of Stygonectes are situated much closer than 

 300 mUes from this ancient shoreline, and several ranges either lie 

 contiguous to this line or overlap it. It is of further interest to com- 

 pare species distributions to the contemporary shoreline of North 

 America. If species which range closer to the present coastline are 

 compared to those which range further inland, they are seen, with few 

 exceptions, to have much wider distribution, to demonstrate fewer 

 evolutionary modifications or specializations, and almost exclusively 

 to inhabit interstitial biotopes. Species occm-ring further inland 



