The subterranean amphipod crustacean fauna of an artesian well in Texas

Subterranean amphipod crustaceans of the suborder Gammaridea have been sampled on a regular basis from the artesian well in San Marcos, Texas, since late 1973. Water in the artesian well comes from the extensive, subterranean Edwards Aquifer, which is associated with the Balcones Escarpment and Fault Zone of south-central Texas. The taxonomically diverse amphipod fauna of the well is believed to be one of the richest of its kind in the world and is composed of five families, six genera, and 10 species. One family, four genera, and six species are newly described herein. Four distinct phylogenetic lineages are represented in the amphipod fauna: crangonyctids, hadzioids, bogidielloids, and sebids.
The zoogeographic and ecological implications of the artesian well amphipod fauna are profound. Approximately 80 percent of the species have obvious affinities with marine or brackish water species and are presumably derivatives of marine ancestors that colonized newly opened freshwater habitats during recession of epeiric seas in the Late Cretaceous. The high taxonomic diversity of the amphipod fauna is probably explained by: (1) exposure of south-central Texas to an extensive marine embayment during the Cretaceous period, (2) complex geological structure of the Balcones Fault Zone and large size of the Edwards Aquifer, and (3) ecological complexity of aquatic communities in the Edwards Aquifer.