Some freshwater ostracoda from Esmeralda Formation (Neogene) of Western Nevada

• F.M. Swain ^[1] ; J.L. Wagoner ^[2]
  1. [1] University of Delaware

     University of Delaware

     Estados Unidos

     
  2. [2] Lawrence Livermore National Laboratory

     Lawrence Livermore National Laboratory

     Estados Unidos

     
• Localización: Revista española de micropaleontología, ISSN 0556-655X, Vol. 17, Nº. 1, 1985, págs. 123-149
• Idioma: inglés
• Texto completo no disponible (Saber más ...)
• Resumen

  • In a section of the Esmeralda Formation (Late Miocene-Pliocene) 161 m. thick, exposed near Blair Junction, Esmeralda County, Nevada, 16 species of freshwater Ostracoda are recorded. Eight of the species are new and represent the genera Cypricercus, Kassinina, Heterocypris, Tuberocypris, Dogelinella?, Candona, Cypridopsella?, and Ussuriocypris; several of these are reported for the first time in North America. Other genera represented by species in open nomenclature are Paracypria?, Bradycypris?, Eucypris, Pontoniella?, Pseudocandona?, Darwinula?, and Limnocythere.

    The span of time represented by each of the ostracode species was controlled, at least partly, by ecological factors related to recurring intervals of volcanic activity in the area. In general, Candona parabretzi n. sp., Cypricercus oblongatus n. sp., Darwinula? sp. and Limnocythere sp. are long ranging in, or occur in the upper part of the Esmeralda lake sediments of the Blair Junction section. These forms were finally eliminated due to unfavorable environmental conditions in the lake waters, about 90 m. above the base of the measured section.

    Most of the other species are restricted to a thin basal 1.5 m. unit of the Blair Junction section. Detailed sampling within this unit showed that several of the species range throughout, or nearly throughout it: Heterocypris blairensis n. sp., Kassinina microreticulata n. sp., Tuberocypris cancellatus n. sp., and Dogelinella? coaldalensis n. sp. Several other species range only part way through or occur in the middle part of the unit (Cypridosella? esmeraldensis, n. sp., Bradycypris? sp., Ussuriocypris subreniformis n. sp., Herpetocypris stenoformis Dickinson and Swain, Pseudocandona? sp., and Pontoniella? sp.). At present it cannot be stated whether two faunules were entirely environmentally controlled, or are in part due to evolutionary modifications. The principal causes of the disappearance of both ostracodes and mollusks in the upper part of the exposed Esmeralda Formation in the Blair Junction section were probably a combination of recurring volcanic activity and increasing aridity during pre-Pleistocene time in this area, that eventually destroyed the habitats of ostracodes and mollusks. A variety of other ostracode species are present in additional exposures of the Esmeralda Formation in western Nevada. Cyprideis sp., a brackish water ostracode, is among those at other localities, although absent at Blair Junction. Its presence indicates places in which Esmeralda lake waters became alkaline. Possible zonation of the Neogene freshwater ostracodes of this region is presented.